DMVPN - Phase 2 and summarization

In a DMVPN topology, when configuring routing on the hub, one may apply summarization to simplify and reduce the size of the routing table. However, when using Phase 2, summarization on the hub is not recommended.

Phase 2 enables the direct communication between spokes, however, this can be achieved only if the following two prerequisites are fulfilled:

  • Spoke routers need to have a route for the network that they are trying to reach.
  • The next hop IP address of the route has to be the remote spoke.

If summarization were configured on the hub in a Phase 2 DMVPN network, the spokes would not learn about the specific networks behind other spokes through the summarized routes. This is because summarization introduces a loss of specificity in route information. In other words, the hub would advertise only the summary route to the spokes, not the individual subnet routes that make up the summary. Thus, the prerequisite is not fulfilled.

https://networklessons.com/cisco/ccie-enterprise-infrastructure/introduction-to-dmvpn#Phase_2