I want talk a little bit about a less known feature of EIGRP called leak-maps. Similarly to the BGP unsupress-map feature, the EIGRP leak-map is a method in which we allow routes that have been summarized to be “leaked” to the routing table. This technique is also commonly referred to as route-leaking. Today, I will try to explain a couple of cases where you would want to use this feature.
Case A: Route leaking to control routing
Route-leaking can help you control packet flow by using the shortest prefix match rule. Let’s first examine the following diagram.
In this topology, you can see that R2 and R3 are both announcing the summary of 192.168.0.0/20 to R1. If R1 needs to reach 192.168.1.0/24 or 192.168.2.0/24 or even 192.168.3.0/24 it will load-balance between R2 and R3 because of the equal path due to the summary-address.
You can see here in R1’s routing table that for prefix 192.168.1.0/24, we are load balancing with a share count of 1 (so 50/50) between R2 (192.168.13.3) and R3 (192.168.12.2). The shortest prefix match rule says that more specific prefixes are preferred over less specific prefixes. In this case, if we route-leak the 192.168.1.0/24 prefix on R2, we should see the route preferred towards R2. Let’s see the configuration:
And the routing table now on R1 once the configuration applied:
As expected, routing to 192.168.1.0/24 prefix goes through R2 (192.168.12.2) because of the shortest prefix match rule. As you can see by this example, we can use leak-map’s to influence routing.
Case B: Route leaking in a stub area
Another feature of leak-maps that I think is less-known is the stub area leak-map. As you probably know, the “eigrp stub connected” command permits EIGRP to advertise the directly connected routes only. By using the leak-map command, you can “leak” non-directly connected networks from the stub area. Let’s take the same topology and make R2 and R3 stub routers.
Now let’s take a look at R1’s routing table:
As expected, only directly connected routes (192.168.0.0/30 and 192.168.0.4/30) are learned through EIGRP. Now let’s use the leak-map feature to leak from R2 the 192.168.1.0/24 prefix and let’s check the result on R1’s routing table.
As expected, once we applied the leak-map LEAK, the prefix is announced through the stub network even if it is defined as a stub connected device.
I hope these two cases helped clear up on when and how we would use the EIGRP leak-map feature.