A network administrator is reviewing the Rapid PVST+ configuration on a Cisco Catalyst switch. The output of 'show spanning-tree vlan 10' shows a port Gi0/1 with the role 'Alt', state 'BLK', and type 'P2p'. Which of the following statements accurately describes the 'Alt' role and 'P2p' type for this port?
- AThe port is an Alternate port, indicating it is forwarding traffic, and it is a shared link.
- BThe port is an Alternate port, indicating it is blocking traffic as a backup path, and it is configured as a point-to-point link.
- CThe port is a Designated port, indicating it is blocking traffic, and it is configured as a point-to-point link.
- DThe port is a Root port, indicating it is forwarding traffic, and it is a shared link.
Show answer & explanationAnswer & explanation
Correct answer: B. The port is an Alternate port, indicating it is blocking traffic as a backup path, and it is configured as a point-to-point link.
An 'Alt' (Alternate) port in Rapid PVST+ is a port that is blocking traffic to prevent loops but is ready to transition to forwarding if the current Root or Designated port fails. The 'P2p' (Point-to-point) link type indicates that the port is connected to another switch directly, typically via a full-duplex link, allowing for faster STP convergence.
Why the other options are wrong
- A. Alternate ports block traffic, they do not forward. P2p is for point-to-point, not shared links.
- C. Designated ports forward traffic, they are not typically blocking (unless in a learning or listening state temporarily).
- D. Root ports forward traffic and are the best path to the root bridge. P2p is for point-to-point, not shared links.
RSTP Alternate Port (Alt) and Link Type (P2p)
In Rapid PVST+, an Alternate (Alt) port is a blocking port that provides a redundant path to the root bridge. A Point-to-point (P2p) link type indicates a full-duplex connection to another switch, enabling rapid transition to forwarding.
- Alternate (Alt) port: A non-designated port that is blocking and has a valid path to the root bridge.
- P2p link type: Indicates a full-duplex, point-to-point connection, allowing for rapid state transitions (e.g., from blocking to forwarding in 2-3 seconds).
- Shared link type: Indicates a half-duplex or shared segment (e.g., hub), which uses traditional STP timers.
Memory trick: Alternate Blocks, Point-to-Point Powers Past.