About Bridge Assurance - Cisco Catalyst 4500 Series Software Configuration Manual

Chapter 26 Configuring Optional STP Features

About Bridge Assurance

You can use Bridge Assurance to help prevent looping conditions that are caused by unidirectional links
(one-way traffic on a link or port), or a malfunction in a neighboring switch. Here, a malfunction refers
to a switch that is not able to run STP any more, while still forwarding traffic (a brain dead switch).
BPDUs are sent out on all operational network ports, including alternate and backup ports, for each hello
time period. Bridge Assurance monitors the receipt of BPDUs on point-to-point links on all network
ports. When a port does not receive BPDUs within the alloted hello time period, the port is put into a
blocked state (the same as a port inconsistent state, which stops forwarding of frames). When the port
resumes receipt of BPDUs, the port resumes normal spanning tree operations.
Note
This example shows how Bridge Assurance protects your network from bridging loops. Here,
Figure 26-2
when the device fails (brain dead) and Bridge Assurance is not enabled on the network.
Figure 26-2
Figure 26-3
Only Rapid PVST+ and MST spanning tree protocols support Bridge Assurance. PVST+ does
not support Bridge Assurance.
shows a normal STP topology, and
Network with Normal STP Topology
Root
BPDUs
BPDUs
Edge
Network Loop Due to a Malfunctioning Switch
Malfunctioning
Loop
Root
BPDUs
Edge
Catalyst 4500 Series Switch, Cisco IOS Software Configuration Guide - Cisco IOS XE 3.9.xE and IOS 15.2(5)Ex
Figure 26-3
BPDUs
Blocked
switch
X
About Bridge Assurance
demonstrates a potential network problem
26-11