ip summary-address eigrp command, as long as a more specific route exists in the routing table. An EIGRP router does not autosummarize networks in which it does not participate.
Content 2.4 Implementing Advanced EIGRP Features 2.4.3 Configuring Manual Route Summarization Example Figure shows the discontiguous network 172.16.0.0. In the configuration examples for routers A and B, automatic summarization has been disabled, so the 172.16.1.0 and 172.16.2.0 subnets are advertised into network 10.0.0.0. The routing tables of routers in the 10.0.0.0 network, including router C as shown in Figure , now include these discontiguous subnets.An EIGRP router autosummarizes routes only for networks to which it is attached. If a network was not autosummarized at the major network boundary, as is the case in this example on routers A and B because autosummarization was turned off, all the subnet routes are carried into the router C routing table. Router C will not autosummarize the 172.16.1.0 and 172.16.2.0 subnets, because it does not own the 172.16.0.0 network. Therefore, router C would send routes to the 172.16.1.0 and 172.16.2.0 subnets to the WAN. Configuring a summary route on the router C interface serial 0/0/0, as shown in the figure, means that only one route is sent on the WAN, representing all subnets that belong to network 172.16.0.0. To configure manual route summarization, use the following procedure: Step 1 Select the interface to propagate the summary route. Step 2 Specify the EIGRP routing protocol, the AS number, and the summary address and the mask of the routes being summarized.
Note
For manual route summarization, the summary route is advertised only if a component (a more specific entry) of the summary route is present in the routing table.
Content 2.4 Implementing Advanced EIGRP Features 2.4.4 Load Balancing Across Equal Cost Paths Equal-cost load balancing is the ability of a router to distribute traffic over all its network ports that are the same metric from the destination address. EIGRP automatically load balances across equal cost paths. Load balancing increases the use of network segments and increases effective network bandwidth. For IP, Cisco IOS software by default will install up to four equal-cost paths in the routing table for most routing protocols. The maximum-paths maximum-path command can be used to allow up to six equal-cost paths. (Setting the maximum-path option to 1 disables load balancing.) When a packet is process-switched, load balancing over equal-cost paths occurs on a per-packet basis. When packets are fast-switched, load balancing over equal-cost paths occurs on a per-destination basis. Therefore, if you are testing load balancing, do not ping to or from routers with the fast-switching interfaces, because these locally router-generated packets are process-switched rather than fast-switched, and might produce confusing results.
Content 2.4 Implementing Advanced EIGRP Features 2.4.5 Load Balancing Across Unequal Cost Paths EIGRP can also balance traffic across multiple routes that have different metrics, which is called unequal-cost load balancing. The degree to which EIGRP performs load balancing is controlled with the variance command, as shown in Figure .
Content 2.4 Implementing Advanced EIGRP Features 2.4.6 Load Balancing Across Unequal Cost Paths Example In Figure , a variance of 2 is configured, and the range of the metric values, which are the FDs for router E to get to network Z, is 20 to 45. This range of values determines the feasibility of a potential route.A route is feasible if the next router in the path is closer to the destination than the current router, and if the metric of the alternate path is within the variance. Load balancing can use only feasible paths, and the routing table includes only these paths. The two feasibility conditions are: If both of these conditions are met, the route is considered feasible and can be added to the routing table. In the figure, there are three paths to network Z with the following metrics: By default, the router places only path 2, via C, in the routing table, because it is the least cost path. To load balance over paths 1 and 2, use a variance of 2, because 20 * 2 = 40, which is greater than the metric through path 1. In this example, router E uses router C as the successor because it has the lowest FD (20). With the variance 2 command applied to router E, the path through router B meets the criteria for load balancing. In this case, the FD through router B is less than twice the FD for the successor (router C). Router D is not considered for load balancing with this variance, because the FD through router D is greater than twice the FD for the successor (router C). In this example, however, router D would never be a feasible successor no matter what the variance is, because router D’s AD of 25 is greater than router E’s FD of 20. To avoid a potential routing loop, router D is not considered a feasible successor.
Content 2.4 Implementing Advanced EIGRP Features 2.4.7 EIGRP Bandwidth Use Across WAN Links EIGRP operates efficiently in WAN environments. It is scalable on both point-to-point links and multipoint nonbroadcast multiaccess (NBMA) links. Because of the inherent differences in the operational characteristics of WAN links, the default configuration parameters may not be the best option for all WAN links. A solid understanding of EIGRP operation, coupled with knowledge of available link speeds, can yield an efficient, reliable, and scalable router configuration. By default, EIGRP may use up to 50 percent of the bandwidth of an interface or subinterface for routing traffic. EIGRP uses the bandwidth specified with the bandwidth command, or the default bandwidth of the link if none is configured, when calculating how much bandwidth to use. This percentage can be changed on a per-interface basis by using the ip bandwidth-percent eigrp as-number percent command, where as-number is the autonomous system number, and percent is the percentage of the configured bandwidth that EIGRP can use. This percentage can be greater than 100, which may be useful if the bandwidth is configured artificially low for routing-policy reasons. For example, assume that the actual bandwidth of a router serial link is 64 kbps, but the bandwidth value is set artificially low at 32 kbps. Figure shows how to modify the behavior of EIGRP so that it limits routing protocol traffic according to the actual bandwidth of the serial interface. The example configuration sets the bandwidth-percent on serial 0/0 to 100 percent for the EIGRP process running in AS 24. Since 100 percent of 32 kbps is 32, EIGRP can use half of the actual bandwidth of 64 kbps.
Content 2.4 Implementing Advanced EIGRP Features 2.4.8 Bandwidth Utilization over WAN Interfaces Cisco IOS software assumes that point-to-point Frame Relay subinterfaces (like all serial interfaces) operate at full T1 link speed. In many implementations, however, only fractional T1 speeds are available. Therefore, when configuring these subinterfaces, set the bandwidth to match the contracted committed information rate (CIR) of the permanent virtual circuit (PVC).When configuring multipoint interfaces, especially for Frame Relay