DMVPN Phase 3 EIGRP Routing

Lessons Discussion
22

Hi,

Am I right in assuming that when you advertise a default route from the hub, disabling split-horizon on the tunnel interface is no longer necessary?

I just noticed the answer to my question is in the Conclusions section, but just to clarify, you only don’t need to worry about split horizon if a default route is used - correct?

Regards,

Sam

23

Hello Samir

Yes, you are correct. Split horizon can become an obstacle when you want one spoke to advertise networks to another spoke. In such a case, it is necessary for an update to enter a particular interface on the hub, and be readvertised out of the same interface, which violates split horizon.

In the scenario in the lesson, we don’t have this situation because spokes don’t need to learn about each other’s networks.

I hope this has been helpful!

Laz

1 Like
24

Hello, everyone!

I was playing around with DMVPN and wanted to see the redirection process in action after it has already happened, so I issued the clear ip nhrp command to clear the NHRP cache and noticed the following packet pop up in Wireshark

Note: This is the link between the Hub and the two spokes. 3.3.3.3 is the hub and the rest are the spokes.

obrázok
obrázok995×37 1.12 KB

What exactly does this message do, please? Google says that when a network entry is deleted in the RIB, NHRP is notified of the event. But why do we care? For all we know, the Spoke could just send a packet to the hub, which would attempt to redirect it again.

Thank you.

David

25

Hello David

The NHRP purge request, as you correctly shared, is a message that is sent when a network entry is removed from the RIB. The message instructs the receiving device to remove a particular entry from the NHRP cache. What is the purpose of this? Of course, a spoke could send a packet once again to the hub, which would renew and update the NHRP entries. However, it is always preferable to keep the NHRP cache updated.

Over time, the NHRP cache may accumulate entries that are no longer valid. For example, a spoke might change its physical IP address, or a tunnel might become inactive. Keeping outdated entries can lead to inefficient routing and potential security issues.

The NHRP Purge Request is used to trigger the removal of such stale or invalid entries from the NHRP cache. It can be initiated by a network device (like a router) that detects a change in the network topology, or as a routine maintenance procedure.

Regular purging of the NHRP cache helps maintain optimal network performance in a DMVPN setup. It ensures that the path information is current, which is critical for the efficient and secure routing of traffic within the VPN network.

I hope this has been helpful!

Laz