[c-nsp] EIGRP SoO question

[Ivan Pepelnjak]

You'll probably find enough details here:

http://wiki.nil.com/Multihomed_MPLS_VPN_sites_running_EIGRP

If that's not the case, let me know and I'll fix the article.

Ivan
 
http://www.ioshints.info/about
http://blog.ioshints.info/ 

> -----Original Message-----
> From: Derick Winkworth [mailto:dwinkworth at att.net] 
> Sent: Sunday, July 12, 2009 9:38 PM
> To: cisco-nsp at puck.nether.net
> Subject: [c-nsp] EIGRP SoO question
> 
> I'm trying to wrap my head around how this works.
> 
> There is BGP SOO.  This is where routes are tagged as they 
> are redistributed into BGP so that other PEs attached to the 
> same customer site do not push the routes back into the site. 
>  This accounts for the PE ->  CE direction.
> 
> In the opposite direction, it seems there are actually two 
> different mechanisms.
> 
> There is
> 
> a) EIGRP SOO.  This is an EIGRP extension/tag that the PE 
> uses so it does not re-introduce a route back into the PE 
> iBGP cloud.  Routes are tagged going into a site, and if the 
> site is dual-homed and the route comes back to another PE 
> that is appropriately configured, this other PE will see the 
> tag and not re-advertise that route back into BGP.
> 
> b)  BGP cost community.  This attribute carries the EIGRP 
> metric of the route that is being redistributed into BGP.  At 
> another PE (presumable a PE attached to a multihomed site), 
> this attribute tells BGP to compare the EIGRP cost embedded 
> in the attribute directly to an EIGRP route learned from the 
> CE.  This attribute is compared before any other BGP attribute.
> 
> 
> So I guess why do we need both (a) and (b)?
> 
> The documentation for this is shoddy.
> 
> Derick Winkworth
> CCIE #15672
>