How a PE Ring Works with Different Types of Traffic
Unicast Traffic
Ingress traffic from VM3: Takes the shortest upstream path to the CB, SPX Link B.
Egress out of VM4: Takes the shortest downstream path to PE5, selects SPX Link B.
Ingress traffic from VM1: Takes the shortest upstream path to the CB, SPX Link A.
Egress out of VM2: Takes the shortest downstream path to PE4, selects SPX Link B.
When the ring is in the Active state, each PE unit is mapped to one of the SPX links (either SPX Link A or SPX Link B) to carry both upstream and downstream traffic. Any known unicast traffic flowing in and out of that PE uses the same SPX link until the ring breaks or there is a change in the ring topology. If the ring breaks at a point where the mapped SPX link cannot be reached by this PE, the PE unit is remapped to the alternate SPX link (if it is reachable). This will cause some minimal traffic loss until the remapping is complete. The convergence time depends on the number of PE data ports and traffic streams to be moved from the existing SPX link to the newer SPX link.
Multicast/Broadcast and Unknown Unicast Traffic
Ingress traffic from VM3: Takes the shortest upstream path to the CB, SPX Link B.
Egress out of VM4, VM2: Downstream traffic takes both SPX Link A and SPX Link B.
Ingress traffic from VM1: Takes the shortest upstream path to the CB, SPX Link A.
Egress out of VM3: Downstream traffic takes both SPX Link A and SPX Link B.
When the ring is active, each PE unit is mapped to one of the SPX links (either SPX Link A or SPX Link B) to carry the upstream traffic. Downstream traffic is carried over both SPX links (SPX Link A and SPX Link B) to achieve fast reconvergence. If the ring breaks at a point where the mapped SPX link cannot be reached from this PE, the PE unit is remapped to the alternate SPX link (if it is reachable) to carry the upstream traffic. This causes traffic loss only until the remapping is complete. Reprogramming the SPX link for downstream traffic is not required. This helps in achieving faster reconvergence and reducing traffic loss to a minimum in most failover scenarios.