Leak-maps

A leak-map is a method of using a route-map.
It can be helpful to use a leak-map to "leak" in or only allow certain routes to the stub. The benefit of route leaking (advertising a route) is that the leaked route is more specific than a summarized route and is therefore the preferred route.

Consider the example below:

Routers A,B, C and stub are participating in the same EIGRP AS.
Notice that past router A and B, the addresses can be summarized as a /23 (as noted above.)
So, we can summarize these routes by adding the following command on A and B on the interfaces facing router c:
 Router(config-if)#ip summary-address eigrp 100 192.168.0.0 255.255.254.0

So what does this mean for router C?
Without any special configuration (without any leak-maps configured), that would mean that router C would see two paths to the route 192.168.0.0 /23 with an equal cost.

What if you wanted to force the routers to prefer taking router A to get to the stub LAN (192.168.1.0/24)?
This is where the leak map comes in handy.
By putting the leak-map on router A, and "leaking" just that 192.168.1.0/24 as well as the summary route, we can ensure that router C will prefer that path.

To do this add the following in router A instead of the summary address command above. There are a few steps to getting this work:

1. An ACL is required to restrict the address you want to "leak"
2. A route map is required to match the ACL
3. the leak map parameter has to be added to the summary address command.

These are the commands needed:
RouterA(config)#access-list 1 permit 192.168.1.0 255.255.255.0
RouterA(config)#route-map LEAK-STUB-LAN permit
                               #match ip address 1
RouterA(config)#int fa0/0
RouterA(config-if)#ip summary-address eigrp 100 192.168.0.0 255.255.254.0 leak-map LEAK-STUB-LAN


So what else can you do with a leak-map?

Another alternative method of using a leak-map (which i believe is less commonly known) is using as part of
Router(config-router)# eigrp stub connected leak-map <LEAK-MAP>
As you probably know, "eigrp stub connected" command permits EIGRP to advertise the directly connected routes. However, by using the leak-map command, you can "leak" non directly connected network on the stub.

Troubleshooting Leak Maps>> http://networkqueen.blogspot.com/2011/01/troubleshooting-leak-maps.html
info source: http://www.cisco.com/en/US/docs/ios/12_3t/12_3t14/feature/guide/gt_esflr.html