STP - RSTP port roles
Rapid Spanning Tree Protocol (RSTP), defined in IEEE 802.1w, improves upon the original Spanning Tree Protocol (STP) by providing faster convergence after a topology change. Among other things, RSTP differs from STP in the port roles used. Here are the roles defined in RSTP:
- Root Port: Just like in STP, this is the port on a non-root switch with the lowest cost path to the root switch. It's always in the forwarding state if RSTP is converged.
- Designated Port: This is the port on a network segment (collision domain) that has the lowest cost path to the root switch. It's also always in the forwarding state if RSTP is converged.
- Alternate Port: This is a new role introduced in RSTP. An alternate port is one that provides an alternative path to the root switch, different from the path that the root port takes. It's in a blocking state but can transition to forwarding quickly if the network topology changes and the primary path (via the root port) fails.
- Backup Port: This is another new role introduced in RSTP. A backup port provides a redundant (backup) path to a segment where another switch port already connects. This port is also in a blocking state, and will only become active if the designated port for that segment fails.
- Disabled Port: Similar to STP, this port doesn’t participate in STP/RSTP and doesn't forward frames because it's been administratively shut down.
RSTP improves upon STP by introducing these new port roles and by defining new port states, effectively merging the blocking, listening, and learning states of STP into a single "discarding" state in RSTP. This allows for much faster convergence following a topology change.
The port roles should not be confused with port states.