This document contains frequently asked questions (FAQs) about IP Enhanced Interior Gateway Routing Protocol (EIGRP).

A. Although EIGRP can propagate a default route using the default network method, it is not required. EIGRP redistributes default routes directly.

A. Yes, this command makes it easy to determine why an EIGRP neighbor was reset. This reduces troubleshooting time.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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 summarized).

• External Route—Routes that are redistributed to EIGRP.

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

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.

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.

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.

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.

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.