Cisco Catalyst 4500 Series, Software Configuration Manual

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Catalyst 4500 Series Switch, Cisco IOS Software Configuration Guide - Cisco IOS XE 3.9.xE and IOS 15.2(5)Ex
Chapter 26 Configuring Optional STP Features
About Bridge Assurance
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
Only Rapid PVST+ and MST spanning tree protocols support Bridge Assurance. PVST+ does
not support Bridge Assurance.
This example shows how Bridge Assurance protects your network from bridging loops. Here,
Figure 26-2 shows a normal STP topology, and Figure 26-3 demonstrates a potential network problem
when the device fails (brain dead) and Bridge Assurance is not enabled on the network.
Figure 26-2 Network with Normal STP Topology
Figure 26-3 Network Loop Due to a Malfunctioning Switch
Blocked
354159
Edge
BPDUs
BPDUs BPDUs
Root
Malfunctioning
switch
Root
BPDUs
Loop
354160
Edge
X