Subject: RE: RD in 2547 bis

From: "Sylvia Sugar" <[email-masked]>
Date: Thu, 17 Apr 2003 07:53:49 +0000

Hello Mr. Jim,

I must thank you for taking time to mail back on my query. It really feels 
nice to have the author of a book on MPLS, answer me my queries. I am really 
thankful to you for your time and patience.

I still have a few more things to clarify.

> > >>
> > >> > > > >RD is local to the router /used to distinguish VPNv4 routes
> > >>locally, is my
> > >> > >understanding.
> > >> > > RD becomes crucial with the presence of RRs.
> > >>
> > >>How come? Can you please give me some pointers as to how RD becomes
> > >>"crucial" in presence
> > >>of RRs.
>
>the RD is not really local to the router in the sense that it can affect
>route selection at the RRs. If you use the same RD on say PE1 and PE2 (lets
>call it RD 100:2) and then export route 1.1.1.1 from both PEs, the RR will
>see two identical routes e.g. 100:2:1.1.1.1. It will select one as best 
>path
>and therefore one of the routes is lost from downstream PEs.

But Mr Jim, how could one use different Route Distiguisher for the same 
prefix? In the book it seems to say its unique to the customer.

On the other had, when one passes routes across VPNs, it seems to say that 
the Route Target is used to identify the "VRF" of customer. There is 
something about multiple service providers, who each would have control of 
RD which would be local to them, but Route Target would be unique to a 
customer across all service providers.

The reason cited for the same is the size of the Route Distinguisher is very 
small.

This is my understanding from 2547 and 2547bis.

I am not too sure where this was mentioned in your book.

However, it seems to make Route Distinguisher redundant.
One could simply replace Route Distinguisher with Route Target and it seems 
to achieve the same functionality

Please correct me if I am wrong. This is all sooo confusing. :-((

-SylviaXO

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From: "Jim Guichard" <[email-masked]>
Date: Thu, 17 Apr 2003 11:36:24 -0400

Hi Sylvia,

> >-----Original Message-----
> >From: Sylvia Sugar [mailto:[email-masked]]
> >Sent: Thursday, April 17, 2003 3:54 AM
> >To: [email-masked]; [email-masked]
> >Subject: RE: RD in 2547 bis
> >
> >
> >Hello Mr. Jim,
> >

this can happen if you export the same route from two different PEs - note
that this will happen if you have a dual-attached site advertising the same
set of prefixes.

> >customer across all service providers.

not necessarily - you can rewrite the RT values at the ASBR routers or you
can carry them unchanged between AS's - this is a deployment choice.

> >The reason cited for the same is the size of the Route

the RD is 64 bits = 8 bytes

> >to achieve the same functionality

no, it does not. The RT is used for import/export policy. The RD is used to
make the IPv4 address unique. An update can carry multiple RT values but a
single RD for each NLRI within the update. Jim