Subject: Route Distinguisher in MPLS/BGP VPN

From: "Ju, Yu" <[email-masked]>
Date: Fri, 18 May 2001 16:55:14 -0400

Hi,

I have the following question.

Suppose  CE routers Ca, Cb, Cc are connected to PE routers Pa, Pb, Pc
respectively and the CE routers are all in one VPN. Ca and Cb both advertise
a route to a prefix P.  Then PE routers Pa and Pb will advertise VPN-IPv4
route to Pc. If Pa and Pb use different route distinguisher for the same
prefix P, then at PE router Pc, which route should Pc choose? Note that the
two VPN-IPV4 route are not comparable since they have different route
distinguisher.

Thanks,

Yu

---

From: "Shah, Himanshu" <[email-masked]>
Date: Mon, 21 May 2001 08:34:41 -0400

Assuming that import RT on Pc matches RT sent from Pa and
Pb, both of these routes will end up in same VRF in Pc. They
will appear as different routes in VRF because of different RD.

This is a misconfiguration (on Pa or Pb). RD should have been
configured same for the VRF that belongs to that VPN.

As a result, the later arrived VPN-IPV4 will prevail for the 
purposes of route redistribution in IGP and packet forwarding.
Ocillation will continue due to subsequent advertisements...

my 2c,
/himanshu

Himanshu Shah
Tenor Networks
Acton, MA

-----Original Message-----
From: Ju, Yu [mailto:[email-masked]]
Sent: Friday, May 18, 2001 4:55 PM
To: [email-masked]'
Subject: Route Distinguisher in MPLS/BGP VPN

Hi,

Thanks,

Yu

---

From: Dan Tappan <[email-masked]>
Date: Mon, 21 May 2001 09:10:12 -0400

>configured same for the VRF that belongs to that VPN.

This is incorrect. There is no requirement that routes in the same VRF use 
the same RD. The RD serves to make routes unique within BGP. The only 
requirements are
a. if two VPNv4 routes have the same RD and IPv4 parts, then they must be 
comparable routes.
b. if two routes are imported into the same VRF and have the same IPv4 
parts (but possibly different RDs) then they must be comparable routes

A correct implementation will ignore the RD when comparing routes in the 
same VRF in case (b).

>Ocillation will continue due to subsequent advertisements...

And in any case oscillation should not occur, same or different RD, since 
the BGP bestpath should produce a deterministic result between the two routes.