MPLS - RDs, RTs, and Import/Export

Hi all,

I'm having some trouble wrapping my head around how VRF RDs and RTs are
being handled by the router.

First, some things I know (I think...):
- A VRF holds a set of routes, and is a copy of the standard routing table
- A RD is set per-vrf and its sole purpose is to create a way for BGP to
distinguish between identical prefixes from different VRFs
- Routers do not use the RD to make import/export decisions into VRFs
- An export RT on a VRF means that if the router advertises a route from
that VRF into BGP, it will attach the extended community specified in the
export statement
- An import RT on a VRF means that if the router receives a prefix from BGP
that has the specified community set, it will import it into the VRF where
the import statement is configured

It came as a revalation to me that the RD doesn't have ANYTHING to do with
importing/exporting routes -- so much of the docs I've read seem to imply
that it does!

Okay, so here's what I don't get. Say we're doing a central services MPLS
VPN. Two customers, one central services VPN so there are three VRFs in
play. Assuming only one PE per customer/server site to simplify a bit.

Exports:
Customer 1's routes are exported from their PE into MP-BGP with a RT of 1
(not valid, but this is an example).
Customer 2's routes are exported from their PE into MP-BGP with a RT of 2.
Central services's routes are exported from their PE with a RT of 3.

Imports:
Customer 1 imports routes with a RT of 3 to receive central services routes.
Customer 2 imports routes with a RT of 3 to receive central services routes.
Central services imports routes with an RT of 1 and 2 to receive Customer 1
and 2's routes.

Based on the reading I've done, at this point Customer 1 and Customer 2
should be able to communicate with Central Services but not with each
other. But that doesn't really make sense to me! Wouldn't central
services *import* routes from Customer 1 with an RT of 1 and then
*export* those
same routes with an RT of 2? Wouldn't that mean that both Customer 1 and
Customer 2 are receiving each other's routes, with the Central Services PE
acting as a route bridge?

Any lead on what I might be missing, as well as confirmation of what I
think I know, would be very much appreciated.

Thanks, experts!

Keller Giacomarro
keller.g_at_gmail.com

Blogs and organic groups at http://www.ccie.net
Received on Sat Dec 17 2011 - 22:45:54 ART