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Monday, October 17, 2011

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.

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