DMVPN Phase 2 and Phase 3

Routing & Switching
1

Hi Rene,

I gone through the DMVPN Lesson.But am not able to still understand what exactly happens in DMVPN phase 2 and 3 and why DMVPN phane 2 will not support Summarization and default routing?Can you please help me with some more examples and Phase 2,Phase 3 packet captures.

Thanks in Advance!

2

Hi Sushanth,

Let’s take a look at the difference between DMVPN phase 2 and phase 3.

DMVPN phase 2

  • Spoke-to-spoke traffic is possible but spoke routers need to have a route for the network they try to reach.
  • The next hop IP address of the route has to be the remote spoke router.

If you add a summary or default route, then all traffic that matches your summary/default route will use the next hop that you configured in your summary/default route. If you want to reach networks behind other spoke routers, you need specific routes.

Take a look at RIP over DMVPN phase 2 here:

In that lesson, I added a debug of NHRP that shows that the spoke tries to resolve the NBMA address for a remote spoke. That’s the only thing it does for DMVPN phase 2.

DMVPN phase 3

  • We don’t need specific routes for networks behind other spoke routers anymore.
  • Spoke router forwards traffic for spoke-to-spoke traffic initially to the hub router.
  • Hub sends NHRP redirect to spoke router.
  • Spoke receives NHRP redirect and installs new entry in routing table.

We now use NHRP to figure out how to reach a remote network so you don’t need specific routes anymore. You can see this in this lesson:

In the NHRP debug you can see that NHRP resolves the NBMA address but we also receive from our hub how to get to the remote spoke. The spoke then installs an entry like this in the routing table:

Spoke1#show ip route nhrp | include 3.3.3.3
H        3.3.3.3 [250/255] via 172.16.123.3, 00:07:19, Tunnel0

Rene

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