Cisco Support Community
cancel
Showing results for 
Search instead for 
Did you mean: 
Announcements
Webcast-Catalyst9k

Why is my simple OSPF convergence so slow?

Ok, here is the setup:

  • Router A is a Cat 3750X stack in the data centre
  • Router B is a Cat 3750X stack in the data centre
  • Router C is a Cat 4500 in the user access distribution closet

All three routers are joined on VLAN 999 which is my OSPF backbone. RouterA and RouterC speak OSPF, but RouterB does not do so yet; I am waiting for a maintenance slot so I can enable the feature set. The backbone adjacency between RouterA and RouterC is up and running.

RouterA and RouterB have SVIs in Vlan 15,with HSRP for the servers to send to. On RouterA, the SVI is in OSPF area 0 as a passive interface. I can see the OSPF route turning up in RouterC, and a host on RouterC can get to Vlan 15 OK.

Because I don't have OSPF on RouterB yet, I have put a floating static route on RouterC for Vlan 15, via RouterB's backbone address, with AD=150. Of course, this normally gets overwritten by the OSPF route from RouterA.

So I decided to check it out by shutting down the Vlan 15 SVI on RouterA. The HSRP failed over to RouterB pretty well instantly; I guess RouterA sends a resignation PDU just before it goes down. But the OSPF took a good 5 seconds to fail over. That is, the time from when I shut the interface on RouterA, to the time that RouterC removed the OSPF route from its routing table and replaced it with the floating static. And of course all that time, Router A was blackholing the traffic intended for Vlan 15. (Actually, not quite true; it was sending it out its default route.)

I think the delay is on RouterA. Debug tells me that from the time I shut down the SVI on RouterA, it takes five seconds before RouterA gets round to generating an LSA to delete the route. Can anyone tell me why it takes so long?

Thanks in advance.

Kevin DORRELL

Luxembourg

1 ACCEPTED SOLUTION

Accepted Solutions

Why is my simple OSPF convergence so slow?

The Spf-start delay is 5000 ms by default.

2 REPLIES

Why is my simple OSPF convergence so slow?

The Spf-start delay is 5000 ms by default.

Why is my simple OSPF convergence so slow?

Thank you, that is precisely the answer I was looking for.

This delay timer seems to have some bizzare consequences. If the SVIs on RouterA and RouterB are in area 0, and RouterC is using the route via RouterA, and I shut down the SVI of RouterA, then RouterC sees a black hole for 5 seconds before the LSA turns up to say that RouterA no longer has a route.

BUT, if the SVIs on RouterA and RouterB are in area 15, then the convergence at RouterC is pretty well instantaneous. That means the inter-area routes converge faster than the intra-area routes, which is kinda counter-intuitive. It would seem that the delay timer applies only to SPF calculation, not to inter-area routes.

Kevin Dorrell

Luxembourg

798
Views
0
Helpful
2
Replies
CreatePlease to create content