Re: A noobs thoughts on RDs and RTs

John, one thing you left out is that it's not just the iBGP, but the use of MP-BGP and tdp -or- ldp that makes all this magic come together.

Regards,
 Joe Sanchez

On Apr 9, 2012, at 11:20 PM, John Neiberger <jneiberger_at_gmail.com> wrote:

> This is sort of an addendum to the discussion a week or so ago about
> the difference between route targets and route descriptors. Now that
> I've had a chance to study them a bit more I thought I would check to
> see if my understanding is correct. As I mentioned in the other
> thread, I always like to approach learning and teaching from this
> standpoint: what problem are we trying to solve? We have two primary
> problems to solve, but it's easier to them once you've encountered
> them.
>
> I'll assume you have a basic understanding of VRFs. Let's assume you
> have two customers assigned to two VRFs and they have overlapping IP
> address space, e.g. 10/8. You configure your VRFs and then (let's
> pretend) you configure iBGP and somehow start passing routes around
> your core network. When a remote iBGP peer receives a 10.0.0.0/8 route
> twice, what is it supposed to do with them? You know that they're from
> two different customers, but how is the router supposed to know that?
> That's where the Route Distinguisher comes in. It is a tag of sorts
> that you prepend to the routes from your VRFs so that receiving
> routers know that they're to be treated differently. They are, in
> fact, different routes.
>
> Okay, the router has received them. Now what is it supposed to do with
> them? You may have the same two VRFs configured on this receiving
> router, but is the router just supposed to automatically put all
> routes with matching route descriptors into the same VRFs? Sure,
> that's one way it could work. But what happens if down the road you
> need to share some routes between customers? How could you control
> which VRFs get which routes? Or what if you have a load-balancing
> situation where you have routes from the same customer being
> advertised with two different Route Distinguishers?
>
> Well, my friends, that's where the Route Target comes into play, and
> even though so many examples of RTs and RDs make them the same, they
> have nothing whatsoever to do with each other. The RT is a BGP
> community that rides along with your route advertisements that tells
> the receiving router which VPN they belong to. That gives you some
> serious flexibility.
>
> But wait. Why do we need two different types of "labels" to
> differentiate between these routes. Now that we have a Route Target,
> why can't we just use that? Well, because the RT is a community
> attribute. It is not prepended to the beginning of the route like the
> Route Distinguisher. If you take away the RD, you're back to a
> situation where the receiving router doesn't know that these routes
> came from different customers. It sees the exact same routes with
> different route targets. That might confuse it.
>
> So, the RD does NOTHING but distinguish routes. Clever, huh? It makes
> them unique within your iBGP network. The RT is what tells the
> receiving router which VPN those routes belong to, which gives you
> much more flexibility with regard to importing and exporting.
>
> So, you RD/RT experts, is that basically correct?
>
> Thanks,
> John
>
>
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Received on Tue Apr 10 2012 - 05:54:23 ART