Subject: Re: RD in 2547 bis

From: "Sylvia Sugar" <[email-masked]>
Date: Wed, 16 Apr 2003 10:22:41 +0000

Hello,

We were involved in an MPLS design for a service provider here.

I would like to know the following:

> > > > Also, with RT, we can simply check for one RT attribute and decide 
>whether
> > > > we want 100s of MP_NLRI or not (the way BGP update is packed). If we 
>had
> > >to
> > > > do this based on RD, then we really would have to go through each 
>VPNv4
> > > > prefix to find out whether it was a candidate for the import or not.
> > >
>
>Agree with this point of yours.

This is what is confusing me.
From what my book says authored by cisco press,  use route targets to assign 
routes to put routes into a VRF. So we anyways use the "Route Target 
attribute" for this.

>
> > > > >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.
>
> > > > > I think it comes down to what gives us more granularity and 
>control over
> > > > what we want to do.
>
>I think this is it. And nothing else. We can also work out with having just 
>one RT but
>then having both gives us some greater control over what we want to do!
>

Yes Mr Smith, but how does not having Route Distinguisher affect things.

I have no example in this book which shows me how to use "Route 
Distinguisher" can help. It doesnt get used to filter routes either. It 
seems to be nothing but local.

Maybe its not in this book. Can someone please suggest some good book to 
read to clarify this?

Can you please explain the "what we want to do" bit?

-SylviaXO

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From: John Smith <[email-masked]>
Date: Wed, 16 Apr 2003 11:47:17 +0100 (BST)

Sugar,

> routes to put routes into a VRF. So we anyways use the "Route Target 

Righto! What i agreed to was that searching for which prefixes need to be inserted in
which VRFs by looking at the RDs is not only unpractical .. but down right stupidity and
brain dead also!

How?

Let me explain you.

Suppose you decide one day to look at the RD to decide which routes should go into which
VRF. The problem is that if a BGP peer sends you 10,000 prefixes then you will need to
lookup 10000 times to insert routes into your VRF.

With the RT concept, all the routes are tagged with just one RT extended community
attribute and you by looking at this can decide which VRF to insert the routes to.

This does increase the scalability and also gives you more control over the routes which
are advertised using BGP.

> Yes Mr Smith, but how does not having Route Distinguisher affect things.

Did i ever say that Sugar?

> Maybe its not in this book. Can someone please suggest some good book to 

Try one by Ivan Pepelnjak and Jim Guichard

> Can you please explain the "what we want to do" bit?

couldnt get you?

JS

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---

From: "alok" <[email-masked]>
Date: Wed, 16 Apr 2003 17:10:06 +0530

Hi John,

----- Original Message -----
From: John Smith <[email-masked]>
To: Sylvia Sugar <[email-masked]>;
<[email-masked]>
Sent: Wednesday, April 16, 2003 4:17 PM
Subject: Re: RD in 2547 bis

> are advertised using BGP.

thanks for clarifying this.

*I never realised* that we dont look at "MBGP NLRIs- *one by one*"  for BGP
decision process to compare things with the existing entries in the
LOCAL_RIB.

thanks again,
Alok

---

From: "Jim Guichard" <[email-masked]>
Date: Wed, 16 Apr 2003 15:20:05 -0400

Sylvia,

> >-----Original Message-----
> >From: Sylvia Sugar [mailto:[email-masked]]
> >Sent: Wednesday, April 16, 2003 6:23 AM
> >To: [email-masked]

This is correct - the RT controls the import/export policies - the RD has
nothing to do with this, period.

> >>How come? Can you please give me some pointers as to how RD becomes

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.

> >Yes Mr Smith, but how does not having Route Distinguisher affect things.

see above RR discussion.

> >seems to be nothing but local.

it is not in the book as it has nothing to do with import/export policies.

Jim

---

From: vrishab sikand <[email-masked]>
Date: Wed, 16 Apr 2003 12:43:12 -0700 (PDT)

[snip]... > >This is what is confusing me.

>nothing to do with this, period.
What about the case: Where routes are being injected into the back bone via a connection at a PE from the CE. At that point (unless BGP is being used between PE - CE and its a provider of provider case) there is no RT or RD coming with route. The association with the VRF and the interface is done on basis of configured RD. So in a way the import policy there is based on RD.

Jim Guichard <[email-masked]> wrote:Sylvia,

see above RR discussion.

Jim

> >Can you please explain the "what we want to do" bit?

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---

From: "Jim Guichard" <[email-masked]>
Date: Wed, 16 Apr 2003 15:45:49 -0400

-----Original Message-----
  From: vrishab sikand [mailto:[email-masked]]
  Sent: Wednesday, April 16, 2003 3:43 PM
  To: Jim Guichard; Sylvia Sugar; [email-masked]
  Subject: RE: RD in 2547 bis

  [snip]...

  >nothing to do with this, period.

  What about the case: Where routes are being injected into the back bone
via a connection at a PE from the CE. At that point (unless BGP is being
used between PE - CE and its a provider of provider case) there is no RT or
RD coming with route. The association with the VRF and the interface is done
on basis of configured RD. So in a way the import policy there is based on
RD.

  I think you are confused with the RT - the RT provides the association
with an importing VRF from the backbone. For routes coming from the CE e.g.
there is no RD or RT association, the association with the VRF is based on
the routing protocol context and/or interface. Jim

  Jim Guichard <[email-masked]> wrote:
    Sylvia,

    > >>VP! Nv4

    This is correct - the RT controls the import/export policies - the RD
has
    nothing to do with this, period.

    > >>! ;"crucial" in presence

    the RD ! is not r eally 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.

    see above RR discussion.

    > >
    > >I have! no example in this book which shows me how to use "Route

    it is not in the book as it has nothing to do with import/export
policies.

    Jim

    > >Can you please explain the "what we want to do" bit?

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