Bridge Assurance - Cisco Catalyst 2960 series Configuration Manual

Note
• A PortFast network port—is connected only to a Layer 2 switch or bridge. Bridge Assurance is enabled
only on PortFast network ports. For more information, refer to Bridge Assurance.
Note
• A PortFast normal port—is the default type of spanning tree port.
Note
Related Topics
Enabling PortFast Port Types, on page 314

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.
Only Rapid PVST+ and MST spanning tree protocols support Bridge Assurance. PVST+ does not support
Note
Bridge Assurance.
The following example shows how Bridge Assurance protects your network from bridging loops.
If you configure a port connected to a Layer 2 switch or bridge as an edge port, you
might create a bridging loop.
If you configure a port that is connected to a Layer 2 host as a spanning tree network
port, the port will automatically move into the blocking state.
Beginning with Cisco IOS Release 15.2(4)E, or IOS XE 3.8.0E, if you enter the
spanning-tree portfast [trunk] command in the global or interface configuration mode,
the system automatically saves it as spanning-tree portfast edge [trunk].
Consolidated Platform Configuration Guide, Cisco IOS Release 15.2(4)E (Catalyst 2960-X Switches)
Information About Optional Spanning-Tree Features
299