auto-cost reference-bandwidth could affect on Convergence?

engr.J.K.Vasvani · ‎11-16-2009

Dear Experts,

Could you please explain what exactly the mis-matched refrence-bandwidth and Incremental SPF (commands) on different routers could affect? ( I mean few are having defaults and others having non-default)

Do these parameters affect on convergence in ospf domain?

I'd appreciate yours earliest replies.

ThankYou

Giuseppe Larosa · ‎11-16-2009

Hello Jagdesh,

having different autocost reference bandwidth is not recommended it can lead to suboptimal routing :

two different routers can make opposite choices and this can create loops in some cases.

It is more a question of consistency then a problem of convergence.

incremental SPF timers can be different in your routers but this clearly means that overall convergence is that of bigger timers.

Hope to help

Giuseppe

Joseph W. Doherty · ‎11-17-2009

"two different routers can make opposite choices and this can create loops in some cases. "

I can see that as possible during convergence, but could you provide an example after convergence (i.e. a loop)?

AbrahamLincoln · ‎04-29-2021

Hi Giuseppe Larosa. I have a question.

If we change auto-cost reference bandwidth from 10000 to 40000, is it impacted for network and endusers?

In other word, after changing the reference bandwidth value, does OSPF re-convergence the topology?

Giuseppe Larosa · ‎04-30-2021

Hello,

>> In other word, after changing the reference bandwidth value, does OSPF re-convergence the topology?

Each router will create a new version of its own Router LSA with new costs on links as per new reference bandwidth.

The new Router LSA has to be propagated = flooded to all the routers in the same area.

Reception of new Router LSAs will trigger new SPF calculation(s).

Hope to help

Giuseppe

Joseph W. Doherty · ‎11-17-2009

"Could you please explain what exactly the mis-matched refrence-bandwidth and Incremental SPF (commands) on different routers could affect? ( I mean few are having defaults and others having non-default)"

Mismatched reference bandwidths will will impact the link costs used by routers in their SPF calculation (and route path selection). For example, say you have two peer routers, both with 100 Mbps Ethernet interfaces. Default OSPF reference cost would assign such links as 1. However, changing reference bandwidth on one would change it's cost for the same interface bandwidths. Perhaps its 100 Mbps links now cost 10 (i.e. gig ref). Both routers would treat one router's 100 Mbps links like the other's 10 Mbps links.

Incremental SPF is a method to attempt to "short circuit" doing SPF for the whole topology. End result should be identical, however router with Incremental SPF CPU enabled might use less CPU for SPF calculations and finish such calculations faster. Further information on this feature can be found here: https://www.cisco.com/en/US/docs/ios/12_0s/feature/guide/ospfispf.html

"Do these parameters affect on convergence in ospf domain? "

Yes, with "correctness" for reference-bandwidth and CPU resouce usage and perhaps time to converge for the latter.