The granularity of protection for N:M is per tunnel-group. For each to-be-protected tunnel group there is a redundancy domain and tunnel group. Each domain has 1:1 mapping. Each domain contains up to four nodes. One node is elected to be the active node for the tunnel group, the other nodes are standby. Tunnel states are synchronized between all nodes in the domain.
A node may have multiple tunnel groups, and in turn, participates in multiple redundancy domains.