Rapid Spanning Tree Notes

Just been reading a bit on RSTP, there are plenty of excellent detailed resources around. Here's my summary.




PDF version available here

Rapid Spanning Tree - 802.1w

Port Roles

Role	Description	Detailed
Root	Path to root bridge	The port that is closest to the root bridge in terms of path cost.
Designated	Forwarding to a segment	The port that sends the best BPDU on the segment to which it is connected.
Alternate	Alternative path to root	Alternate route to the root bridge. This port receives more useful BPDUs from another bridge. State blocked. Can be activated if the root port fails (RSTP equivalent to uplinkfast).
Backup	Redundant port	This port receives more useful BPDUs from the local bridge, therefore is redundant. State is blocked.
Disabled	Shutdown	Administratively shut down.

Port States

State	Description
Discarding	Combines the 802.1D disabled, blocking and listening states.
Learning	Drops frames but learns MAC addresses.
Forwarding	Sends frames

Port Types

Type	Description
Edge	End host, selected when portfast is enabled.
Root	Path to root bridge.
Point-to-point	Designated port to another switch. Selected when non-edge port is in full duplex mode.

• It is assumed that a non-edge port in half-duplex mode may connect to a shared medium with multiple peer switches. These ports will fall back to 802.1D compatibility mode.

• A non-edge port transitioning to forwarding will generate a TCN.

BPDUs

• All switches send BPDUs out of all ports.

• Loss of 3 BPDUs from a neighbour is taken to mean the neighbour is down. All information on that port is aged out.

• If an 802.1D BPDU is received then that port falls back to 802.1D mode from RSTP.