Monday, October 17, 2011
About Juniper - Feel it !!!
Guys, recently I passed Juniper JNCIA, It was kinda cool exam. My juniper endeavor started when I stepped into ISP domain. You cannot survive without knowing about Juniper in ISP world. I started working on all service Routers, most of them are very powerful High end routers. It was bit challenging when I first started using this product, now its fun working on this box. What I learned from using different vendor product is " deeper understanding of technology ", To paraphrase it, I would say you learn better what you learned in the first place with Cisco. JNCIA is equivalent to CCNA, but from exam perspective it is lot simpler than CCNA. The CLI method in JunOS is totally ironic to Cisco CLI. It will take some time and practice to master it. I am just working on this area as well. I suggest you guys to keep an eye on multiple vendor products rather than sticking on to one product. To start with Cisco (CCNA) is right choice. As you move on the ladder of networking, try to taste and feel all competitive products.
EIGRP Frequently Asked Questions
Questions
Introduction
Does EIGRP require an ip default−network command to propagate a default route?
Should I always use the eigrp log−neighbor−changes command when I configure EIGRP?
Does EIGRP support secondary addresses?
What debugging capabilities does EIGRP have?
What does the word serno mean on the end of an EIGRP topology entry when you issue the show ip
eigrp topology command?
What percent of bandwidth and processor resources does EIGRP use?
Does EIGRP support aggregation and variable length subnet masks?
Does EIGRP support areas?
Can I configure more than one EIGRP autonomous system on the same router?
If there are two EIGRP processes that run and two equal paths are learned, one by each EIGRP
process, do both routes get installed?
What does the EIGRP stuck in active message mean?
What does the neighbor statement in the EIGRP configuration section do?
Why does the EIGRP passive−interface command remove all neighbors for an interface?
Why are routes received from one neighbor on a point−to−multipoint interface that runs EIGRP not
propagated to another neighbor on the same point−to−multipoint interface?
When I configure EIGRP, how can I configure a network statement with a mask?
I have two routes: 172.16.1.0/24 and 172.16.1.0/28. How can I deny 172.16.1.0/28 while I allow
172.16.1.0/24 in EIGRP?
I have a router that runs Cisco Express Forwarding (CEF) and EIGRP. Who does load−balancing
when there are multiple links to a destination?
How can I use only one path when a router has two equal cost paths?
What is the difference in metric calculation between EIGRP and IGRP?
What is the EIGRP Stub Routing feature?
How can I send a default route to the Stub router from the hub?
How EIGRP behaves over a GRE tunnel compared to a directly connected network?
What is an offset−list, and how is it useful?
How can I tag external routes in EIGRP?
What are the primary functions of the PDM?
What are the various load−balancing options available in EIGRP?
Related Information
ANSWERS FOR THE ABOVE QUESTIONS
Introduction
This document contains frequently asked questions (FAQs) about IP Enhanced Interior Gateway Routing
Protocol (EIGRP).
Q. Does EIGRP require an ip default−network command to propagate a
default route?
A. Although EIGRP can propagate a default route using the default network method, it is not
required. EIGRP redistributes default routes directly.
Q. Should I always use the eigrp log−neighbor−changes command when
I configure EIGRP?
A. Yes, this command makes it easy to determine why an EIGRP neighbor was reset. This
reduces troubleshooting time.
Q. Does EIGRP support secondary addresses?
A. EIGRP does support secondary addresses. Since EIGRP always sources data packets from
the primary address, Cisco recommends that you configure all routers on a particular subnet
with primary addresses that belong to the same subnet. Routers do not form EIGRP neighbors
over secondary networks. Therefore, if all of the primary IP addresses of routers do not agree,
problems can arise with neighbor adjacencies.
Q. What debugging capabilities does EIGRP have?
A. There are protocol−independent and −dependent debug commands. There is also a suite of
show commands that display neighbor table status, topology table status, and EIGRP traffic
statistics. Some of these commands are:
¨ show ip eigrp neighbors
¨ show ip eigrp interfaces
¨ show ip eigrp topology
¨ show ip eigrp traffic
Q. What does the word serno mean on the end of an EIGRP topology
entry when you issue the show ip eigrp topology command?
A. For example:
show ip eigrp topology
P 172.22.71.208/29, 2 successors, FD is 46163456
via 172.30.1.42 (46163456/45651456), Serial0.2, serno 7539273
via 172.30.2.49 (46163456/45651456), Serial2.6, serno 7539266
Serno stands for serial number. When DRDBs are threaded to be sent, they are assigned a
serial number. If you display the topology table at the time an entry is threaded, it shows you
the serial number associated with the DRDB.
Threading is the technique used inside the router to queue items up for transmission to
neighbors. The updates are not created until it is time for them to go out the interface. Before
that, a linked list of pointers to items to send is created (for example, the thread).
These sernos are local to the router and are not passed with the routing update.
Q. What percent of bandwidth and processor resources does EIGRP
use?
A. EIGRP version 1 introduced a feature that prevents any single EIGRP process from using
more than fifty percent of the configured bandwidth on any link during periods of network
convergence. Each AS or protocol (for instance, IP, IPX, or Appletalk) serviced by EIGRP is
a separate process. You can use the ip bandwidth−percent eigrp interface configuration
command in order to properly configure the bandwidth percentage on each WAN interface.
Refer to the EIGRP White Paper for more information on how this feature works.
In addition, the implementation of partial and incremental updates means that EIGRP sends
routing information only when a topology change occurs. This feature significantly reduces
bandwidth use.
The feasible successor feature of EIGRP reduces the amount of processor resources used by
an autonomous system (AS). It requires only the routers affected by a topology change to
perform route re−computation. The route re−computation only occurs for routes that were
affected, which reduces search time in complex data structures.
Q. Does EIGRP support aggregation and variable length subnet masks?
A. Yes, EIGRP supports aggregation and variable length subnet masks (VLSM). Unlike Open
Shortest Path First (OSPF), EIGRP allows summarization and aggregation at any point in the
network. EIGRP supports aggregation to any bit. This allows properly designed EIGRP
networks to scale exceptionally well without the use of areas. EIGRP also supports automatic
summarization of network addresses at major network borders.
Q. Does EIGRP support areas?
A. No, a single EIGRP process is analogous to an area of a link−state protocol. However,
within the process, information can be filtered and aggregated at any interface boundary. In
order to bound the propagation of routing information, you can use summarization to create a
hierarchy.
Q. Can I configure more than one EIGRP autonomous system on the
same router?
A. Yes, you can configure more than one EIGRP autonomous system on the same router. This
is typically done at a redistribution point where two EIGRP autonomous systems are
interconnected. Individual router interfaces should only be included within a single EIGRP
autonomous system.
Cisco does not recommend running multiple EIGRP autonomous systems on the same set of
interfaces on the router. If multiple EIGRP autonomous systems are used with multiple points
of mutual redistribution, it can cause discrepancies in the EIGRP topology table if correct
filtering is not performed at the redistribution points. If possible, Cisco recommends you
configure only one EIGRP autonomous system in any single autonomous system. You can
also use another protocol, such as Border Gateway Protocol (BGP), in order to connect the
two EIGRP autonomous systems.
Q. If there are two EIGRP processes that run and two equal paths are
learned, one by each EIGRP process, do both routes get installed?
A. No, only one route is installed. The router installs the route that was learned through the
EIGRP process with the lower Autonomous System (AS) number. In Cisco IOS Software
Releases earlier than 12.2(7)T, the router installed the path with the latest timestamp received
from either of the EIGRP processes. The change in behavior is tracked by Cisco bug ID
CSCdm47037.
Q. What does the EIGRP stuck in active message mean?
A. When EIGRP returns a stuck in active (SIA) message, it means that it has not received a
reply to a query. EIGRP sends a query when a route is lost and another feasible route does not
exist in the topology table. The SIA is caused by two sequential events:
¨ The route reported by the SIA has gone away.
¨ An EIGRP neighbor (or neighbors) have not replied to the query for that route.
When the SIA occurs, the router clears the neighbor that did not reply to the query. When this
happens, determine which neighbor has been cleared. Keep in mind that this router can be
many hops away. Refer to What Does the EIGRP DUAL−3−SIA Error Message Mean? for
more information.
Q. What does the neighbor statement in the EIGRP configuration section
do?
A. The neighbor command is used in EIGRP in order to define a neighboring router with
which to exchange routing information. Due to the current behavior of this command, EIGRP
exchanges routing information with the neighbors in the form of unicast packets whenever the
neighbor command is configured for an interface. EIGRP stops processing all multicast
packets that come inbound on that interface. Also, EIGRP stops sending multicast packets on
that interface.
The ideal behavior of this command is for EIGRP to start sending EIGRP packets as unicast
packets to the specified neighbor, but not stop sending and receiving multicast packets on that
interface. Since the command does not behave as intended, the neighbor command should be
used carefully, understanding the impact of the command on the network.
Q. Why does the EIGRP passive−interface command remove all
neighbors for an interface?
A. The passive−interface command disables the transmission and receipt of EIGRP hello
packets on an interface. Unlike IGRP or RIP, EIGRP sends hello packets in order to form and
sustain neighbor adjacencies. Without a neighbor adjacency, EIGRP cannot exchange routes
with a neighbor. Therefore, the passive−interface command prevents the exchange of routes
on the interface. Although EIGRP does not send or receive routing updates on an interface
configured with the passive−interface command, it still includes the address of the interface
in routing updates sent out of other non−passive interfaces. Refer to How Does the Passive
Interface Feature Work in EIGRP? for more information.
Q. Why are routes received from one neighbor on a point−to−multipoint
interface that runs EIGRP not propagated to another neighbor on the
same point−to−multipoint interface?
A. The split horizon rule prohibits a router from advertising a route through an interface that
the router itself uses to reach the destination. In order to disable the split horizon behavior,
use the no ip split−horizon eigrp as−number interface command. Some important points to
remember about EIGRP split horizon are:
¨ Split horizon behavior is turned on by default.
When you change the EIGRP split horizon setting on an interface, it resets all
adjacencies with EIGRP neighbors reachable over that interface.
¨ Split horizon should only be disabled on a hub site in a hub−and−spoke network.
Disabling split horizon on the spokes radically increases EIGRP memory
consumption on the hub router, as well as the amount of traffic generated on the
spoke routers.
¨
The EIGRP split horizon behavior is not controlled or influenced by the ip
split−horizon command.
¨
For more information on split horizon and poison reverse, refer to Split Horizon and Poison
Reverse. For more information on commands, refer to EIGRP Commands.
Q. When I configure EIGRP, how can I configure a network statement
with a mask?
A. The optional network−mask argument was first added to the network statement in Cisco
IOS Software Release 12.0(4)T. The mask argument can be configured in any format (such as
in a network mask or in wild card bits). For example, you can use network 10.10.10.0
255.255.255.252 or network 10.10.10.0 0.0.0.3.
Q. I have two routes: 172.16.1.0/24 and 172.16.1.0/28. How can I deny
172.16.1.0/28 while I allow 172.16.1.0/24 in EIGRP?
A. In order to do this you need to use a prefix−list as shown here:
router eigrp 100
network 172.16.0.0
distribute−list prefix test in
auto−summary
no eigrp log−neighbor−changes
!
ip prefix−list test seq 5 permit 172.16.1.0/24
This allows only the 172.16.1.0/24 prefix and therefore denies 172.16.1.0/28.
Note: The use of ACL and distribute−list under EIGRP does not work in this case. This is
because ACLs do not check the mask, they just check the network portion. Since the network
portion is the same, when you allow 172.16.1.0/24, you also allow 172.16.1.0/28.
Q. I have a router that runs Cisco Express Forwarding (CEF) and EIGRP.
Who does load−balancing when there are multiple links to a destination?
A. The way in which CEF works is that CEF does the switching of the packet based on the
routing table which is populated by the routing protocols such as EIGRP. In short, CEF does
the load−balancing once the routing protocol table is calculated. Refer to How Does Load
Balancing Work? for more information on load balancing.
Q. How can I use only one path when a router has two equal cost paths?
A. Configure the bandwidth value on the interfaces to default, and increase the delay on the
backup interface so that the router does not see two equal cost paths.
Q. What is the difference in metric calculation between EIGRP and IGRP?
A. The EIGRP metric is obtained when you multiply the IGRP metric by 256. The IGRP uses
only 24 bits in its update packet for the metric field, but EIGRP uses 32 bits in its update packet for the metric field. For example, the IGRP metric to a destination network is 8586,
but the EIGRP metric is 8586 x 256 = 2,198,016. Integer division is used when you divide
10^7 by minimum BW, so the calculation involves integer division, which leads to a variation
from manual calculation.
Q. What is the EIGRP Stub Routing feature?
A. The Stub routing feature is used to conserve bandwidth by summarizing and filtering
routes. Only specified routes are propagated from the remote (Stub) router to the distribution
router because of the Stub routing feature. For more information about the stub routing
feature, refer to EIGRP Stub Routing. The EIGRP stub feature can be configured on the
switch with the eigrp stub command, and it can be removed with the no eigrp stub. When
you remove the eigrp stub command from the switch, the switch that runs the IP Base image
throws the error:
EIGRP is restricted to stub configurations only
This issue can be resolved if you upgrade to Advanced Enterprise Images. This error is
documented in CSCeh58135.
Q. How can I send a default route to the Stub router from the hub?
A. Do this under the outbound interface on the hub router with the ip summary−address
eigrp X 0.0.0.0 0.0.0.0 command. This command suppresses all the more specific routes and
only sends the summary route. In the case of the 0.0.0.0 0.0.0.0, it means it suppresses
everything, and the only route that is in the outbound update is 0.0.0.0/0. One drawback to
this method is that EIGRP installs a 0.0.0.0/0 route to Null0 is the local routing table with an
admin distance of 5.
Q. How EIGRP behaves over a GRE tunnel compared to a directly
connected network?
A. EIGRP will use the same administrative distance and metric calculation for the GRE
tunnel. The cost calculation is based on bandwidth and delay. The bandwidth and delay of the
GRE tunnel will be taken from the tunnel interface configured on the router. The tunnel will
also be treated like a directly connected network. If there are two paths to reach a network
either through a VLAN interface or tunnel interface, EIGRP prefers the Virtual−Access
Interface (VAI) VLAN interface because the VLAN interface has greater bandwidth than the
tunnel interface. In order to influence the routing through the tunnel interface, increase the
bandwidth parameter of the tunnel interface, or increase the delay parameter of the VLAN
interface.
Q. What is an offset−list, and how is it useful?
A. The offset−list is an feature used to modify the composite metrics in EIGRP. The value
configured in the offset−list command is added to the delay value calculated by the router for
the route matched by an access−list. An offset−list is the preferred method to influence a
particular path that is advertised and/or chosen.
Q. How can I tag external routes in EIGRP?
A. You can tag routes that EIGRP has learned from another routing protocol using a 32 bit tag
value. Starting with ddts CSCdw22585, internal routes can also be tagged. However, the tag value cannot exceed 255 due to packet limitations for internal routes.
Q. What are the primary functions of the PDM?
A. EIGRP supports 3 protocol suites: IP, IPv6, and IPX. Each of them has its own PDM.
These are the primary functions of PDM:
Maintaining the neighbor and topology tables of EIGRP routers that belong to that
protocol suite
¨
¨ Building and translating protocol specific packets for DUAL
¨ Interfacing DUAL to the protocol specific routing table
Computing the metric and passing this information to DUAL; DUAL handles only
the picking of the feasible successors (FSs)
¨
¨ Implement filtering and access lists.
¨ Perform redistribution functions to/from other routing protocols.
Q. What are the various load−balancing options available in EIGRP?
A. The offset−list can be used to modify the metrics of routes that EIGRP learns through a
particular interface, or PBR can be used.
Introduction
Does EIGRP require an ip default−network command to propagate a default route?
Should I always use the eigrp log−neighbor−changes command when I configure EIGRP?
Does EIGRP support secondary addresses?
What debugging capabilities does EIGRP have?
What does the word serno mean on the end of an EIGRP topology entry when you issue the show ip
eigrp topology command?
What percent of bandwidth and processor resources does EIGRP use?
Does EIGRP support aggregation and variable length subnet masks?
Does EIGRP support areas?
Can I configure more than one EIGRP autonomous system on the same router?
If there are two EIGRP processes that run and two equal paths are learned, one by each EIGRP
process, do both routes get installed?
What does the EIGRP stuck in active message mean?
What does the neighbor statement in the EIGRP configuration section do?
Why does the EIGRP passive−interface command remove all neighbors for an interface?
Why are routes received from one neighbor on a point−to−multipoint interface that runs EIGRP not
propagated to another neighbor on the same point−to−multipoint interface?
When I configure EIGRP, how can I configure a network statement with a mask?
I have two routes: 172.16.1.0/24 and 172.16.1.0/28. How can I deny 172.16.1.0/28 while I allow
172.16.1.0/24 in EIGRP?
I have a router that runs Cisco Express Forwarding (CEF) and EIGRP. Who does load−balancing
when there are multiple links to a destination?
How can I use only one path when a router has two equal cost paths?
What is the difference in metric calculation between EIGRP and IGRP?
What is the EIGRP Stub Routing feature?
How can I send a default route to the Stub router from the hub?
How EIGRP behaves over a GRE tunnel compared to a directly connected network?
What is an offset−list, and how is it useful?
How can I tag external routes in EIGRP?
What are the primary functions of the PDM?
What are the various load−balancing options available in EIGRP?
Related Information
ANSWERS FOR THE ABOVE QUESTIONS
Introduction
This document contains frequently asked questions (FAQs) about IP Enhanced Interior Gateway Routing
Protocol (EIGRP).
Q. Does EIGRP require an ip default−network command to propagate a
default route?
A. Although EIGRP can propagate a default route using the default network method, it is not
required. EIGRP redistributes default routes directly.
Q. Should I always use the eigrp log−neighbor−changes command when
I configure EIGRP?
A. Yes, this command makes it easy to determine why an EIGRP neighbor was reset. This
reduces troubleshooting time.
Q. Does EIGRP support secondary addresses?
A. EIGRP does support secondary addresses. Since EIGRP always sources data packets from
the primary address, Cisco recommends that you configure all routers on a particular subnet
with primary addresses that belong to the same subnet. Routers do not form EIGRP neighbors
over secondary networks. Therefore, if all of the primary IP addresses of routers do not agree,
problems can arise with neighbor adjacencies.
Q. What debugging capabilities does EIGRP have?
A. There are protocol−independent and −dependent debug commands. There is also a suite of
show commands that display neighbor table status, topology table status, and EIGRP traffic
statistics. Some of these commands are:
¨ show ip eigrp neighbors
¨ show ip eigrp interfaces
¨ show ip eigrp topology
¨ show ip eigrp traffic
Q. What does the word serno mean on the end of an EIGRP topology
entry when you issue the show ip eigrp topology command?
A. For example:
show ip eigrp topology
P 172.22.71.208/29, 2 successors, FD is 46163456
via 172.30.1.42 (46163456/45651456), Serial0.2, serno 7539273
via 172.30.2.49 (46163456/45651456), Serial2.6, serno 7539266
Serno stands for serial number. When DRDBs are threaded to be sent, they are assigned a
serial number. If you display the topology table at the time an entry is threaded, it shows you
the serial number associated with the DRDB.
Threading is the technique used inside the router to queue items up for transmission to
neighbors. The updates are not created until it is time for them to go out the interface. Before
that, a linked list of pointers to items to send is created (for example, the thread).
These sernos are local to the router and are not passed with the routing update.
Q. What percent of bandwidth and processor resources does EIGRP
use?
A. EIGRP version 1 introduced a feature that prevents any single EIGRP process from using
more than fifty percent of the configured bandwidth on any link during periods of network
convergence. Each AS or protocol (for instance, IP, IPX, or Appletalk) serviced by EIGRP is
a separate process. You can use the ip bandwidth−percent eigrp interface configuration
command in order to properly configure the bandwidth percentage on each WAN interface.
Refer to the EIGRP White Paper for more information on how this feature works.
In addition, the implementation of partial and incremental updates means that EIGRP sends
routing information only when a topology change occurs. This feature significantly reduces
bandwidth use.
The feasible successor feature of EIGRP reduces the amount of processor resources used by
an autonomous system (AS). It requires only the routers affected by a topology change to
perform route re−computation. The route re−computation only occurs for routes that were
affected, which reduces search time in complex data structures.
Q. Does EIGRP support aggregation and variable length subnet masks?
A. Yes, EIGRP supports aggregation and variable length subnet masks (VLSM). Unlike Open
Shortest Path First (OSPF), EIGRP allows summarization and aggregation at any point in the
network. EIGRP supports aggregation to any bit. This allows properly designed EIGRP
networks to scale exceptionally well without the use of areas. EIGRP also supports automatic
summarization of network addresses at major network borders.
Q. Does EIGRP support areas?
A. No, a single EIGRP process is analogous to an area of a link−state protocol. However,
within the process, information can be filtered and aggregated at any interface boundary. In
order to bound the propagation of routing information, you can use summarization to create a
hierarchy.
Q. Can I configure more than one EIGRP autonomous system on the
same router?
A. Yes, you can configure more than one EIGRP autonomous system on the same router. This
is typically done at a redistribution point where two EIGRP autonomous systems are
interconnected. Individual router interfaces should only be included within a single EIGRP
autonomous system.
Cisco does not recommend running multiple EIGRP autonomous systems on the same set of
interfaces on the router. If multiple EIGRP autonomous systems are used with multiple points
of mutual redistribution, it can cause discrepancies in the EIGRP topology table if correct
filtering is not performed at the redistribution points. If possible, Cisco recommends you
configure only one EIGRP autonomous system in any single autonomous system. You can
also use another protocol, such as Border Gateway Protocol (BGP), in order to connect the
two EIGRP autonomous systems.
Q. If there are two EIGRP processes that run and two equal paths are
learned, one by each EIGRP process, do both routes get installed?
A. No, only one route is installed. The router installs the route that was learned through the
EIGRP process with the lower Autonomous System (AS) number. In Cisco IOS Software
Releases earlier than 12.2(7)T, the router installed the path with the latest timestamp received
from either of the EIGRP processes. The change in behavior is tracked by Cisco bug ID
CSCdm47037.
Q. What does the EIGRP stuck in active message mean?
A. When EIGRP returns a stuck in active (SIA) message, it means that it has not received a
reply to a query. EIGRP sends a query when a route is lost and another feasible route does not
exist in the topology table. The SIA is caused by two sequential events:
¨ The route reported by the SIA has gone away.
¨ An EIGRP neighbor (or neighbors) have not replied to the query for that route.
When the SIA occurs, the router clears the neighbor that did not reply to the query. When this
happens, determine which neighbor has been cleared. Keep in mind that this router can be
many hops away. Refer to What Does the EIGRP DUAL−3−SIA Error Message Mean? for
more information.
Q. What does the neighbor statement in the EIGRP configuration section
do?
A. The neighbor command is used in EIGRP in order to define a neighboring router with
which to exchange routing information. Due to the current behavior of this command, EIGRP
exchanges routing information with the neighbors in the form of unicast packets whenever the
neighbor command is configured for an interface. EIGRP stops processing all multicast
packets that come inbound on that interface. Also, EIGRP stops sending multicast packets on
that interface.
The ideal behavior of this command is for EIGRP to start sending EIGRP packets as unicast
packets to the specified neighbor, but not stop sending and receiving multicast packets on that
interface. Since the command does not behave as intended, the neighbor command should be
used carefully, understanding the impact of the command on the network.
Q. Why does the EIGRP passive−interface command remove all
neighbors for an interface?
A. The passive−interface command disables the transmission and receipt of EIGRP hello
packets on an interface. Unlike IGRP or RIP, EIGRP sends hello packets in order to form and
sustain neighbor adjacencies. Without a neighbor adjacency, EIGRP cannot exchange routes
with a neighbor. Therefore, the passive−interface command prevents the exchange of routes
on the interface. Although EIGRP does not send or receive routing updates on an interface
configured with the passive−interface command, it still includes the address of the interface
in routing updates sent out of other non−passive interfaces. Refer to How Does the Passive
Interface Feature Work in EIGRP? for more information.
Q. Why are routes received from one neighbor on a point−to−multipoint
interface that runs EIGRP not propagated to another neighbor on the
same point−to−multipoint interface?
A. The split horizon rule prohibits a router from advertising a route through an interface that
the router itself uses to reach the destination. In order to disable the split horizon behavior,
use the no ip split−horizon eigrp as−number interface command. Some important points to
remember about EIGRP split horizon are:
¨ Split horizon behavior is turned on by default.
When you change the EIGRP split horizon setting on an interface, it resets all
adjacencies with EIGRP neighbors reachable over that interface.
¨ Split horizon should only be disabled on a hub site in a hub−and−spoke network.
Disabling split horizon on the spokes radically increases EIGRP memory
consumption on the hub router, as well as the amount of traffic generated on the
spoke routers.
¨
The EIGRP split horizon behavior is not controlled or influenced by the ip
split−horizon command.
¨
For more information on split horizon and poison reverse, refer to Split Horizon and Poison
Reverse. For more information on commands, refer to EIGRP Commands.
Q. When I configure EIGRP, how can I configure a network statement
with a mask?
A. The optional network−mask argument was first added to the network statement in Cisco
IOS Software Release 12.0(4)T. The mask argument can be configured in any format (such as
in a network mask or in wild card bits). For example, you can use network 10.10.10.0
255.255.255.252 or network 10.10.10.0 0.0.0.3.
Q. I have two routes: 172.16.1.0/24 and 172.16.1.0/28. How can I deny
172.16.1.0/28 while I allow 172.16.1.0/24 in EIGRP?
A. In order to do this you need to use a prefix−list as shown here:
router eigrp 100
network 172.16.0.0
distribute−list prefix test in
auto−summary
no eigrp log−neighbor−changes
!
ip prefix−list test seq 5 permit 172.16.1.0/24
This allows only the 172.16.1.0/24 prefix and therefore denies 172.16.1.0/28.
Note: The use of ACL and distribute−list under EIGRP does not work in this case. This is
because ACLs do not check the mask, they just check the network portion. Since the network
portion is the same, when you allow 172.16.1.0/24, you also allow 172.16.1.0/28.
Q. I have a router that runs Cisco Express Forwarding (CEF) and EIGRP.
Who does load−balancing when there are multiple links to a destination?
A. The way in which CEF works is that CEF does the switching of the packet based on the
routing table which is populated by the routing protocols such as EIGRP. In short, CEF does
the load−balancing once the routing protocol table is calculated. Refer to How Does Load
Balancing Work? for more information on load balancing.
Q. How can I use only one path when a router has two equal cost paths?
A. Configure the bandwidth value on the interfaces to default, and increase the delay on the
backup interface so that the router does not see two equal cost paths.
Q. What is the difference in metric calculation between EIGRP and IGRP?
A. The EIGRP metric is obtained when you multiply the IGRP metric by 256. The IGRP uses
only 24 bits in its update packet for the metric field, but EIGRP uses 32 bits in its update packet for the metric field. For example, the IGRP metric to a destination network is 8586,
but the EIGRP metric is 8586 x 256 = 2,198,016. Integer division is used when you divide
10^7 by minimum BW, so the calculation involves integer division, which leads to a variation
from manual calculation.
Q. What is the EIGRP Stub Routing feature?
A. The Stub routing feature is used to conserve bandwidth by summarizing and filtering
routes. Only specified routes are propagated from the remote (Stub) router to the distribution
router because of the Stub routing feature. For more information about the stub routing
feature, refer to EIGRP Stub Routing. The EIGRP stub feature can be configured on the
switch with the eigrp stub command, and it can be removed with the no eigrp stub. When
you remove the eigrp stub command from the switch, the switch that runs the IP Base image
throws the error:
EIGRP is restricted to stub configurations only
This issue can be resolved if you upgrade to Advanced Enterprise Images. This error is
documented in CSCeh58135.
Q. How can I send a default route to the Stub router from the hub?
A. Do this under the outbound interface on the hub router with the ip summary−address
eigrp X 0.0.0.0 0.0.0.0 command. This command suppresses all the more specific routes and
only sends the summary route. In the case of the 0.0.0.0 0.0.0.0, it means it suppresses
everything, and the only route that is in the outbound update is 0.0.0.0/0. One drawback to
this method is that EIGRP installs a 0.0.0.0/0 route to Null0 is the local routing table with an
admin distance of 5.
Q. How EIGRP behaves over a GRE tunnel compared to a directly
connected network?
A. EIGRP will use the same administrative distance and metric calculation for the GRE
tunnel. The cost calculation is based on bandwidth and delay. The bandwidth and delay of the
GRE tunnel will be taken from the tunnel interface configured on the router. The tunnel will
also be treated like a directly connected network. If there are two paths to reach a network
either through a VLAN interface or tunnel interface, EIGRP prefers the Virtual−Access
Interface (VAI) VLAN interface because the VLAN interface has greater bandwidth than the
tunnel interface. In order to influence the routing through the tunnel interface, increase the
bandwidth parameter of the tunnel interface, or increase the delay parameter of the VLAN
interface.
Q. What is an offset−list, and how is it useful?
A. The offset−list is an feature used to modify the composite metrics in EIGRP. The value
configured in the offset−list command is added to the delay value calculated by the router for
the route matched by an access−list. An offset−list is the preferred method to influence a
particular path that is advertised and/or chosen.
Q. How can I tag external routes in EIGRP?
A. You can tag routes that EIGRP has learned from another routing protocol using a 32 bit tag
value. Starting with ddts CSCdw22585, internal routes can also be tagged. However, the tag value cannot exceed 255 due to packet limitations for internal routes.
Q. What are the primary functions of the PDM?
A. EIGRP supports 3 protocol suites: IP, IPv6, and IPX. Each of them has its own PDM.
These are the primary functions of PDM:
Maintaining the neighbor and topology tables of EIGRP routers that belong to that
protocol suite
¨
¨ Building and translating protocol specific packets for DUAL
¨ Interfacing DUAL to the protocol specific routing table
Computing the metric and passing this information to DUAL; DUAL handles only
the picking of the feasible successors (FSs)
¨
¨ Implement filtering and access lists.
¨ Perform redistribution functions to/from other routing protocols.
Q. What are the various load−balancing options available in EIGRP?
A. The offset−list can be used to modify the metrics of routes that EIGRP learns through a
particular interface, or PBR can be used.
OSPF: Frequently Asked Questions
Questions
Introduction
Why are loopbacks advertised as /32 host routes in OSPF?
How do I change the reference bandwidth in OSPF?
How does OSPF calculate its metric or cost?
Are OSPF routing protocol exchanges authenticated?
What is the link−state retransmit interval, and what is the command to set it?
What is the purpose of the variable IP−OSPF−Transmit−Delay?
Is it true that only the static option of the virtual link in OSPF allows discontiguous
networks, regardless of the mask propagation properties?
Are the multicast IP addresses mapped to MAC−level multicast addresses?
Does the Cisco OSPF implementation support IP TOS−based routing?
Does the offset−list subcommand work for OSPF?
Can an OSPF default be originated into the system based on external information on a
router that does not itself have a default?
Can I use the distribute−list in/out command with OSPF to filter routes?
How can I give preference to OSPF interarea routes over intra−area routes?
Do I need to manually set up adjacencies for routers on the Switched Multimegabit Data
Service (SMDS) cloud with the OSPF neighbor subcommand?
When routes are redistributed between OSPF processes, are all shortest path first
algorithm (SPF) metrics preserved, or is the default metric value used?
How does Cisco accommodate OSPF routing on partial−mesh Frame Relay networks?
Which address−wild−mask pair should I use for assigning an unnumbered interface to
an area?
Can I have one numbered side and leave the other side unnumbered in OSPF?
Why do I receive the "cannot allocate router id" error message when I configure Router
OSPF One?
Why do I receive the "unknown routing protocol" error message when I configure
Router OSPF One?
What do the states DR, BDR, and DROTHER mean in show ip ospf interface command
output?
When I issue the show ip ospf neighbor command, why do I only see FULL/DR and
FULL/BDR, with all other neighbors showing 2−WAY/DROTHER?
Why do I not see OSPF neighbors as FULL/DR or FULL/BDR on my serial link?
Do I need any special commands to run OSPF over BRI/PRI links?
Do I need any special commands to run OSPF over asynchronous links?
Which Cisco IOS Software release began support for per−interface authentication type
in OSPF?
Can I control the P−bit when importing external routes into a not−so−stubby area
(NSSA)?
Why are OSPF show commands responding so slowly?
What does the clear ip ospf redistribution command do?
Does OSPF form adjacencies with neighbors that are not on the same subnet?
How often does OSPF send out link−state advertisements (LSAs)?
How do I stop individual interfaces from developing adjacencies in an OSPF network?
When I have two type 5 link−state advertisements (LSAs) for the same external network
in the OSPF database, which path should be installed in the IP routing table?
Why is it that my Cisco 1600 router does not recognize the OSPF protocol?
Why is it that my Cisco 800 router does not run OSPF?
Should I use the same process number while configuring OSPF on multiple routers
within the same network?
I have a router that runs Cisco Express Forwarding (CEF) and OSPF, who does
load−balancing when there are multiple links to a destination?
How does OSPF use two Multilink paths to transfer packets?
How can you detect the topological changes rapidly?
Does the 3825 Series Router support the OSPF Stub feature?
What does the error message %OSPF−4−FLOOD_WAR: Process process−id
re−originates LSA ID ip address type−2 adv−rtr ip address in area area id means?
Can we have OSPF run over a GRE tunnel?
ANSWERS FOR THE ABOVE QUESTIONS
Introduction
The document addresses the most frequently asked questions (FAQ) associated with Open Shortest Path First
(OSPF). The document covers OSPF version 2 only. OSPF version 3, introduced in Cisco IOS® Software
Releases 12.0(24)S, 12.2(18)S, and 12.2(15)T, is used for distributing IP version 6 routing information; it is
not explicitly covered in this document. In the scope of this document, "OSPF" refers to OSPF version 2 and
"IP" refers to IP version 4.
Q. Why are loopbacks advertised as /32 host routes in OSPF?
A. Loopbacks are considered host routes in OSPF, and they are advertised as /32. For more
information, refer to section 9.1 of RFC 2328 . In Cisco IOS Software Releases 11.3T and
12.0, if the ip ospf network point−to−point command is configured under loopbacks, OSPF
advertises the loopback subnet as the actual subnet configured on loopbacks. ISDN dialer
interface advertises /32 subnet instead of its configured subnet mask. This is an expected
behavior if ip ospf network point−to−multipoint is configured.
Q. How do I change the reference bandwidth in OSPF?
A. You can change the reference bandwidth in Cisco IOS Software Release 11.2 and later
using the ospf auto−cost reference−bandwidth command under router ospf. By default,
reference bandwidth is 100 Mbps.
Q. How does OSPF calculate its metric or cost?
A. OSPF uses a reference bandwidth of 100 Mbps for cost calculation. The formula to
calculate the cost is reference bandwidth divided by interface bandwidth. For example, in the
case of Ethernet, it is 100 Mbps / 10 Mbps = 10.
Note: If ip ospf cost cost is used on the interface, it overrides this formulated cost.
Q. Are OSPF routing protocol exchanges authenticated?
A. Yes, OSPF can authenticate all packets exchanged between neighbors. Authentication may
be through simple passwords or through MD5 cryptographic checksums. To configure simple
password authentication for an area, use the command ip ospf authentication−key to assign
a password of up to eight octets to each interface attached to the area. Then, issue the area x
authentication command to the OSPF router configuration to enable authentication. (In the
command, x is the area number.)
Cisco IOS Software Release 12.x also supports the enabling of authentication on a
per−interface basis. If you want to enable authentication on some interfaces only, or if you
want different authentication methods on different interfaces that belong to the same area, use
the ip ospf authentication interface mode command.
Q. What is the link−state retransmit interval, and what is the command to
set it?
A. OSPF must send acknowledgment of each newly received link−state advertisement (LSA).
It does this by sending LSA packets. LSAs are retransmitted until they are acknowledged.
The link−state retransmit interval defines the time between retransmissions. You can use the
command ip ospf retransmit−interval to set the retransmit interval. The default value is 5
seconds.
Q. What is the purpose of the variable IP−OSPF−Transmit−Delay?
A. This variable adds a specified time to the age field of an update. If the delay is not added
before transmission over a link, the time in which the link−state advertisement (LSA)
propagates over the link is not considered. The default value is 1 second. This parameter has
more significance on very low−speed links.
Q. Is it true that only the static option of the virtual link in OSPF allows
discontiguous networks, regardless of the mask propagation
properties?
A. No, virtual links in OSPF maintain connectivity to the backbone from nonbackbone areas,
but they are unnecessary for discontiguous addressing. OSPF provides support for
discontiguous networks because every area has a collection of networks, and OSPF attaches a
mask to each advertisement.
Q. Are the multicast IP addresses mapped to MAC−level multicast
addresses?
A. OSPF sends all advertisements using multicast addressing. Except for Token Ring, the
multicast IP addresses are mapped to MAC−level multicast addresses. Cisco maps Token
Ring to MAC−level broadcast addresses.
Q. Does the Cisco OSPF implementation support IP TOS−based routing?
A. Cisco OSPF only supports TOS 0. This means that routers route all packets on the TOS 0
path, eliminating the need to calculate nonzero TOS paths.
Q. Does the offset−list subcommand work for OSPF?
A. The offset−list command does not work for OSPF. It is used for distance vector protocols
such as Interior Gateway Routing Protocol (IGRP), Routing Information Protocol (RIP), and
RIP version 2.
Q. Can an OSPF default be originated into the system based on external
information on a router that does not itself have a default?
A. OSPF generates a default only if it is configured using the command default−information
originate and if there is a default network in the box from a different process. The default
route in OSPF is 0.0.0.0. If you want an OSPF−enabled router to generate a default route even
if it does not have a default route itself, use the command default−information originate
always.
Q. Can I use the distribute−list in/out command with OSPF to filter
routes?
A. The distribute−list commands are supported in OSPF but work differently than
distance−vector routing protocols such as Routing Information Protocol (RIP) and Enhanced
Interior Gateway Routing Protocol (EIGRP). OSPF routes cannot be filtered from entering
the OSPF database. The distribute−list in command only filters routes from entering the
routing table; it does not prevent link−state packets from being propagated. Therefore, this
command does not help conserve router memory, and it does not prohibit a router from
propagating filtered routes to other routers.
Caution: Use of the distribute−list in command in OSPF may lead to routing loops in
the network if not implemented carefully.
The command distribute−list out works only on the routes being redistributed by the
Autonomous System Boundary Routers (ASBRs) into OSPF. It can be applied to external
type 2 and external type 1 routes, but not to intra−area and interarea routes.
Q. How can I give preference to OSPF interarea routes over intra−area
routes?
A. According to section 11 of RFC 2328 , the order of preference for OSPF routes is:
¨ intra−area routes, O
¨ interarea routes, O IA
¨ external routes type 1, O E1
¨ external routes type 2, O E2
This rule of preference cannot be changed. However, it applies only within a single OSPF
process. If a router is running more than one OSPF process, route comparison occurs. With
route comparison, the metrics and administrative distances (if they have been changed) of the
OSPF processes are compared. Route types are disregarded when routes supplied by two
different OSPF processes are compared.
Q. Do I need to manually set up adjacencies for routers on the Switched
Multimegabit Data Service (SMDS) cloud with the OSPF neighbor
subcommand?
A. In Cisco IOS Software releases earlier than Cisco IOS Software Release 10.0, the
neighbor command was required to establish adjacencies over nonbroadcast multiaccess
(NBMA) networks (such as Frame Relay, X.25, and SMDS). With Cisco IOS Software
Release 10.0 and later, you can use the ip ospf network broadcast command to define the
network as a broadcast network, eliminating the need for the neighbor command. If you are not using a fully meshed SMDS cloud, you must use the ip ospf network
point−to−multipoint command.
Q. When routes are redistributed between OSPF processes, are all
shortest path first algorithm (SPF) metrics preserved, or is the default
metric value used?
A. The SPF metrics are preserved. The redistribution between them is like redistribution
between any two IP routing processes.
Q. How does Cisco accommodate OSPF routing on partial−mesh Frame
Relay networks?
A. You can configure OSPF to understand whether it should attempt to use multicast facilities
on a multi−access interface. Also, if multicast is available, OSPF uses it for its normal
multicasts.
Cisco IOS Software Release 10.0 includes a feature called subinterfaces. You can use
subinterfaces with Frame Relay to tie together a set of virtual circuits (VCs) to form a virtual
interface, which acts as a single IP subnet. All systems within the subnet should be fully
meshed. With Cisco IOS Software Releases 10.3, 11.0 and later, the ip ospf
point−to−multipoint command is also available.
Q. Which address−wild−mask pair should I use for assigning an
unnumbered interface to an area?
A. When an unnumbered interface is configured, it references another interface on the router.
When enabling OSPF on the unnumbered interface, use the address−wild−mask pair of
interfaces to which the unnumbered interface is pointing.
Q. Can I have one numbered side and leave the other side unnumbered
in OSPF?
A. No, OSPF does not work if you have one side numbered and the other side unnumbered.
This creates a discrepancy in the OSPF database that prevents routes from being installed in
the routing table.
Q. Why do I receive the "cannot allocate router id" error message when I
configure Router OSPF One?
A. OSPF picks up the highest IP address as a router ID. If there are no interfaces in up/up
mode with an IP address, it returns this error message. To correct the problem, configure a
loopback interface.
Q. Why do I receive the "unknown routing protocol" error message when
I configure Router OSPF One?
A. Your software may not support OSPF. This error message occurs most frequently with the
Cisco 1600 series routers. If you are using a 1600 router, you need a Plus image to run OSPF.
Q. What do the states DR, BDR, and DROTHER mean in show ip ospf
interface command output?
A. DR means designated router. BDR means backup designated router. DROTHER indicates a
router that is neither the DR or the BDR. The DR generates a Network Link−State
Advertisement, which lists all the routers on that network.
Q. When I issue the show ip ospf neighbor command, why do I only see
FULL/DR and FULL/BDR, with all other neighbors showing
2−WAY/DROTHER?
A. To reduce the amount of flooding on broadcast media, such as Ethernet, FDDI, and Token
Ring, the router becomes full with only designated router (DR) and backup designated router
(BDR), and it shows 2−WAY for all other routers.
Q. Why do I not see OSPF neighbors as FULL/DR or FULL/BDR on my
serial link?
A. This is normal. On point−to−point and point−to−multipoint networks, there are no
designated routers (DRs) or backup designated routers (BDRs).
Q. Do I need any special commands to run OSPF over BRI/PRI links?
A. In addition to the normal OSPF configuration commands, you should use the dialer map
command. When using the dialer map command, use the broadcast keyword to indicate that
broadcasts should be forwarded to the protocol address.
Q. Do I need any special commands to run OSPF over asynchronous
links?
A. In addition to the normal OSPF configuration commands, you should use the async
default routing command on the asynchronous interface. This command enables the router to
pass routing updates to other routers over the asynchronous interface. Also, when using the
dialer map command, use the broadcast keyword to indicate that broadcasts should be
forwarded to the protocol address.
Q. Which Cisco IOS Software release began support for per−interface
authentication type in OSPF?
A. Per−interface authentication type, as described in RFC 2178 , was added in Cisco IOS
Software Release 12.0(8).
Q. Can I control the P−bit when importing external routes into a
not−so−stubby area (NSSA)?
A. When external routing information is imported into an NSSA in a type 7 link−state
advertisement (LSA), the type 7 LSA has only area flooding scope. To further distribute the
external information, type 7 LSAs are translated into type 5 LSAs at the NSSA border. The
P−bit in the type 7 LSA Options field indicates whether the type 7 LSA should be translated.
Only those LSAs with the P−bit set are translated. When you redistribute information into the NSSA, the P−bit is automatically set. A possible workaround applies when the Autonomous
System Boundary Router (ASBR) is also an Area Border Router (ABR). The NSSA ASBR
can then summarize with the not−advertise keyword, which results in not advertising the
translated type 7 LSAs.
Q. Why are OSPF show commands responding so slowly?
A. You may experience a slow response when issuing OSPF show commands, but not with
other commands. The most common reason for this delay is that you have the ip ospf
name−lookup configuration command configured on the router. This command causes the
router to look up the device Domain Name System (DNS) names for all OSPF show
commands, making it easier to identify devices, but resulting in a slowed response time for
the commands. If you are experiencing slow response on commands other than just OSPF
show commands, you may want to start looking at other possible causes, such as the CPU
utilization.
Q. What does the clear ip ospf redistribution command do?
A. The clear ip ospf redistribution command flushes all the type 5 and type 7 link−state
advertisements (LSAs) and scans the routing table for the redistributed routes. This causes a
partial shortest path first algorithm (SPF) in all the routers on the network that receive the
flushed/renewed LSAs. When the expected redistributed route is not in OSPF, this command
may help to renew the LSA and get the route into OSPF.
Q. Does OSPF form adjacencies with neighbors that are not on the same
subnet?
A. The only time that OSPF forms adjacencies between neighbors that are not on the same
subnet is when the neighbors are connected through point−to−point links. This may be
desired when using the ip unnumbered command, but in all other cases, the neighbors must
be on the same subnet.
Q. How often does OSPF send out link−state advertisements (LSAs)?
A. OSPF sends out its self−originated LSAs when the LSA age reaches the link−state refresh
time, which is 1800 seconds.
Q. How do I stop individual interfaces from developing adjacencies in an
OSPF network?
A. To stop routers from becoming OSPF neighbors on a particular interface, issue the
passive−interface command at the interface.
In Internet service provider (ISP) and large enterprise networks, many of the distribution
routers have more than 200 interfaces. Configuring passive−interface on each of the 200
interfaces can be difficult. The solution in such situations is to configure all the interfaces as
passive by default using a single passive−interface default command. Then, configure
individual interfaces where adjacencies are desired using the no passive−interface command.
For more information, refer to Default Passive Interface Feature.
There are some known problems with the passive−interface default command. Workarounds
are listed in Cisco bug ID CSCdr09263 ( registered customers only) .
Q. When I have two type 5 link−state advertisements (LSAs) for the same
external network in the OSPF database, which path should be installed
in the IP routing table?
A. When you have two type 5 LSAs for the same external network in the OSPF database,
prefer the external LSA that has the shortest path to the Autonomous System Boundary
Router (ASBR) and install that into the IP routing table. Use the show ip ospf
border−routers command to check the cost to the ASBR.
Q. Why is it that my Cisco 1600 router does not recognize the OSPF
protocol?
A. Cisco 1600 routers require the Plus feature set image of Cisco IOS Software to run OSPF.
Refer to Table 3: Cisco 1600 Series Routers Feature Sets in the Release Notes for Cisco IOS
Release 11.2(11) Software Feature Packs for Cisco 1600 Series Routers for more information.
Q. Why is it that my Cisco 800 router does not run OSPF?
A. Cisco 800 routers do not support OSPF. However, they do support Routing Information
Protocol (RIP) and Enhanced Interior Gateway Routing Protocol (EIGRP). You can use the
Software Advisor ( registered customers only) tool for more information on feature support.
Q. Should I use the same process number while configuring OSPF on
multiple routers within the same network?
A. OSPF, unlike Border Gateway Protocol (BGP) or Enhanced Interior Gateway Routing
Protocol (EIGRP), does not check the process number (or autonomous system number) when
adjacencies are formed between neighboring routers and routing information is exchanged.
The only case in which the OSPF process number is taken into account is when OSPF is used
as the routing protocol on a Provider Edge to Customer Edge (PE−CE) link in a Multiprotocol
Label Switching (MPLS) VPN. PE routers mark OSPF routes with the domain attribute
derived from the OSPF process number to indicate whether the route originated within the
same OSPF domain or from outside it. If the OSPF process numbering is inconsistent on PE
routers in the MPLS VPN, the domain−id OSPF mode command should be used to mark that
the OSPF processes with different numbers belong to the same OSPF domain.
This means that, in many practical cases, you can use different autonomous system numbers
for the same OSPF domain in your network. However, it is best to use consistent
OSPF−process numbering as much as possible. This consistency simplifies network
maintenance and complies with the network designer intention to keep routers in the same
OSPF domain.
Q. I have a router that runs Cisco Express Forwarding (CEF) and OSPF,
who does load−balancing when there are multiple links to a destination?
A. CEF works by performing the switching of the packet based on the routing table which is
populated by the routing protocols such as OSPF. CEF does the load−balancing once the
routing protocol table has been calculated. For more details on load balancing, refer to How
does load−balancing work?
Q. How does OSPF use two Multilink paths to transfer packets?
A. OSPF uses the metric aCost, which is related to the bandwidth. If there are equal cost paths
(the same bandwidth on both multilinks), OSPF installs both routes in the routing table. The
routing table tries to use both links equally, regardless of the interface utilization. If one of the
links in the first multilink fails, OSPF does not send all the traffic down the second multilink.
If the first multilink peaks 100%, OSPF does not send any traffic down the second multilink
because OSPF tries to use both links equally, regardless of the interface utilization. The
second is used fully only when the first multilink goes down.
Q. How can you detect the topological changes rapidly?
A. In order to have a rapid fault detection of topology changes, the hello timer value needs to
be set to 1 second. The hold timer value, which is is four times that of the hello timer, also
needs to be configured. There is a possibility of more routing traffic if the hello and hold
timer values are reduced from their default values.
Q. Does the 3825 Series Router support the OSPF Stub feature?
A. Yes, the 3800 Series Router that runs Advanced IPServices image supports the OSPF Stub
feature.
Q. What does the error message %OSPF−4−FLOOD_WAR: Process
process−id re−originates LSA ID ip address type−2 adv−rtr ip address in
area area id means?
A. The error message is due to the some router that is flushing the network LSA because the
network LSA received by the router whose LSA ID conflicts with the IP address of one of the
router's interfaces and flushes the LSA out of the network. For OSPF to function correctly the
IP addresses of transit networks must be unique. If it is not unique the conflicting routers
reports this error message. In the error message the router with the OSPF router ID reported
as adv−rtr reports this message.
Q. Can we have OSPF run over a GRE tunnel?
A. Yes, refer to Configuring a GRE Tunnel over IPSec with OSPF.
Introduction
Why are loopbacks advertised as /32 host routes in OSPF?
How do I change the reference bandwidth in OSPF?
How does OSPF calculate its metric or cost?
Are OSPF routing protocol exchanges authenticated?
What is the link−state retransmit interval, and what is the command to set it?
What is the purpose of the variable IP−OSPF−Transmit−Delay?
Is it true that only the static option of the virtual link in OSPF allows discontiguous
networks, regardless of the mask propagation properties?
Are the multicast IP addresses mapped to MAC−level multicast addresses?
Does the Cisco OSPF implementation support IP TOS−based routing?
Does the offset−list subcommand work for OSPF?
Can an OSPF default be originated into the system based on external information on a
router that does not itself have a default?
Can I use the distribute−list in/out command with OSPF to filter routes?
How can I give preference to OSPF interarea routes over intra−area routes?
Do I need to manually set up adjacencies for routers on the Switched Multimegabit Data
Service (SMDS) cloud with the OSPF neighbor subcommand?
When routes are redistributed between OSPF processes, are all shortest path first
algorithm (SPF) metrics preserved, or is the default metric value used?
How does Cisco accommodate OSPF routing on partial−mesh Frame Relay networks?
Which address−wild−mask pair should I use for assigning an unnumbered interface to
an area?
Can I have one numbered side and leave the other side unnumbered in OSPF?
Why do I receive the "cannot allocate router id" error message when I configure Router
OSPF One?
Why do I receive the "unknown routing protocol" error message when I configure
Router OSPF One?
What do the states DR, BDR, and DROTHER mean in show ip ospf interface command
output?
When I issue the show ip ospf neighbor command, why do I only see FULL/DR and
FULL/BDR, with all other neighbors showing 2−WAY/DROTHER?
Why do I not see OSPF neighbors as FULL/DR or FULL/BDR on my serial link?
Do I need any special commands to run OSPF over BRI/PRI links?
Do I need any special commands to run OSPF over asynchronous links?
Which Cisco IOS Software release began support for per−interface authentication type
in OSPF?
Can I control the P−bit when importing external routes into a not−so−stubby area
(NSSA)?
Why are OSPF show commands responding so slowly?
What does the clear ip ospf redistribution command do?
Does OSPF form adjacencies with neighbors that are not on the same subnet?
How often does OSPF send out link−state advertisements (LSAs)?
How do I stop individual interfaces from developing adjacencies in an OSPF network?
When I have two type 5 link−state advertisements (LSAs) for the same external network
in the OSPF database, which path should be installed in the IP routing table?
Why is it that my Cisco 1600 router does not recognize the OSPF protocol?
Why is it that my Cisco 800 router does not run OSPF?
Should I use the same process number while configuring OSPF on multiple routers
within the same network?
I have a router that runs Cisco Express Forwarding (CEF) and OSPF, who does
load−balancing when there are multiple links to a destination?
How does OSPF use two Multilink paths to transfer packets?
How can you detect the topological changes rapidly?
Does the 3825 Series Router support the OSPF Stub feature?
What does the error message %OSPF−4−FLOOD_WAR: Process process−id
re−originates LSA ID ip address type−2 adv−rtr ip address in area area id means?
Can we have OSPF run over a GRE tunnel?
ANSWERS FOR THE ABOVE QUESTIONS
Introduction
The document addresses the most frequently asked questions (FAQ) associated with Open Shortest Path First
(OSPF). The document covers OSPF version 2 only. OSPF version 3, introduced in Cisco IOS® Software
Releases 12.0(24)S, 12.2(18)S, and 12.2(15)T, is used for distributing IP version 6 routing information; it is
not explicitly covered in this document. In the scope of this document, "OSPF" refers to OSPF version 2 and
"IP" refers to IP version 4.
Q. Why are loopbacks advertised as /32 host routes in OSPF?
A. Loopbacks are considered host routes in OSPF, and they are advertised as /32. For more
information, refer to section 9.1 of RFC 2328 . In Cisco IOS Software Releases 11.3T and
12.0, if the ip ospf network point−to−point command is configured under loopbacks, OSPF
advertises the loopback subnet as the actual subnet configured on loopbacks. ISDN dialer
interface advertises /32 subnet instead of its configured subnet mask. This is an expected
behavior if ip ospf network point−to−multipoint is configured.
Q. How do I change the reference bandwidth in OSPF?
A. You can change the reference bandwidth in Cisco IOS Software Release 11.2 and later
using the ospf auto−cost reference−bandwidth command under router ospf. By default,
reference bandwidth is 100 Mbps.
Q. How does OSPF calculate its metric or cost?
A. OSPF uses a reference bandwidth of 100 Mbps for cost calculation. The formula to
calculate the cost is reference bandwidth divided by interface bandwidth. For example, in the
case of Ethernet, it is 100 Mbps / 10 Mbps = 10.
Note: If ip ospf cost cost is used on the interface, it overrides this formulated cost.
Q. Are OSPF routing protocol exchanges authenticated?
A. Yes, OSPF can authenticate all packets exchanged between neighbors. Authentication may
be through simple passwords or through MD5 cryptographic checksums. To configure simple
password authentication for an area, use the command ip ospf authentication−key to assign
a password of up to eight octets to each interface attached to the area. Then, issue the area x
authentication command to the OSPF router configuration to enable authentication. (In the
command, x is the area number.)
Cisco IOS Software Release 12.x also supports the enabling of authentication on a
per−interface basis. If you want to enable authentication on some interfaces only, or if you
want different authentication methods on different interfaces that belong to the same area, use
the ip ospf authentication interface mode command.
Q. What is the link−state retransmit interval, and what is the command to
set it?
A. OSPF must send acknowledgment of each newly received link−state advertisement (LSA).
It does this by sending LSA packets. LSAs are retransmitted until they are acknowledged.
The link−state retransmit interval defines the time between retransmissions. You can use the
command ip ospf retransmit−interval to set the retransmit interval. The default value is 5
seconds.
Q. What is the purpose of the variable IP−OSPF−Transmit−Delay?
A. This variable adds a specified time to the age field of an update. If the delay is not added
before transmission over a link, the time in which the link−state advertisement (LSA)
propagates over the link is not considered. The default value is 1 second. This parameter has
more significance on very low−speed links.
Q. Is it true that only the static option of the virtual link in OSPF allows
discontiguous networks, regardless of the mask propagation
properties?
A. No, virtual links in OSPF maintain connectivity to the backbone from nonbackbone areas,
but they are unnecessary for discontiguous addressing. OSPF provides support for
discontiguous networks because every area has a collection of networks, and OSPF attaches a
mask to each advertisement.
Q. Are the multicast IP addresses mapped to MAC−level multicast
addresses?
A. OSPF sends all advertisements using multicast addressing. Except for Token Ring, the
multicast IP addresses are mapped to MAC−level multicast addresses. Cisco maps Token
Ring to MAC−level broadcast addresses.
Q. Does the Cisco OSPF implementation support IP TOS−based routing?
A. Cisco OSPF only supports TOS 0. This means that routers route all packets on the TOS 0
path, eliminating the need to calculate nonzero TOS paths.
Q. Does the offset−list subcommand work for OSPF?
A. The offset−list command does not work for OSPF. It is used for distance vector protocols
such as Interior Gateway Routing Protocol (IGRP), Routing Information Protocol (RIP), and
RIP version 2.
Q. Can an OSPF default be originated into the system based on external
information on a router that does not itself have a default?
A. OSPF generates a default only if it is configured using the command default−information
originate and if there is a default network in the box from a different process. The default
route in OSPF is 0.0.0.0. If you want an OSPF−enabled router to generate a default route even
if it does not have a default route itself, use the command default−information originate
always.
Q. Can I use the distribute−list in/out command with OSPF to filter
routes?
A. The distribute−list commands are supported in OSPF but work differently than
distance−vector routing protocols such as Routing Information Protocol (RIP) and Enhanced
Interior Gateway Routing Protocol (EIGRP). OSPF routes cannot be filtered from entering
the OSPF database. The distribute−list in command only filters routes from entering the
routing table; it does not prevent link−state packets from being propagated. Therefore, this
command does not help conserve router memory, and it does not prohibit a router from
propagating filtered routes to other routers.
Caution: Use of the distribute−list in command in OSPF may lead to routing loops in
the network if not implemented carefully.
The command distribute−list out works only on the routes being redistributed by the
Autonomous System Boundary Routers (ASBRs) into OSPF. It can be applied to external
type 2 and external type 1 routes, but not to intra−area and interarea routes.
Q. How can I give preference to OSPF interarea routes over intra−area
routes?
A. According to section 11 of RFC 2328 , the order of preference for OSPF routes is:
¨ intra−area routes, O
¨ interarea routes, O IA
¨ external routes type 1, O E1
¨ external routes type 2, O E2
This rule of preference cannot be changed. However, it applies only within a single OSPF
process. If a router is running more than one OSPF process, route comparison occurs. With
route comparison, the metrics and administrative distances (if they have been changed) of the
OSPF processes are compared. Route types are disregarded when routes supplied by two
different OSPF processes are compared.
Q. Do I need to manually set up adjacencies for routers on the Switched
Multimegabit Data Service (SMDS) cloud with the OSPF neighbor
subcommand?
A. In Cisco IOS Software releases earlier than Cisco IOS Software Release 10.0, the
neighbor command was required to establish adjacencies over nonbroadcast multiaccess
(NBMA) networks (such as Frame Relay, X.25, and SMDS). With Cisco IOS Software
Release 10.0 and later, you can use the ip ospf network broadcast command to define the
network as a broadcast network, eliminating the need for the neighbor command. If you are not using a fully meshed SMDS cloud, you must use the ip ospf network
point−to−multipoint command.
Q. When routes are redistributed between OSPF processes, are all
shortest path first algorithm (SPF) metrics preserved, or is the default
metric value used?
A. The SPF metrics are preserved. The redistribution between them is like redistribution
between any two IP routing processes.
Q. How does Cisco accommodate OSPF routing on partial−mesh Frame
Relay networks?
A. You can configure OSPF to understand whether it should attempt to use multicast facilities
on a multi−access interface. Also, if multicast is available, OSPF uses it for its normal
multicasts.
Cisco IOS Software Release 10.0 includes a feature called subinterfaces. You can use
subinterfaces with Frame Relay to tie together a set of virtual circuits (VCs) to form a virtual
interface, which acts as a single IP subnet. All systems within the subnet should be fully
meshed. With Cisco IOS Software Releases 10.3, 11.0 and later, the ip ospf
point−to−multipoint command is also available.
Q. Which address−wild−mask pair should I use for assigning an
unnumbered interface to an area?
A. When an unnumbered interface is configured, it references another interface on the router.
When enabling OSPF on the unnumbered interface, use the address−wild−mask pair of
interfaces to which the unnumbered interface is pointing.
Q. Can I have one numbered side and leave the other side unnumbered
in OSPF?
A. No, OSPF does not work if you have one side numbered and the other side unnumbered.
This creates a discrepancy in the OSPF database that prevents routes from being installed in
the routing table.
Q. Why do I receive the "cannot allocate router id" error message when I
configure Router OSPF One?
A. OSPF picks up the highest IP address as a router ID. If there are no interfaces in up/up
mode with an IP address, it returns this error message. To correct the problem, configure a
loopback interface.
Q. Why do I receive the "unknown routing protocol" error message when
I configure Router OSPF One?
A. Your software may not support OSPF. This error message occurs most frequently with the
Cisco 1600 series routers. If you are using a 1600 router, you need a Plus image to run OSPF.
Q. What do the states DR, BDR, and DROTHER mean in show ip ospf
interface command output?
A. DR means designated router. BDR means backup designated router. DROTHER indicates a
router that is neither the DR or the BDR. The DR generates a Network Link−State
Advertisement, which lists all the routers on that network.
Q. When I issue the show ip ospf neighbor command, why do I only see
FULL/DR and FULL/BDR, with all other neighbors showing
2−WAY/DROTHER?
A. To reduce the amount of flooding on broadcast media, such as Ethernet, FDDI, and Token
Ring, the router becomes full with only designated router (DR) and backup designated router
(BDR), and it shows 2−WAY for all other routers.
Q. Why do I not see OSPF neighbors as FULL/DR or FULL/BDR on my
serial link?
A. This is normal. On point−to−point and point−to−multipoint networks, there are no
designated routers (DRs) or backup designated routers (BDRs).
Q. Do I need any special commands to run OSPF over BRI/PRI links?
A. In addition to the normal OSPF configuration commands, you should use the dialer map
command. When using the dialer map command, use the broadcast keyword to indicate that
broadcasts should be forwarded to the protocol address.
Q. Do I need any special commands to run OSPF over asynchronous
links?
A. In addition to the normal OSPF configuration commands, you should use the async
default routing command on the asynchronous interface. This command enables the router to
pass routing updates to other routers over the asynchronous interface. Also, when using the
dialer map command, use the broadcast keyword to indicate that broadcasts should be
forwarded to the protocol address.
Q. Which Cisco IOS Software release began support for per−interface
authentication type in OSPF?
A. Per−interface authentication type, as described in RFC 2178 , was added in Cisco IOS
Software Release 12.0(8).
Q. Can I control the P−bit when importing external routes into a
not−so−stubby area (NSSA)?
A. When external routing information is imported into an NSSA in a type 7 link−state
advertisement (LSA), the type 7 LSA has only area flooding scope. To further distribute the
external information, type 7 LSAs are translated into type 5 LSAs at the NSSA border. The
P−bit in the type 7 LSA Options field indicates whether the type 7 LSA should be translated.
Only those LSAs with the P−bit set are translated. When you redistribute information into the NSSA, the P−bit is automatically set. A possible workaround applies when the Autonomous
System Boundary Router (ASBR) is also an Area Border Router (ABR). The NSSA ASBR
can then summarize with the not−advertise keyword, which results in not advertising the
translated type 7 LSAs.
Q. Why are OSPF show commands responding so slowly?
A. You may experience a slow response when issuing OSPF show commands, but not with
other commands. The most common reason for this delay is that you have the ip ospf
name−lookup configuration command configured on the router. This command causes the
router to look up the device Domain Name System (DNS) names for all OSPF show
commands, making it easier to identify devices, but resulting in a slowed response time for
the commands. If you are experiencing slow response on commands other than just OSPF
show commands, you may want to start looking at other possible causes, such as the CPU
utilization.
Q. What does the clear ip ospf redistribution command do?
A. The clear ip ospf redistribution command flushes all the type 5 and type 7 link−state
advertisements (LSAs) and scans the routing table for the redistributed routes. This causes a
partial shortest path first algorithm (SPF) in all the routers on the network that receive the
flushed/renewed LSAs. When the expected redistributed route is not in OSPF, this command
may help to renew the LSA and get the route into OSPF.
Q. Does OSPF form adjacencies with neighbors that are not on the same
subnet?
A. The only time that OSPF forms adjacencies between neighbors that are not on the same
subnet is when the neighbors are connected through point−to−point links. This may be
desired when using the ip unnumbered command, but in all other cases, the neighbors must
be on the same subnet.
Q. How often does OSPF send out link−state advertisements (LSAs)?
A. OSPF sends out its self−originated LSAs when the LSA age reaches the link−state refresh
time, which is 1800 seconds.
Q. How do I stop individual interfaces from developing adjacencies in an
OSPF network?
A. To stop routers from becoming OSPF neighbors on a particular interface, issue the
passive−interface command at the interface.
In Internet service provider (ISP) and large enterprise networks, many of the distribution
routers have more than 200 interfaces. Configuring passive−interface on each of the 200
interfaces can be difficult. The solution in such situations is to configure all the interfaces as
passive by default using a single passive−interface default command. Then, configure
individual interfaces where adjacencies are desired using the no passive−interface command.
For more information, refer to Default Passive Interface Feature.
There are some known problems with the passive−interface default command. Workarounds
are listed in Cisco bug ID CSCdr09263 ( registered customers only) .
Q. When I have two type 5 link−state advertisements (LSAs) for the same
external network in the OSPF database, which path should be installed
in the IP routing table?
A. When you have two type 5 LSAs for the same external network in the OSPF database,
prefer the external LSA that has the shortest path to the Autonomous System Boundary
Router (ASBR) and install that into the IP routing table. Use the show ip ospf
border−routers command to check the cost to the ASBR.
Q. Why is it that my Cisco 1600 router does not recognize the OSPF
protocol?
A. Cisco 1600 routers require the Plus feature set image of Cisco IOS Software to run OSPF.
Refer to Table 3: Cisco 1600 Series Routers Feature Sets in the Release Notes for Cisco IOS
Release 11.2(11) Software Feature Packs for Cisco 1600 Series Routers for more information.
Q. Why is it that my Cisco 800 router does not run OSPF?
A. Cisco 800 routers do not support OSPF. However, they do support Routing Information
Protocol (RIP) and Enhanced Interior Gateway Routing Protocol (EIGRP). You can use the
Software Advisor ( registered customers only) tool for more information on feature support.
Q. Should I use the same process number while configuring OSPF on
multiple routers within the same network?
A. OSPF, unlike Border Gateway Protocol (BGP) or Enhanced Interior Gateway Routing
Protocol (EIGRP), does not check the process number (or autonomous system number) when
adjacencies are formed between neighboring routers and routing information is exchanged.
The only case in which the OSPF process number is taken into account is when OSPF is used
as the routing protocol on a Provider Edge to Customer Edge (PE−CE) link in a Multiprotocol
Label Switching (MPLS) VPN. PE routers mark OSPF routes with the domain attribute
derived from the OSPF process number to indicate whether the route originated within the
same OSPF domain or from outside it. If the OSPF process numbering is inconsistent on PE
routers in the MPLS VPN, the domain−id OSPF mode command should be used to mark that
the OSPF processes with different numbers belong to the same OSPF domain.
This means that, in many practical cases, you can use different autonomous system numbers
for the same OSPF domain in your network. However, it is best to use consistent
OSPF−process numbering as much as possible. This consistency simplifies network
maintenance and complies with the network designer intention to keep routers in the same
OSPF domain.
Q. I have a router that runs Cisco Express Forwarding (CEF) and OSPF,
who does load−balancing when there are multiple links to a destination?
A. CEF works by performing the switching of the packet based on the routing table which is
populated by the routing protocols such as OSPF. CEF does the load−balancing once the
routing protocol table has been calculated. For more details on load balancing, refer to How
does load−balancing work?
Q. How does OSPF use two Multilink paths to transfer packets?
A. OSPF uses the metric aCost, which is related to the bandwidth. If there are equal cost paths
(the same bandwidth on both multilinks), OSPF installs both routes in the routing table. The
routing table tries to use both links equally, regardless of the interface utilization. If one of the
links in the first multilink fails, OSPF does not send all the traffic down the second multilink.
If the first multilink peaks 100%, OSPF does not send any traffic down the second multilink
because OSPF tries to use both links equally, regardless of the interface utilization. The
second is used fully only when the first multilink goes down.
Q. How can you detect the topological changes rapidly?
A. In order to have a rapid fault detection of topology changes, the hello timer value needs to
be set to 1 second. The hold timer value, which is is four times that of the hello timer, also
needs to be configured. There is a possibility of more routing traffic if the hello and hold
timer values are reduced from their default values.
Q. Does the 3825 Series Router support the OSPF Stub feature?
A. Yes, the 3800 Series Router that runs Advanced IPServices image supports the OSPF Stub
feature.
Q. What does the error message %OSPF−4−FLOOD_WAR: Process
process−id re−originates LSA ID ip address type−2 adv−rtr ip address in
area area id means?
A. The error message is due to the some router that is flushing the network LSA because the
network LSA received by the router whose LSA ID conflicts with the IP address of one of the
router's interfaces and flushes the LSA out of the network. For OSPF to function correctly the
IP addresses of transit networks must be unique. If it is not unique the conflicting routers
reports this error message. In the error message the router with the OSPF router ID reported
as adv−rtr reports this message.
Q. Can we have OSPF run over a GRE tunnel?
A. Yes, refer to Configuring a GRE Tunnel over IPSec with OSPF.
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