The MPLS-OPS Archive

[Roger Clark Williams <rogerw@nordlink.com>]

John, it is my understanding that it works this way: The RT import 
statement allows the routing update to be accepted into a given egress PE. 
If there is no RT import statement, the update is ignored by that PE. 
Therefore the update is not put into the VRF on that egress PE even if the 
VRF exists and is part of a given VPN. Once the update is allowed (via the 
RT) and has entered the PE, the RD identifies the incoming routing update 
as being part of a particular VPN, and the update information is put into 
the proper VRF.

As to your other question ("why do you need both?"), consider that you 
might want a VPN that shared certain routes with only a few VPN members, 
not the whole VRF routing  table with all members. A good example might be 
Company A Accounting Department. The accounting department wants to be a 
member of a VPN that connects all Company A's sites, but the accounting 
department only wants to be visible to certain other members of the VPN, 
not to everyone in the VPN. By using RTs properly, the routing information 
to the accounting subnet can be delivered to only those sites that have the 
proper RT import statement specific to the AccDept subnet. The RD will 
still allow those updates into the proper VRF. With the same RD but 
different RTs, that routing information would show up in some CE routing 
tables but not others. Yet all are members of the same VPN. So the short 
answer is that the combination of RD and RT allows a segmentation of the 
routing within a single VPN, or as the vernacular has it, better granularity.

Having said that, I think my explanation is perhaps overly general (and 
maybe even wrong), and I would be happy to hear others' more detailed 
explanations.

Roger Williams

>X-Originating-IP: [210.214.114.78]
>From: "john smith" <johnsmith0302@hotmail.com>
>To: "Roger Clark Williams" <rogerw@nordlink.com>
>Subject: Re: RE: [MPLS-OPS]: Route Distinguisher Questions
>Date: Wed, 19 Feb 2003 23:51:38 +0530
>X-Mailer: Microsoft Outlook Express 5.00.2615.200
>X-OriginalArrivalTime: 19 Feb 2003 18:02:52.0186 (UTC) 
>FILETIME=[1ED9CFA0:01C2D841]
>
>
> > In other words, they are announced by the
> > same "word" but their function and use within the system are not the same.
> > They play on the same team (RD and RT within one VRF) but there is no
> > correlation between them other than that fact and their names. So no,
>there
> > is no underlying reason the RD and RTs have to be the same number. In
>fact,
> > there are all kinds of instances in which there are multiple RTs within a
> > single VRF and yet perhaps only one pair (if that) have the same "name" as
> > the RD.
>
>
>so what identifies the route "exclusively" RD or RT?
>if i say that given a route and it came from some VPN, would you use RD or
>RT to identify it..what would you say?
>why do you need both?

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