Sunday, August 23, 2009

DMVPN - EIGRP, Disable Split-Horizon

I was ask to tell the solution for the following question regarding DMVPN.

Q: I am deploying a DMVPN hub and spokes with mGRE tunnels protected by IPSEC. I activated EIGRP on it, and I noticed that on the spoke EIGRP
installs in its routing table only the route to the hub, while the spoke-to-spoke routes don't appear neither in the routing table, neither in the EIGRP topology. Nevertheless, the spokes communicate between them through the dynamic tunnels.
I couldn't find any Cisco document contemplating the issue. I wonder if this is the way it's supposed to work, or if I have to search for some misconfiguration.


Solution:

There are certain rules which we have to remember while configuring DMVPN with EIGRP, One of these rules is split horizon. The DMVPN hub tunnel interface serves multiple spokes. Also the tunnel ip address is on the same subnet as its spokes. When configuring a EIGRP AS you use the tunnel network id as a network you want to participate on with EIGRP.

Split Horizon was designed to not allow a router to advertise a route out the same interface in which the route was originally learned. This conflicts in the case of the tunnel interface because it needs to be able to make neighbors with the spokes on the same subnet and also re-advertise routes learned from one spoke to the next.

Again, If you dont disable split horizon on the tunnel interface you will only see the routes the hub itself is responsible for from a spoke router. This can be done by a single command under tunnel interface.
# no ip split-horizon eigrp 2

Now you can see all routes in the client routing table.

Cheers :)

5 comments:

kamrangul87 said...

its very nice

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