I just recently upgraded from 8.3.2 to 9.1P20 and started to see these alert messages being sent.
Message: cpeer.addr.warn.host: Address 172.31.x.x is not any of the addresses that peer cluster <vserver> considers valid for the cluster peer relationship; Details: An introductory RPC to the peer address "172.31.x.x" failed to connect: RPC: Remote system error - No route to host. Verify that the peer address is correct and try again.
Description: This message occurs when a stable IP address for the peer cluster is no longer valid, making the peer cluster unreachable and causing cross-cluster operations to fail.
I'm sure there was a change in the upgrade, however how do I narrow down what's causing this issue?