- You have only Peer-gateway configured:
If peer-gateway is configuring the MAC address of the vpc peer (SVI of the vpc peer) as static mac address pointing to local sup.
This means, if traffic needs to be routed (destination IP address is an external network) the local vpc peer will route the traffic even if the destination mac is the mac of the vpc peer.
If the destination IP is the IP of vpc peer (for example BGP neighborship messages, with TTL =1), the local vpc peer will try to route the traffic the vpc peer, but it will decrement the TTL and because of TTL=0, there will be a TTL exceeded message being returned. And neighborship will not form
- You have peer-gateway and layer3 peer-router configured:
To avoid the decrement of TTL when dIP is the ip of the vpc peer, you configure l3 peer router. This way, neighborship can form even if the dmac (mac of vpc peer) is pointing to local sup.
- You have only layer3 peer-router configured:
If you do not have peer-gateway configured, the mac address of the peer will not point to local sup, and the traffic will be forwarded as per mac address table. This means layer3 peer-router has no effect since anyway traffic destined to the vpc peer will not be locally routed.
Stay safe,
Sergiu
