Re: MPLS - RDs, RTs, and Import/Export

Here is a good blog post that talks of RD, RT, VPNv4, MPBGP etc

http://uyota.blogspot.com/2011/12/mpls-vpn-fundamentals.html

Cheers

On Sat, Dec 17, 2011 at 11:45 PM, Keller Giacomarro <keller.g_at_gmail.com>wrote:

> 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
>
>
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Received on Mon Dec 19 2011 - 10:30:26 ART