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DMVPN Phase 3:
This phase allows spokes to build a spoke-to-spoke tunnel and to overcomes the phase2 restriction using NHRP traffic indication messages from the hub to signal to the spokes that a better path exists to reach the target network.
1) NHRP requests are no longer triggered by invalid CEF entries. This means that routing information could be effectively summarized.
2) Hub is no longer used as the only source of NHRP information. Instead of this, all spokes participate in NHRP information exchange. This model is less “server-based” but rather more “peer-to-peer”.
3) NHRP replies contain whole routing prefixes, instead of just next-hop information.
Another good news is that initial spoke2spoke packet is now switched using CEF, not process switching like it was before. With NHRP Phase 2, the initial packet has to be switches via process path, as the CEF adjacency is not yet valid (in “glean” state) but with NHRP Phase 2 we’re using CEF all the time.
“ip nhrp redirect” informs the spoke that it can communicate to other intended spoke directly.
IP NHRP Shortcut Switching to discover shorter paths to a destination network after receiving a n NHRP redirect message from the hub.This allows the routers to communicate directly with each other without the need for an intermediate hop