Task
● Load the Initial EIGRP configuration in your lab
● Configure EIGRP AS 100 on all devices in the topology.
● Enable EIGRP on all links in the 170.1.0.0/16 and 172.1.0.0/16 networks.
● Enable Auto-Summary under the EIGRP process on all devices in the topology.
● Disable EIGRP split-horizon on R5's tunnel interface connecting to the DMVPN network.
● Test IPv4 reachability throughout the network, and note any connectivity problems
Configuration
R1-R10:
!
router eigrp 100
auto-summary
network 170.1.0.0 0.0.255.255
network 172.16.0.0 0.0.255.255
!
end
!
R5:
!
interface Tunnel0
no ip split-horizon eigrp 100
!
end
!
Verification:
As of IOS release 15.0, EIGRP auto-summary now defaults to disabled. With EIGRP auto-summary enabled, VLSM is supported, but only for subnets that are within the same major network boundary. As network advertisements pass between major network boundaries, they are automatically summarized to their classful mask. The result of this behavior is that EIGRP cannot support discontiguous major networks, as seen in the reachability problems in this task.
Note that in the routing table output of devices in this topology, two automatic summaries are generated, one for the 170.1.0.0/16 network and one for the 172.16.0.0/16 network. Although all of the routers are advertising the classful summary of their Loopback subnet 170.1.0.0/16 to all of their neighbors, each router also installs a local EIGRP summary route to Null0. The end result is that they cannot learn about each other's summaries to 170.1.0.0/16, and reachability is not obtained between these networks
R2:
!
show ip route eigrp
!
R10:
!
show ip route 170.1.0.0
!
R10:
!
configure terminal
!
interface Loopback0
!
shutdown
!
R10:
!
show ip route 170.1.0.0
!
In this case, R10 can learn the summary 150.1.0.0/16 from R8, but only because it does not have a locally installed Null0 route that matches the summary as well. From a reachability standpoint, R10 should now be able to reach R8's Loopback0, but none further than that, because R8 will null route any other packets matching 170.1.0.0/16:
R10:
!
ping 170.1.8.8
!
R10:
!
debug ip icmp
!
R10:
!
ping 170.1.5.5
!
Although there is little practical application to having EIGRP auto-summary enabled, it is important to understand how its behaviour can negatively affect a routing design. For example, if a router's configuration is from a 15.x IOS version were to be rolled back to an IOS release of 12.4T or earlier, the default EIGRP auto-summary behavior would be re-enabled and potentially harm reachability throughout the network.
