Subject: Re: Route Distinguisher in MPLS/BGP VPN

From: "krishna mohan doddapaneni" <[email-masked]>
Date: Fri, 18 May 2001 21:58:02

Hi Yu,

I dont think you can use overlapping addresses in the same VPN or in VPN's that communicate with each other.

Krishna

>From: "Ju, Yu"

>To: "[email-masked]'"

>Subject: Route Distinguisher in MPLS/BGP VPN

>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

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From: "Alexander Marhold" <[email-masked]>
Date: Sat, 19 May 2001 07:55:29 +0200

Hello !

I consider this to be a valid question, as maybe Ca and Cb are the Customer
Edge router for the same customer networks ( redundancy and/or loadsharing)

In principle MP-BGP with vpnv4 -routes behave exactly like normal BGP
regarding path selection.
It will select the better path according to the 11 (12) step BGP path
selection process, the better one will win and only the best path will be
advertised, observing split-horizon.

If you use different RDs then the routes will come together in the VRF where
the same selection will take place, as the path is from MP-BGP to
VRF-specific BGP and from there via the BGP-scan task to the VRF-specific
Routing table and from there to the customer via ( RIPV2, OSPF) the PE-CE
routing protocol.

Loadsharing in that case will take place whenever you have implemented
routing protocols which allow loadsharing or if you enable BGP-multipath
support.

--> RD Route Distinguisher

aim: make routes unique, distinguish between VRF (virtual routing tables) on
same PE

set: on the vrf and attached to all routes when redistributed into MP-BGP

rule 1: no 2 VRFs on the same PE may have the same RD

rule 2: the combination of <RD>:<ipv4 address> (=VPNV4-address) needs to be
unique in the provider network as BGP (or better MP-BGP) only forwards the
best route (except for redundant connections or multipath)

regards

Alexander Marhold
PROIN

-----Original Message-----
From: [email-masked] [mailto:[email-masked]]On Behalf Of krishna mohan
doddapaneni
Sent: Friday, May 18, 2001 9:58 PM
To: [email-masked]; [email-masked]
Subject: Re: Route Distinguisher in MPLS/BGP VPN

  Hi Yu,

  I dont think you can use overlapping addresses in the same VPN or in VPN's
that communicate with each other.

  Krishna

  >two VPN-IPV4 route are not comparable since they have different route

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