UDLD Timers with RPVST+

[umeshagroya]

Hi Guys,

I have four 6509 switches connected in full mesh. The IOS is 12.2(33)SXI4a. Spanning tree mode configured is RPVST+. The switches are connected using fiber cables.

UDLD default hello interval is 15 sec. Going through Cisco documentation regarding UDLD, I noticed that holdtime of UDLD should be less than convergence time of spanning tree. Taking this into consideration, what should I set UDLD hello interval so that UDLD will take care of unidirectional links without causing a spanning tree loop to occur.

[Shashank Singh]

Hi Umesh,

Convergence time depends on how well the network is configured with uplinkfast/backbonefast/portfast features. Normally the convergence time with RPVST+ is about 5 sec.

Configuring fast UDLD is a good idea. If one of the links becomes unidirectional, Fast UDLD detects this situation faster than regular UDLD and errdisables the link. Traffic is switched over to the second link by EtherChannel. Because this occurs very quickly, traffic loss is minimized.

Configuration steps are available at http://www.cisco.com/en/US/docs/switches/lan/catalyst4500/12.2/54sg/configuration/guide/udld.html#wp1063130

Hope this helps.

Shashank

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