Explain DMVPN phase 1, phase 2 and phase 3.
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Phase 1 is hub-and-spoke only: spokes use point-to-point GRE to the hub and all spoke-to-spoke traffic goes through the hub. Phase 2 adds direct spoke-to-spoke tunnels: spokes use mGRE, the routing protocol keeps the remote spoke as next hop, and the spoke resolves that next hop with NHRP. The catch is that the hub cannot summarize. Phase 3 keeps spoke-to-spoke but lets the hub summarize: the hub sends an NHRP redirect when traffic hairpins through it, and the spoke installs a shortcut for the specific prefix.
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| Phase 1 | Phase 2 | Phase 3 | |
|---|---|---|---|
| Spoke tunnel | point-to-point GRE | mGRE | mGRE |
| Spoke to spoke | through the hub | direct, triggered by the next hop in the routing table | direct, triggered by a hub redirect |
| Hub summarization | yes | no | yes, even just a default |
| Key commands | EIGRP: no next-hop-self, no split horizon; OSPF: broadcast, hub is DR | hub ip nhrp redirect, spokes ip nhrp shortcut | |
| Hierarchical hubs | no | awkward | supported |
Why phase 3 is the default choice today
Spokes carry only a summary or default plus the shortcuts they use, routing tables stay small, and the hub design can grow into regions without changing spokes.
They will ask next
Why can the hub not summarize in phase 2?
In phase 2 a spoke builds a direct tunnel only when its routing table shows the other spoke's tunnel address as next hop. A summary from the hub has the hub as next hop, so all traffic follows it through the hub.
Why do spokes need OSPF priority 0 in phase 2?
Phase 2 uses OSPF network type broadcast so the next hop is preserved. The DR must be able to reach every router with multicast, and only the hub can. A spoke elected DR breaks the adjacencies, so spokes get priority 0 and the hub a high priority.
Traps
- Saying phase 3 means three tunnels or three hubs. The phases are design generations.