EIGRP

 Q. Does EIGRP require a default network 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 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 from a circular number space shared by all interfaces. 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. Prior to 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 statement on each WAN interface. Refer to the EIGRP White Paper for more details 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. You can configure more than one EIGRP autonomous system on the same router, but Cisco does not recommend it. Multiple EIGRP autonomous systems on the same router that use mutual redistribution can cause discrepancies in the EIGRP topology table. Cisco recommends you configure only one EIGRP autonomous system in any single autonomous system. Also use another protocol, such as 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 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 details 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 statementwith 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 deny172.16.1.0/28 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 details on load balancing.