[AusNOG] VPLS OSPF question

[Mark Smith]

----- Original Message -----
> From: Mark Smith <markzzzsmith at yahoo.com.au>
> To: Johann Lo <Johann.Lo at aptel.com.au>; "ausnog at lists.ausnog.net" <ausnog at lists.ausnog.net>
> Cc: 
> Sent: Tuesday, 23 April 2013 7:20 AM
> Subject: Re: [AusNOG] VPLS OSPF question
> 
> 


<snip>

> 
> 
> If some of your VPLS sites had redundant access links to the VPLS, you now have 
> the possibility of Equal Cost Multiple Paths, and that further increases the SPF 
> run time because SPF doesn't just stop when it has found the first shortest 
> path (or shortest path first ;-) ) As supporting ECMP on Ciscos is a default 
> (and likely on other vendors implementations of OSPF), the SPF runs for this 200 
> site VPLS are going to be relatively long. On a Cisco you can change that by 
> specifying 'maximum-paths 1' under router ospf.
> 

Thinking about it a bit more, complete evaluation of all the paths for ECMP purposes is actually going to happen regardless of whether you have redundant equal paths or not, as it is the only way for the router to determine if equal cost paths exist. So if you don't have them, or don't want to use equal cost paths even if they do exist (which can make troubleshooting harder), switching it off via the 'maximum-paths' command is the only option.