MPLS VPN Label and Route Target

Unanswered Question
Aug 27th, 2007

Hi all,

My quesation are realted to MPLS and are as follws:-

1-> Between the Pe Routers there is a MP-BGP Session . The MP-BGP protocal carries the VPNV4 routes in the Extended Communtiy. Now between the P routers there is Label Swapping done. But along with the Local Label and the next hop label there is a Vpn Label also being attached to the packet. Now what is that VPN Label for?

2---> Secondly if someone says that it is used for identifying the VPN or the VRF at the Egress PE Router then what does RT does. Coz as far as I know that it is the RT which identifies the VRF or the VPN at the Egress Router. So wat does this VPN Lable is used for?

3--> Thirdly what is the difference between the Penultimate Hop Popping Tag (PHP) tag and Untagged Tag.

Warm Regards

Raghav

I have this problem too.
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mohammedmahmoud Mon, 08/27/2007 - 22:27

Hi Raghav,

An MPLS VPN packet contains 2 labels, the LDP label and the VPN label. The VPN label is only understood and exchanged between PE routers via MP-BGP, the packet is switched over the P router via the LDP Top label, thus the LDP top label will switch the packet from the ingress PE to the egress PE passing over all the P routers, and finally the VPN label delivers the packet to the customer interface.

You must differentiate between the job of the VPN label and the RT. The RT is used to identify the VPN routes in order to be installed in the appropriate VRF table, while the VPN label is used to forward the packets to the appropriate customer interface, thus both have different jobs.

PHP is the action where the P router just before the egress PE pops the label before it sends the packet to the egress PE (the label is popped (removed) one hop earlier). Without PHP a double lookup is required on the egress PE, the first lookup in the LFIB (indicates Pop for the label from the labeled packet) and then a lookup in the FIB to forward the IP packet based on the next-hop address to the CE router assuming ordinary MPLS packet (optimizing the MPLS performance by decreasing the lookups).

HTH,

Mohammed Mahmoud.

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