Summarization should be performed on the interfaces outbound to the remote sites, as well. The purpose of this summarization is to limit the routing updates to the remote routers so that they contain only a default route or major net routes; without the summarization, all the components in the region will be sent to the remote sites. As explained in the Case Study later in the chapter, "Troubleshooting Stuck-In-Active Routes," sending the intra-region component routes unnecessarily to the remotes causes the remote sites to be included in the query process, which is not a good thing. Increasing the range of the query process beyond what is absolutely necessary increases the amount of work required to reach network convergence and the chances that there will be a problem with convergence due to link or router issues. The more devices or links involved in convergence increases the likelihood that you will have a problem with it.
Additionally, if the routes are not summarized from the distribution routers to the remote routers, a significant amount of more work and traffic are required to start up the neighbor relationship between the distribution and remote routers. Because smaller bandwidth links tend to be used between remote sites and the distribution layer, decreasing EIGRP's bandwidth requirements at startup is a wise move.
The method used to summarize the routes to the remote sites can be either the summary-address statement or the distribute-list statement. Either one of these methods will work fine for this application. For more on how to implement the summary-address and distribute-list statements, refer to "Case Study: Summarization Methods," later in the chapter.
At the end of the earlier section "Summarizing within the Core," the decision of whether to add summarization within the network core was not made. Based on the decision to summarize from the distribution layer into the core via summary-address or distribution-list statements, summarization within the core is unnecessary. Because each distribution layer router is sending only summary information to the core, it is probably unnecessary to further summarize between core routers.
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