Router D: Route by way of Router B is removed from the topology table. This is the successor route. Router D has no feasible successor identified. Router D must complete a new route computation. In Router C: Route to Network A by way of Router D is down. Route by way of Router D is removed from the table. This is the feasible successor route for Router C. In Router D: Router D has no feasible successor. It cannot switch to an identified alternative backup route. Router D must recompute the topology of the network. The path to destination Network A is set to Active. Router D sends a query packet to all connected neighbors, Router C and Router E, requesting topology information. Router C does have a previous entry for Router D. Router D does not have a previous entry for Router E. In Router E: Route to Network A through Router D is down. The route by way of Router D is taken down. This is the successor route for Router E. Router E does not have a feasible route identified. Note that the Reported Distance cost of routing by way of Router C is 3, the same cost as the successor route by way of Router D. In Router C: Router E sends a query packet to Router C. Router C removes Router E from the table. Router C replies to Router D with new route to Network A. In Router D: Route status to destination Network A is still marked as Active. Computing has not been completed yet. Router C has replied to Router D to confirm that a route to destination Network A is available with a cost of 5. Router D is still waiting for a reply from Router E. In Router E: Router E has no feasible successor to reach destination Network A. Router E, therefore, tags the status of the route to destination network as Active. Router E will have to recompute the network topology. Router E removes the route by way of Router D from the table. Router E sends a query to Router C, requesting topology information. Router E already has an entry by way of Router C. It is at a cost of 3, the same as the successor route. In Router E: Router C replies with an RD of 3. Router E can now set the route by way of Router C as the new successor with an FD of 4 and an RD of 3. Router E replaces the “Active” status of the route to destination Network A with a “Passive Status”. Note that a route will have a “Passive Status” by default, as long as hello packets are being received. In this example, only “Active Status” routes are flagged. In Router E: Router E sends a reply to Router D informing of Router E topology information. In Router D: Router D receives the reply packed from Router E, informing of Router E topology information. Router D enters this data for the route to destination Network A by way of Router E. This route becomes an additional successor route as the cost is the same as routing by way of Router C and the RD is less than the FD cost of 5. Convergence has occurred among all EIGRP routers using the DUAL algorithm.

Content 3.2 EIGRP Configuration 3.2.1 Configuring EIGRP Despite the complexity of DUAL, configuring EIGRP can be relatively simple. EIGRP configuration commands vary depending on the protocol that is to be routed. Some examples of these protocols are IP, IPX, and AppleTalk. This section covers EIGRP configuration for the IP protocol. Perform the following steps to configure EIGRP for IP:

Use the following to enable EIGRP and define the autonomous system: router(config)#router eigrp autonomous-system-number The autonomous system number is used to identify all routers that belong within the internetwork. This value must match all routers within the internetwork.
Indicate which networks belong to the EIGRP autonomous system on the local router by using the following command: router(config-router)#network network-number The network-number is the network number that determines which interfaces of the router are participating in EIGRP and which networks are advertised by the router. The network command configures only connected networks. For example, network 3.1.0.0, which is on the far left of the main Figure, is not directly connected to Router A. Consequently, that network is not part of the configuration of Router A.
When configuring serial links using EIGRP, it is important to configure the bandwidth setting on the interface. If the bandwidth for these interfaces is not changed, EIGRP assumes the default bandwidth on the link instead of the true bandwidth. If the link is slower, the router may not be able to converge, routing updates might become lost, or suboptimal path selection may result. To set the interface bandwidth, use the following syntax: router(config-if)#bandwidth kilobits The bandwidth command is only used by the routing process and should be set to match the line speed of the interface.
Cisco also recommends adding the following command to all EIGRP configurations: router(config-if)#eigrp log-neighbor-changes This command enables the logging of neighbor adjacency changes to monitor the stability of the routing system and to help detect problems.

Lab Activity Lab Exercise: Configuring EIGRP Routing This lab is to setup an IP addressing scheme for the network. Lab Activity e-Lab Activity: Configuring EIGRP In this lab, the student will configure EIGRP routing.

Content 3.2 EIGRP Configuration 3.2.2 Configuring EIGRP summarization EIGRP automatically summarizes routes at the classful boundary. This is the boundary where the network address ends, as defined by class-based addressing. This means that even though RTC is connected only to the subnet 2.1.1.0, it will advertise that it is connected to the entire Class A network, 2.0.0.0. In most cases auto summarization is beneficial because it keeps routing tables as compact as possible. However, automatic summarization may not be the preferred option in certain instances. For example, if there are discontiguous subnetworks auto-summarization must be disabled for routing to work properly. To turn off auto-summarization, use the following command: router(config-router)#no auto-summary With EIGRP, a summary address can be manually configured by configuring a prefix network. Manual summary routes are configured on a per-interface basis, so the interface that will propagate the route summary must be selected first. Then the summary address can be defined with the ip summary-address eigrp command: router(config-if)#ip summary-address eigrp autonomous-system-number ip-address mask administrative-distance EIGRP summary routes have an administrative distance of 5 by default. Optionally, they can be configured for a value between 1 and 255. In Figure , RTC can be configured using the commands shown: RTC(config)#router eigrp 2446
RTC(config-router)#no auto-summary
RTC(config-router)#exit
RTC(config)#interface serial 0/0
RTC(config-if)#ip summary-address eigrp 2446 2.1.0.0 255.255.0.0 Therefore, RTC will add a route to its table as follows: D 2.1.0.0/16 is a summary, 00:00:22, Null0 Notice that the summary route is sourced from Null0 and not from an actual interface. This is because this route is used for advertisement purposes and does not represent a path that RTC can take to reach that network. On RTC, this route has an administrative distance of 5. RTD is not aware of the summarization but accepts the route. The route is assigned the administrative distance of a normal EIGRP route, which is 90 by default. In the configuration for RTC, auto-summarization is turned off with the no auto-summary command. If auto-summarization was not turned off, RTD would receive two routes, the manual summary address, which is 2.1.0.0 /16, and the automatic, classful summary address, which is 2.0.0.0 /8. In most cases when manually summarizing, the no auto-summary command should be issued.

Content 3.2 EIGRP Configuration 3.2.3 Verifying basic EIGRP Verifying EIGRP operation is performed by the use of