When deciding where to summarize, follow this rule of thumb: Only provide full topology information where it's needed in the network. In other words, hide any information that isn't necessary to make a good routing decision.
For example, routers in the core don't need to know about every single network in the access layer. Rather than advertising a lot of detailed information about individual destinations into the core, distribution layer routers should summarize each group of access layer destinations into a single shorter prefix route and advertise these summary routes into the core.
Likewise, the access layer routers don't need to know how to reach each and every specific destination in the network; an access layer router should have only enough information to forward its traffic to one of the few (most likely two) distribution routers it is attached to. Typically, an access layer router needs only one route (the default route), although dual-homed access devices may need special consideration to reduce or eliminate suboptimal routing. This topic will be covered more thoroughly in Chapter 4, "Applying the Principles of Network Design."
As you can see from these examples, the distribution layer is the most natural summarization point in a hierarchical network. When being advertised into the core, destinations in the access layer can be summarized by distribution routers, reducing the area through which any topology change must propagate to only the local distribution region. Summarization from the distribution layer toward access layer routers can dramatically reduce the amount of information these routers must deal with.
Look at Figure 2-8 for a more concrete example. Router A, which is in the distribution layer, is receiving advertisements for:
Figure 2-8 Summarizing from the Distribution Layer into the Core
Figure 2-8 Summarizing from the Distribution Layer into
Router A is, in turn, summarizing these four routes into a single destination, 10.1.1.0/24, and advertising this into the core.
Because the four longer prefix networks 10.1.1.0/26, 10.1.1.64/26, 10.1.1.128/26, and 10.1.192/26 are hidden from the core routers, the core won't be affected if one of these networks fails, so none of the routers on the core will need to recalculate their routing tables. Hiding detailed topology information from the core has reduced the area through which the changes in the network must propagate.
Note that all the addresses in a range don't need to be used to summarize that range; they just can't be used elsewhere in the network. You could summarize 10.1.1.0/24, 10.1.2.0/24, and 10.1.3.0/24 into 10.1.0.0/16 as long as 10.1.4.0 through 10.1.255.255 aren't being used.
Figure 2-9 is an example of a distribution layer router summarizing the routing information being advertised to access layer devices. In Figure 2-8, the entire routing table on Router A has been summarized into one destination, 0.0.0.0/0, which is called the default route.
Figure 2-9 Summarizing from the Distribution Layer into the Access Layer
Figure 2-9 Summarizing from the Distribution Layer into
Because this default route is the only route advertised to the access layer routers, a destination that becomes unreachable in another part of the network won't cause these access layer routers to recompute their routing tables. In other words, they won't participate in convergence.
The downside to advertising the default route only to these routers is that suboptimal routing may result from doing so.
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