I recently encountered a problem where misconfigured route summarization resulted in suboptimal routing. Consider the following scenario.

A small LAN of two subnets has two equal-cost paths to its parent WAN through routers A and B, both of which run EIGRP in AS 123. By default, EIGRP will load balance these paths and all will be good. However, each subnet will be advertised separately. This isn't a big deal for just the two we have, but what about ten? Or a hundred?

Enter route summarization. A proactive network engineer may attempt to manually summarize contiguous subnets for maximum efficiency. This limits the overall number of routes in the EIGRP (or other applicable IGP) topology, and contributes to the stability of the network. The two LAN subnets in our scenario can be easily summarized on the WAN-facing interfaces:

Router(config-if)# ip summary-address eigrp 123 192.168.0.0 255.255.254.0

The two /24 routes are now advertised to the WAN as a single /23 route. Here's how the WAN saw our LAN subnets before the summarization:

D    192.168.0.0/24 [90/30720] via 172.16.0.6, 00:01:08, FastEthernet1/1
                    [90/30720] via 172.16.0.2, 00:01:08, FastEthernet1/0
D    192.168.1.0/24 [90/30720] via 172.16.0.6, 00:00:13, FastEthernet1/1
                    [90/30720] via 172.16.0.2, 00:00:13, FastEthernet1/0

And after:

D    192.168.0.0/23 [90/30720] via 172.16.0.6, 00:00:01, FastEthernet1/1
                    [90/30720] via 172.16.0.2, 00:00:01, FastEthernet1/0

Nice and efficient. But now assume that router A fails for some reason and its replacement isn't configured with the summary statement. Here are the routes the WAN sees with only router B summarizing:

D    192.168.0.0/24 [90/30720] via 172.16.0.2, 00:00:04, FastEthernet1/0
D    192.168.1.0/24 [90/30720] via 172.16.0.2, 00:00:04, FastEthernet1/0
D    192.168.0.0/23 [90/30720] via 172.16.0.6, 00:00:04, FastEthernet1/1

As you can see, the new router A is advertising two more-specific routes than router B. As a result, the WAN will always prefer the path through router A over B, and load balancing no longer takes place. When implementing route summarization, always make sure to implement it equally across border routers.