Thursday, June 10, 2010

Understand Bridge Assurance

Bridge assurance comes up often in Nexus troubleshooting. Therefore it is important to understand its design and effect. What is BA? It is a Cisco STP enhancement feature designed to prevent loops, by making sure that a neighboring switch does not malfunction and begin forwarding frames when it shouldn't. Configured incorrectly, BA will likely cause some headaches.

BA monitors receipt of BPDUs on point-to-point links. When the BPDUs stop being received, the port is put into blocking state (actually a port inconsistent state, which stops forwarding). This is typically seen with "show spanning-tree ..."

Now it will make sense to highlight the important characteristics of BA:
• It’s enabled globally by default, but disabled by default on interfaces
• It is enabled only on STP “network” interface
• For it to work, both ends of the link must support BA; Otherwise, the BA side will block
• BA only works on point to point Cisco connections

Here are two examples of BA troubleshooting:

1. If STP "network" type is used with a host VPC, the host side does not support bridge assurance, and we know Nexus 1000v does not even send BPDU, then turning on BA on Nexus 5000 will make the port go into “inconsistency” and blocking.

2. With Nexus 7000 and 5000 back to back VPC connections, it is important to set both sides to type “network”, thus enabling BA consistently. Otherwise, it will also go into blocking state due to inconsistency.

See a more complete picture of spanning tree design is a typical Nexus virtualized data center here.

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