what is the second level label of a mpls packet?

[wlsj]

i thought it was the RT or RD,but i can not associate the MBGP atrribute with the mpls label,confused,

please help me!

recommending a related book would be helpful

thanks

[swaroop.potdar]

Devang, the VPN label is used to identify the customer VPN prefix, in the very same way the IGP/LDP label is used to identify the global prefix. But none of the labels carry the RD or RT or the prefix into the label. Its just a mapping of the label values to the prefixes.

HTH-Cheers,

Swaroop

[devang_etcom]

I got it Swaroop... so you mean to say 2nd label is generated by Egress PE and that will be used by only PE routes right!!! now one more question is as PE to PE forwarding is done by the MPLS Label which is our 1st label now why PE router need 2nd label... as PE will decide which VRF it has to install that route is based on the RT value so why we need 2nd label?

regards

Devang

[swaroop.potdar]

Devang, IGP label(1st label) is required to label switch the path till the IBGP nexthop (which is other PE's advertising the VPN routes).Once it packet reaches the egress PE it has to find the right VRF to which the route belongs and then find the right outgoing interface. So from MPLS perspective it assigns a second label which is the mapping of VPN routes to label value. And as per this label (second label it identifies the right VPN). Without the second label there is no mapping to identify which VPN the packet belongs to.

Its more of the control plane and forwarding plane thing. When BGP sends the update it sends it with RT RD and VPN label to be used.

But when the packet from the customer side(data traffic) arrives that packet in its L3header only has a destination IP. No RD or RT so how would the PE's take the forwarding decision for that packet, so for that purpose the second label which was assigned as the part of control plane for a destination route (where in the routes's RD and RT values were known) is used.

So to conclude when forwarding packets there is no RD or RT information, its just labels which have been assigned when the packet entered the MPLS cloud.(1st label the IGP label for the BGP nexhop, and the second label (VPN label for the VPN prefix) to identify the right VPN for that packet.

HTH-Cheers,

Swaroop

[devang_etcom]

Swaroop,

So it means there are two MPLS label one is for the IPv4 route which is the first label and other is for the VPNv4 route which is the second label right... and second label just only to identify the VPN only and that will be exchanged only between PEs... right!!!

so when packet arrived at the egress PE it will remove the second label and depends on the RT vaule it will put that route in appropriate VRF... so again VPN label will used only for the data transfer but installing route in VRF will only be handl by RT... and in BGP update PE will exchange the VPNv4 route,RT and VPN label...right!!!

regards

Devang

[swaroop.potdar]

Yes Devang you got it right!

HTH-Cheers,

Swaroop

[dangal.43]

Swaroop and Devang,

I got benifit from your conversation thanks to both of you and originator of the post too.

Some are the things are very little but creating lots of confusion but you guys are there to make it clear. coollllllll

[devang_etcom]

Thanks Swaroop for your help...

If i will have any further question then i will ask it again:)... thanks again

regards

Devang

[swaroop.potdar]

Dan, Devang, Thank you :-) and you are welcome.

Cheers,

Swaroop