hoosing Where to Summarize Routes
EIGRP supports route summarization at any router, unlike OSPF, which requires that summarization
be performed only at area border routers (ABR) or autonomous system border
routers (ASBR). EIGRP’s flexibility helps when designing the internetwork, but it also
poses some questions as to where to summarize EIGRP routes.
In some cases, the options are relatively obvious. For example, consider the 10.17.32.0/19
address block in manufacturing in Figure 1-1. The manufacturing division’s router could
summarize all its routes as a single 10.17.32.0/19 route when advertising to Core1. Alternately,
Core1 could summarize all those same routes, advertising a summary for
10.17.32.0/19. In either case, packets from the rest of the internetwork shown in Figure 1-1
will flow toward Core1 and then to the Manufacturing division.
Next, consider the 10.16.0.0/16 address block in the Data Center. Because all these subnets
reside to the right of Layer 3 switches Core1 and Core2, these two devices could summarize
10.16.0.0/16. However, these routes could also be summarized on WAN1/WAN2 for
advertisement to the branches on the left. Summarizing on Core1/Core2 helps reduce the size of the routing tables on WAN1 and WAN2. However, the sheer number of subnets in
a Data Center is typically small compared to the number of small remote sites, so the savings
of routing table space may be small.
One advantage of summarizing 10.16.0.0/16 onWAN1/WAN2 instead of Core1/Core2 in
be performed only at area border routers (ABR) or autonomous system border
routers (ASBR). EIGRP’s flexibility helps when designing the internetwork, but it also
poses some questions as to where to summarize EIGRP routes.
In some cases, the options are relatively obvious. For example, consider the 10.17.32.0/19
address block in manufacturing in Figure 1-1. The manufacturing division’s router could
summarize all its routes as a single 10.17.32.0/19 route when advertising to Core1. Alternately,
Core1 could summarize all those same routes, advertising a summary for
10.17.32.0/19. In either case, packets from the rest of the internetwork shown in Figure 1-1
will flow toward Core1 and then to the Manufacturing division.
Next, consider the 10.16.0.0/16 address block in the Data Center. Because all these subnets
reside to the right of Layer 3 switches Core1 and Core2, these two devices could summarize
10.16.0.0/16. However, these routes could also be summarized on WAN1/WAN2 for
advertisement to the branches on the left. Summarizing on Core1/Core2 helps reduce the size of the routing tables on WAN1 and WAN2. However, the sheer number of subnets in
a Data Center is typically small compared to the number of small remote sites, so the savings
of routing table space may be small.
One advantage of summarizing 10.16.0.0/16 onWAN1/WAN2 instead of Core1/Core2 in
