cancel
Showing results for 
Search instead for 
Did you mean: 
cancel
218
Views
0
Helpful
5
Replies
Highlighted
Beginner

Ospf Convergence between two equal cost routes

Hi all, 

There are two core routers with equal cost paths to them from the distribution switches. 

One core router needs to be taken down. If this is done gracefully how quickly will the distribution switches change its preferred route to use the other core. 

Thanks in advance. Please let me know if I've been unclear. 

Everyone's tags (1)
5 REPLIES 5
Hall of Fame Guru

Re: Ospf Convergence between two equal cost routes

 

If they are equal cost routes then they should be both be in the routing table so there is no convergence. 

 

You may lose a packet but that should be it. 

 

Jon

Beginner

Re: Ospf Convergence between two equal cost routes

Thanks Jon, this is what I would expect. We're planning to lab it next week so we should get a definitive answer.
Engager

Re: Ospf Convergence between two equal cost routes

Hi,

 


@stuart_jones wrote:

If this is done gracefully how quickly will the distribution switches change its preferred route to use the other core. 


Do you know the OSPF Stub Router Advertisement  feature?

In a nutshell, a router advertises LSAs with maximum metrics when you enable it, and makes alternative paths through other routers more preferred by its neighbors. Of course you'll need to check that your IOS supports it.

 

HTH

Rolf

VIP Expert

Re: Ospf Convergence between two equal cost routes

As Jon notes, there should be no "convergence" to just use the remaining link because both links are in the route table for the same destinations. I.e. once the router removes the one link, all traffic will shift to the other.

The forgoing (e.g. shut interface) should be the situation on a router where you shut an interface.

What could happen, though, is OSPF could continue to route to the dropped (failed) link during the time OSPF doesn't know it's down. If OSPF is signaled with a hardware link drop, it should be around 50ms or less. If the link loss detection relies on a timer, you could have traffic sent to the down link for multiple seconds.

You mention doing this "gracefully". If done correctly, your transition time will depend on how fast you configure both neighbor routers, but you shouldn't lose any traffic.

(How to do gracefully? Change OSPF cost on one router's link. Confirm that link dropped from ECMP. Shut link interface on other router. Confirm all okay. [Suggestion, only shut interface on router "nearer" you. You might also reset cost on first router now, so you need not remember to do it to later restore ECMP.])
Beginner

Re: Ospf Convergence between two equal cost routes

Hi Joseph, thanks for your detailed response. Good shout regarding changing the OSPF cost on the link.
CreatePlease to create content
Content for Community-Ad
August's Community Spotlight Awards