Monday, 11 January 2010

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.

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