OSPF Question

[johnelliot]

If there are 2 links between 2 routers (1 Ethernet+1 DSL), and I force OSPF to prefer the ethernet connection via giving the SHDSL link a higher cost - What is the worse case scenario (In seconds) that the redundant SHDSL link will be used in the event of the ethernet link going down?

Would it be the dead-interval?

[sundar.palaniappan]

You are correct. Non-receipt of OSPF hellos within the dead interval period would cause the neighbor to go down. Hellos/dead-interval can be tweaked to meet your situation. You might even be able to set the dead interval to as low as 1 second and use fast hellos for detecting neighbor loss in 1 second. However, be careful in tuning the timers as every environment is different.

Here's a link that you may find useful in this context:

http://www.cisco.com/en/US/products/ps6350/products_configuration_guide_chapter09186a00804556cc.html#wp1027177

HTH

Sundar

[johnelliot]

Thanks for the clarification + link Sundar.

I assume Fast hellos would not function very well over "higher" latency WAN links.....i.e. SHDSL link has ~15m/sec latency(When not loaded)

There would also be a performance hit if fast hellos are enabled?

[sundar.palaniappan · Accepted solution]

As you can see below, you can set the fast hellos multiplier to anything between 3 - 20.

R2(config-if)#ip ospf dead-interval minimal hello-multiplier ?

<3-20> Number of Hellos sent within 1 second

However, you want to be conservative and set the multiplier to a high # as you mayn't want to see the neighbor considered down on non-receipt of 2 or 3 hellos. Performance shouldn't be affected much as hello packets themselves are very small in size and shouldn't consume much bandwidth.

HTH

Sundar