[c-nsp] EIGRP SoO question

[Derick Winkworth]

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