Cisco Systems 3750E manual Mstp and Switch Stacks, Detecting Unidirectional Link Failure, 19-8

Chapter 19 Configuring MSTP

Understanding MSTP

Detecting Unidirectional Link Failure

This feature is not yet present in the IEEE MST standard, but it is included in this Cisco IOS release. The software checks the consistency of the port role and state in the received BPDUs to detect unidirectional link failures that could cause bridging loops.

When a designated port detects a conflict, it keeps its role, but reverts to discarding state because disrupting connectivity in case of inconsistency is preferable to opening a bridging loop.

Figure 19-3illustrates a unidirectional link failure that typically creates a bridging loop. Switch A is the root switch, and its BPDUs are lost on the link leading to switch B. RSTP and MST BPDUs include the role and state of the sending port. With this information, switch A can detect that switch B does not react to the superior BPDUs it sends and that switch B is the designated, not root switch. As a result, switch A blocks (or keeps blocking) its port, thus preventing the bridging loop.

Figure 19-3 Detecting Unidirectional Link Failure

Switch	Superior	Switch
	BPDU
A		B

Inferior BPDU,

Designated + Learning bit set

92722

MSTP and Switch Stacks

A switch stack appears as a single spanning-tree node to the rest of the network, and all stack members use the same switch ID for a given spanning tree. The switch ID is derived from the MAC address of the stack master.

If a switch that does not support MSTP is added to a switch stack that does support MSTP or the reverse, the switch is put into a version mismatch state. If possible, the switch is automatically upgraded or downgraded to the same version of software that is running on the switch stack.

When a new switch joins the stack, it sets its switch ID to the stack master switch ID. If the newly added switch has the lowest ID and if the root path cost is the same among all stack members, the newly added switch becomes the stack root. A topology change occurs if the newly added switch contains a better root port for the switch stack or a better designated port for the LAN connected to the stack. The newly added switch causes a topology change in the network if another switch connected to the newly added switch changes its root port or designated ports.

When a stack member leaves the stack, spanning-tree reconvergence occurs within the stack (and possibly outside the stack). The remaining stack member with the lowest stack port ID becomes the stack root.

If the stack master fails or leaves the stack, the stack members elect a new stack master, and all stack members change their switch IDs of the spanning trees to the new master switch ID.

For more information about switch stacks, see Chapter 5, “Managing Switch Stacks.”

Catalyst 3750-E and 3560-E Switch Software Configuration Guide

19-8 OL-9775-02