The leak-map just allows you to advertise a specific prefix within the range of a summary advertisement, as well as the summary itself. We can see in the diagram below that we are advertising the 10.0.0.0/13 network out to R5 & R6, however, we are also advertising the 10.1.0.0/24 network out to R5 using a leak-map.

EIGRP Leak-Map

I’ve configured everything but the leak-map so far. Basically all routers just have EIGRP 100 running & auto-summary disabled. R3 has been configured to summarise 10.0.0.0/13 out to R5 & R6. Below is the current routing tables of both R5 and R6.

R5#show ip route eigrp
36.0.0.0/24 is subnetted, 1 subnets
D 36.36.36.0 [90/30720] via 35.35.35.3, 04:32:18, FastEthernet0/0
67.0.0.0/24 is subnetted, 1 subnets
D 67.67.67.0 [90/33280] via 45.45.45.4, 04:32:18, FastEthernet0/1
[90/33280] via 35.35.35.3, 04:32:18, FastEthernet0/0
10.0.0.0/13 is subnetted, 1 subnets
D 10.0.0.0 [90/284160] via 35.35.35.3, 00:07:48, FastEthernet0/0
47.0.0.0/24 is subnetted, 1 subnets
D 47.47.47.0 [90/30720] via 45.45.45.4, 04:46:57, FastEthernet0/1
R6#show ip route eigrp
35.0.0.0/24 is subnetted, 1 subnets
D 35.35.35.0 [90/30720] via 36.36.36.3, 04:33:38, FastEthernet0/1
10.0.0.0/13 is subnetted, 1 subnets
D 10.0.0.0 [90/284160] via 36.36.36.3, 00:30:21, FastEthernet0/1
47.0.0.0/24 is subnetted, 1 subnets
D 47.47.47.0 [90/30720] via 67.67.67.7, 04:48:00, FastEthernet0/0
45.0.0.0/24 is subnetted, 1 subnets
D 45.45.45.0 [90/33280] via 67.67.67.7, 04:33:35, FastEthernet0/0
[90/33280] via 36.36.36.3, 04:33:35, FastEthernet0/1
So now we want to make R5 and R6 both prefer the 10.1.0.0/24 network via R3’s Fa0/0 interface. There’s a few ways to do this, and in this lab I’ll make the route more preferred by using a leak-map to advertise a more specific route to the 10.1.0.0/24 network via R3’s Fa0/0. See the configuration below.

R3(config)#ip prefix-list 10_1_0_0_network permit 10.1.0.0/24
R3(config)#
R3(config)#route-map Leak_10_1_0_0 permit 10
R3(config-route-map)#match ip address prefix-list 10_1_0_0_network
R3(config-route-map)#exit
R3(config)#
R3(config)#int range fa0/0
R3(config-if-range)#ip summary-address eigrp 100 10.0.0.0 255.248.0.0 leak-map Leak_10_1_0_0
So now, even on R6 which has a direct connection to R3, it prefers the path via R7-R4-R5-R6 for the 10.1.0.0/24 network, purely because it’s more specific.

R6#show ip route eigrp
35.0.0.0/24 is subnetted, 1 subnets
D 35.35.35.0 [90/30720] via 36.36.36.3, 04:40:41, FastEthernet0/1
10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
D 10.0.0.0/13 [90/284160] via 36.36.36.3, 00:37:24, FastEthernet0/1
D 10.1.0.0/24 [90/296960] via 67.67.67.7, 00:00:17, FastEthernet0/0
47.0.0.0/24 is subnetted, 1 subnets
D 47.47.47.0 [90/30720] via 67.67.67.7, 04:55:03, FastEthernet0/0
45.0.0.0/24 is subnetted, 1 subnets
D 45.45.45.0 [90/33280] via 67.67.67.7, 04:40:38, FastEthernet0/0
[90/33280] via 36.36.36.3, 04:40:38, FastEthernet0/1