Subject: RT Import/Export in BGP/MPLS based VPN

From: Sam Ford <[email-masked]>
Date: Sat, 29 Sep 2001 13:11:04 -0700 (PDT)

Hello,

I have a doubt on the assignment of RT in BGP/MPLS
VPN,
a little bit off topic from the threat.

I was confused about the assignment of RT when going
through examples in various materials.

Let's say, there are 2 VPNs, in ASN 10.

VPN-1: CE-1 - PE-1, and other sites in full mesh
VPN-2: CE-2 - PE-2, and other sites in full mesh

And one server: CE-3 - PE-3

I want to conntect only CE-1 & CE-2 sites with CE-3
in Central Services VPN topology. (CE-3 does not act
as
transit between CE-1 & CE-2, CE-1 & CE-2 cannot
communicate). So, I assigned RTs as follows:

In PE-1:
  RD: 10:1
  Export/Import: 10:1

In PE-2:
  RD: 10:2
  Export/Import: 10:2

In PE-3:
  RD: 10:3
  Export/Import: 10:1
  Export/Import: 10:2

Assuming there's no IP address overlap among those
sites.

Q1, wouldn't this RT assignment work?

In the examples I've seen, RTs are assinged as
follows:

In PE-1:
  RD: 10:1
  Export/Import: 10:1
  Import: 10:3
  Export: 10:4

In PE-2:
  RD: 10:2
  Export/Import: 10:2
  Import: 10:3
  Export: 10:4

In PE-3:
  RD: 10:3
  Export: 10:3
  Import: 10:4

Q2, Is there any reason to do this way?

Q3, Above, am I creating a new VPN (3) by connecting
CE-3
  to CE-1 & CE-2? Or am I making CE-3 to belong to
  both the existing VPNs (VPN-1 & VPN-2)?

I would appreciate any clarification,
-SF

-----Original Message-----
From:	Eric Rosen [SMTP:[email-masked]]
Sent:	Friday, September 28, 2001 12:39 PM
To:	Eric Gray
Cc:	Ajay Simha; Jay Karthik; MPLS mailing list
Subject:	Re: LDP usage in MPLS based VPNs 

In  the  "hub  and spoke"  case  that  Ajay 
mentioned,  the intent  is  the
following. 

Let H be the  hub, S1 and S2 the spokes.  Let A be 
the address prefix for a
subnet that attaches to S2.  What we want is:

- S1 thinks the route to A is via H. 

- H thinks the route to A is via S2. 

Thus traffic from S1 to A goes via H and then on to
S2. 

To make this work, S2 must distribute a route RD1:A,
and H must distribute a 
route RD2:A.   Both these  routes have to  be able  to
pass through  a route
reflector, since  RD1:A must make  it to  H, and RD2:A
 must make it  to S1.
This  implies that  RD1  must be  different  than RD2;
 otherwise the  route
reflector could not pass on both routes. 

Given that S1,  S2, and H are in the  same VPN, this
is a  case in which you
couldn't just replace the RD with a VPN-id.  There
might also be other cases
in which  you want to pass around  two different
routes to  the same address
prefix, using  policy to decide which  to install
where  (presumably in some
manner that ensures loop-freedom). 

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