Phase 1 - In phase 1 we use NHRP so that spokes can register themselves with the HUB. Only Hub uses a multipoint GRE interface, all spokes will be using regular point-to-point GRE tunnel interfaces which means that there will be no direct spoke-to-spoke communication, all traffic has to go via hub.

The only advantage of the phase I setup is the fact the hub router’s configuration is MUCH simpler. Summarization is possible in phase 1.

Phase 2 - In phase 2 all spokes routers also use multipoint GRE tunnels so we do have direct spoke to spoke tunneling. When a spoke router wants to communicate to another spoke it will send an NHRP resolution request to the hub to find the NBMA IP address of the other spoke. Summarization is not possible in phase 2.

Full Process

1.Spoke 1 forwards a packet with a next hop which is another spoke (spoke 2). There is no NHRP map entry for this spoke so an NHRP resolution request is sent to the hub.

2.The request from spoke 1 contains the tunnel IP address of the spoke 2 so the hub relays the request to spoke 2.

3.Spoke 2 receives the request, adds its own address mapping to it and sends it as an NHRP reply directly to spoke 1.

4.Spoke 2 then sends its own NHRP resolution request to the hub that relays it to spoke 1.

5.Spoke 1 receives the request from spoke 2 via the hub and replies by adding its own mapping to it and sending it directly to spoke 2.

Spoke to Spoke tunnel is established.

Phase 3 - In phase 3 NHRP redirect configured on the hub tells the initiator spoke to look for a better PATH to the DESTINATION spoke. On receiving the NHRP redirect message the spokes communicate with each other over the hub and they have their NHRP replies for the NHRP Resolution Requests that they sent out.

NHRP Shortcut configured on the spoke updates the CEF table. It basically changes the next-hop value for a remote spoke from the initial hub tunnel IP address to the NHRP resolved tunnel IP address of remote spoke.

Summarization is possible in phase 3.

Phase 1 - In phase 1 we use NHRP so that spokes can register themselves with the hub. Only Hub uses a multipoint GRE interface, all spokes will be using regular point-to-point GRE tunnel interfaces which means that there will be no direct spoke-to-spoke communication, all traffic has to go via hub.

The only advantage of the phase I setup is the fact the hub router’s configuration is much simpler. Summarization is possible in phase 1.

Phase 2 - In phase 2 all spokes routers also use multipoint GRE tunnels so we do have direct spoke to spoke tunneling. When a spoke router wants to communicate to another spoke it will send an NHRP resolution request to the hub to find the NBMA IP address of the other spoke. Summarization is not possible in phase 2.

Full Process

1.Spoke 1 forwards a packet with a next hop which is another spoke (spoke 2). There is no NHRP map entry for this spoke so an NHRP resolution request is sent to the hub.

2.The request from spoke 1 contains the tunnel IP address of the spoke 2 so the hub relays the request to spoke 2.

3.Spoke 2 receives the request, adds its own address mapping to it and sends it as an NHRP reply directly to spoke 1.

4.Spoke 2 then sends its own NHRP resolution request to the hub that relays it to spoke 1.

5.Spoke 1 receives the request from spoke 2 via the hub and replies by adding its own mapping to it and sending it directly to spoke 2.

Spoke to Spoke tunnel is established.

Phase 3 - In phase 3 NHRP redirect configured on the hub tells the initiator spoke to look for a better path to the destination spoke. On receiving the NHRP redirect message the spokes communicate with each other over the hub and they have their NHRP replies for the NHRP Resolution Requests that they sent out.

NHRP Shortcut configured on the spoke updates the CEF table. It basically changes the next-hop value for a remote spoke from the initial hub tunnel IP address to the NHRP resolved tunnel IP address of remote spoke.

Summarization is possible in phase 3.