WL & Devang,

When a PE router distributes a VPN-IPv4 route via BGP, it uses its own address as the "BGP next hop". It also assigns and distributes

an MPLS label. (Essentially, PE routers distribute not VPN-IPv4 routes, but Labeled VPN-IPv4 routes. When the PE processes a

received packet that has this label at the top of the stack, the PE will pop the stack, and send the packet directly to the site from to which the route leads. This will usually mean that it just sends the packet to the CE router from which it learned the route.

So RD is used to make the IPV4 route unique and the RT is to assign membership of the route to a given VRF. So they have no direct role in creation of the second label. And the second label does not contain the RD or RT values, it just another MPLS label with the same label format as the IGP label.

Reference : Lookup the RFC 2547 section "4.2.2. Route Distribution Among PEs by BGP"

Also what you are trying to do, and what did you manipulate to get what output would be good to start with.

HTH-Cheers,

Swaroop

Swaroop,

I am again confused now in one of my past post you gave explanation is:http://forum.cisco.com/eforum/servlet/NetProf?page=netprof&forum=Service%20Providers&topic=MPLS&CommCmd=MB%3Fcmd%3Ddisplay_location%26location%3D.1ddf971b

so what is the exact 2nd label is?

what will be there in 2nd label?

see if it telll remote PE router which customer he has to send packet using 2nd label then it should be the RT label right??????

Hi Dan,

As Swaroop has stated, VPN labels has no relation with RT and RD, VPN labels are exchanged via MBGP (between PEs only, P router are not aware of the VPN labels), and it is imposed as the second label on a MPLS VPN packet, the first label is the LDP label and it is responsible for delivering the packet to the appropriate egress PE, and the second label (VPN label) is responsible to forward the packet to the desired CE router.

On the other hand RD is used to create the VPNv4 unique address of the customer (VPNv4 = IPv4 + RD) and RT is used to exchange the routes itself between VRFs on PEs, and as it is obvious there is no slight relation between VPN label and RT or RD.

I hope that i've been informative.

HTH,

Mohammed Mahmoud.

Dan, both replies are very much consitent. The second label is not the RT label but the label assigned to the prefix in a VPN/VRF routing table.

So again to conclude, RD is used to make the customer/VPN routes unique, as many customers may be using similar private addresses (192.168/172.16/10.), and the RT is used to exchange routes between VPN's.

And to these VPN routes the owner(the egress PE) of the routes assigns VPN label (second label). And advertises these routes to other PE's, with the label using MPBGP (VPNv4 Add Fly).

I would be glad to clarify any other questions to the best of my knowledge.

HTH-Cheers,

Swaroop

the RT and RD only used int exchanging route(control plane),and no relation with the actual traffic(forwarding plane),right?

Hi,

As i've recommended earlier please check out this book:

MPLS Fundamentals

Luc De Ghein, CCIE

No. 1897

HTH,

Mohammed Mahmoud.

Mohammed

another question arise,when does the PE router insert the vpn label?and how to map this labeled packet to the mpls backbone label?

thank you

wlsj

Hi wlsj,

When the PE (Egress PE) is advertising the vpnv4 prefix via MP-BGP to other PEs, it also signals the VPN label that is associated with the vpnv4 prefix. And if by the mpls backbone label you mean the LDP label (Top label), then its simply discovered as the label corresponding to the next-hop of the BGP routes (or in other words the router advertising the route, which is the Egress router).

I hope that i've been informative.

HTH,

Mohammed Mahmoud.

Wlsj, if you go through this thread, when and how does the LDP label is mapped to the VPN label would be evident.

Without making it repititive, if would be nice if you can let us know what you are trying to do, and why do you want to map the VPN label with a specific LDP label, as this happends automatically as per the label stack formation.

HTH-Cheers,

Swaroop

Swaroop,

will you please take a look at this link:

http://www.cisco.com/application/pdf/en/us/guest/netsol/ns465/c654/cdccont_0900aecd80375d78.pdf page# 8 and step#2... so as per this document one label is used to identify the customer VPN prefix and other is MPLS label for forwarding...

regards

Devang

Second (top in the label stack) label is used by P's to forward packets between PE's. RT is propagated as a BGP ext community and never used for forwarding but to control inter-VRF export/import of routes.