Spanning Tree Protocol Device and Port States and Roles

STP/PVST/PVST+ port states

Disabled
Blocking
Listening
Learning
Forwarding

RSTP/RPVST/RPVST+/MST port states

Disabled
Discarding
Learning
Forwarding

Disabled

Discards everything, including BPDUs.

Blocking

Discards everything except BPDUs.

Listening

Discards traffic and doesn’t learn MAC addresses. Exchanges BPDUs and determines its role (root, designated, alternate, backup, non-designated (blocked)).

Learning

Port prepares itself for forwarding by learning MAC addresses from received frames, but still discards the frames.

Forwarding

Forwards frames received from the attached segment and frames from other ports to the attached segment.

Discarding

Does not forward traffic or learn MAC addresses. It still sends and receives BPDUs for negotiation.

PVST/PVST+/RSTP/RPVST/RPVST+/MST Port Roles

Root
Designated
Alternate
Backup
Edge
Disabled

STP Port Roles

Root
Designated
Non-Designated (Blocked)
Disabled

Root device role

The leader of the network segment and central point of the spanning tree topology.
All ports on this device are designated.
Device with the lowest bridge ID (BID).

BID

Bridge ID
STP Priority.MAC
8 bytes: priority 2 bytes, MAC 6 bytes

Root port role

All bridges other than the root bridge have a root port

It’s the lowest-path-cost port to the root bridge, serving as the single point of entry for traffic from the root bridge.

Designated port role

This port will forward traffic. Between two ports on a segment, it’s the port on the switch with the lower path cost to the root bridge.

Alternate port role

This port provides a secondary path to the root bridge, should the root bridge become unreachable from the root port.

It’s pre-determined to prevent network disruption during a topology recalculation.

It’s in a discarding or blocking state, not forwarding traffic, but processing BPDUs.

Backup port role

This port serves as a backup for a designated port on the same segment. It takes over as the designated port if the designated port becomes invalid, similar to how the alternate port serves as a backup to the root port.

It provides a backup for downstream traffic (away from the root) as opposed to upstream traffic (towards the root).

Edge port role

This port role is intended for direct connection to an end-user device. It’s manually configured and bypasses the initial Blocking and Learning states, transitioning immediately to forwarding for immediate connectivity for the host.

config-if#spanning-tree port type edge

Non-Designated (Blocked) port role

This port is neither a root nor designated. It doesn’t participate in frame forwarding. It only listens to and process BPDUs.

Priority

Lowest priority becomes the root bridge for a given VLAN.

Priority multiples of 4096

Range from 0 to 61,440 (0x to 15× 4096)

Default: 32,768