IRB timers recommended configuration

[alexis.thomasa]

Hi there networkers,

I have configured IRB on my 3845 border router to have redundancy in case of failure. I tested it and it works OK, but the only detail is that the convergence time after the main link failsover to the secondary is too long. I think this is due to the default timers and I'd like to adjust them, but I don't know exactly what values to use to avoid flapping between both links. Any suggestions?

Cheers,

AT

[Giuseppe Larosa]

Hello Alexis,

the convergence time that you see is caused by spanning tree protocol, by enabling IRB you enable an STP instance running on the two interfaces of the router and that interact with the LAN switch ports.

you can try to tune these STP timers if the STP diameter is less then 7 switches in a row.

see

http://www.cisco.com/en/US/tech/tk389/tk815/technologies_tech_note09186a0080094663.shtml

for STP timer tuning see

http://www.cisco.com/en/US/docs/ios/bridging/configuration/guide/br_transprnt_brdg_ps6441_TSD_Products_Configuration_Guide_Chapter.html#wp1003798

but this has to be done carefully

Hope to help

Giuseppe

[alexis.thomasa]

Hello Giuseppe,

Thank you for the feedback.

This router connects to two different switches acting as active - backup and both ports connected to the router are configured as 'spanning-tree portfast'. At this time, all the ports on both switches are forwarding traffic and also one of the router ports is in blocking state so I think what I have to tune is the STP timers on the router. I just wonder why it doesn't take 50secs as in the regular STP behaviour but just 15secs as it was the forwarding time of the IRB timer.

Cheers,

AT

[Giuseppe Larosa]

Hello Alexis,

point of attention here:

>> both ports connected to the router are configured as 'spanning-tree portfast

this is not totally safe as the two router ports are part of a 2 ports LAN switch.

>>  just wonder why it doesn't take 50secs as in the regular STP behaviour but just 15secs as it was the forwarding time of the IRB timer.

this confirms the point of attention above on switch side the port is an edge port ( RSTP terminology) and the switch does not go via the normal port states sequence.

network stability should be the first target, speed of convergence should come later.,

Hope to help

Giuseppe

[alexis.thomasa]

Thank you for your help, Giuseppe.

I'm going to work in the points that you mention to be sure everything is fine.

Cheers,

AT.