This document contains frequently asked questions (FAQs) about IP
Enhanced Interior Gateway Routing Protocol (EIGRP).
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.
Should I always use the eigrp log-neighbor-changes command when I
A. Yes, this command makes it easy to determine why an EIGRP neighbor was
reset. This reduces troubleshooting time.
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.
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
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
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
What percent of bandwidth and processor resources does EIGRP
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
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.
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.
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.
Can I configure more than one EIGRP autonomous system on the same
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.
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.
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:
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
Does the EIGRP DUAL-3-SIA Error Message Mean? for more
What does the neighbor statement in the EIGRP configuration section
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.
Why does the EIGRP passive-interface command remove all neighbors for an
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
Does the Passive Interface Feature Work in EIGRP? for more
Why are routes received from one neighbor on a point-to-multipoint
interface that runs EIGRP not propagated to another neighbor on the same
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
command. Some important points to remember about EIGRP split horizon
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
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
Horizon and Poison Reverse. For more information on commands, refer to
When I configure EIGRP, how can I configure a network statement with a
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
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
distribute-list prefix test in
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
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.
I have a router that runs Cisco Express Forwarding (CEF) and EIGRP. Who
does load-balancing when there are multiple links to a
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
Does Load Balancing Work? for more information on load balancing.
How do you verify if the EIGRP Non Stop Forwarding (NSF) feature is
A. In order to check the EIGRP NSF feature, issue the show
ip protocols command. Here is the sample output:
show ip protocols
Routing Protocol is "eigrp 101"
Outgoing update filter list for all interfaces is not set
Incoming update filter list for all interfaces is not set
Default networks flagged in outgoing updates
Default networks accepted from incoming updates
EIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0
EIGRP maximum hopcount 100
EIGRP maximum metric variance 1
Redistributing: eigrp 101
EIGRP NSF-aware route hold timer is 240s
Automatic network summarization is in effect
Maximum path: 4
Routing for Networks:
Routing Information Sources:
Gateway Distance Last Update
Distance: internal 90 external 170
This output shows that the router is NSF-aware and the route-hold timer
is set to 240 seconds, which is the default value.
How can I use only one path when a router has two equal cost
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.
What is the difference in metric calculation between EIGRP and
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
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
Routing. The EIGRP stub feature can be configured on the switch with the
stub [receive-only] [leak-map name] [connected] [static] [summary]
command. This feature can be removed with
the no eigrp stub command. When you remove the
eigrp stub command from the switch, the switch that
runs the IP Base image throws this 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
How can I send a default route to the Stub router from the
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.
What are different route types in EIGRP?
A. There are three different types of routes in EIGRP:
Internal Route—Routes that are originated within the
Autonomous System (AS).
Summary Route—Routes that are summarized in the
router (for example, internal paths that have been
External Route—Routes that are redistributed to
How do you redistribute an IPv6 default route in
A. For redistributing an IPv6 default route in EIGRP, a sample
configuration is shown here:
ipv6 prefix-list DEFAULT-ONLY-V6 seq 10 permit ::/0
route-map DEFAULT_2EIGRP-V6 permit 10
match ipv6 address prefix-list DEFAULT-ONLY-V6
router eigrp Starz_EIGRP
address-family ipv6 unicast
redistribute static route-map DEFAULT_2EIGRP-V6
How does EIGRP behave over a GRE tunnel compared to a directly connected
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.
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.
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.
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
Interfacing DUAL to the protocol specific routing
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
What are the various load-balancing options available in
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.
What does the %DUAL-5-NBRCHANGE: IP-EIGRP(0) 100: Neighbor
10.254.0.3 (Tunnel0) is down: holding time expired error
A. This message indicates that the router has not heard any EIGRP packets
from the neighbor within the hold-time limit. Because this is a packet-loss
issue, check for a Layer 2 problem.
Is there a IPv6 deployment guide that includes
A. Refer to
IPv6 in Branch Networks for more information.
From the 16:29:14.262 Poison squashed: 10.X.X.X/24
reverse message, what does poison
A. The router threads a topology table entry as a poison in reply to an
update received (the router sets up for poison reverse). While the router is
building the packet that contains the poison reverse, the router realizes that
it does not need to send it. For example, if the router receives a query for
the route from the neighbor, it is currently threaded to poison. Thus, it sends
the poison squashed message.
Is it normal that EIGRP takes over 30 seconds to
A. EIGRP taking longer to converge under heavy CPU usage is a normal
behavior. EIGRP convergence is faster when you lower the hold time. The lowest
values for hello and hold time are 1 second and 3 seconds respectively. For
Router(Config)# interface Fa0/0
!--- (Under an interface directly connected to EIGRP peers.)
Router(Config-if)#ip hello-interval eigrp 1
Router(Config-if)#ip hold-time eigrp 3
Note: Make sure that the hold time is changed on both ends.
For more information on EIGRP performance related issues, refer to
How to resolve EIGRP