Understanding How Loop Guard Works - Cisco WS-X6066-SLB-APC - Content Switching Module Software Manual

Chapter 9 Configuring Spanning Tree PortFast, UplinkFast, BackboneFast, and Loop Guard
Figure 9-4
Switch A
(Root)
If a new switch is introduced into a shared-medium topology, BackboneFast is not activated.
shows a shared-medium topology in which a new switch is added. The new switch begins sending
inferior BPDUs that say it is the root switch. However, the other switches ignore these inferior BPDUs
and the new switch learns that Switch B is the designated bridge to Switch A, the root switch.
Figure 9-5
Switch C

Understanding How Loop Guard Works

Unidirectional link failures may cause a root port or alternate port to become designated as root if
BPDUs are absent. Some software failures may introduce temporary loops in the network. The loop
guard feature checks if a root port or an alternate root port receives BPDUs. If the port is not receiving
BPDUs, the loop guard feature puts the port into an inconsistent state until it starts receiving BPDUs
again. Loop guard isolates the failure and lets spanning tree converge to a stable topology without the
failed link or bridge.
78-13315-02
BackboneFast Example After Indirect Link Failure
L1
Link failure
L2
BackboneFast transitions port
through listening and learning
states to forwarding state
Switch C
Adding a Switch in a Shared-Medium Topology
Switch A
(Root)
Blocked port
Catalyst 6000 Family Software Configuration Guide—Releases 6.3 and 6.4
Switch B
L3
Switch B
(Designated Bridge)
Added switch

Understanding How Loop Guard Works

Figure 9-5
9-5