How to tell the usage of a label in a label stack?

Unanswered Question
Jun 2nd, 2007

for example:

have a topo as below








all net belong to a same vpn.

if,I say if lsr3 have a same label value for lsr4 and net3,then lsr3 how to tell if the label is a vpn label or a mpls forwording label.Looking to a packet procession blow:

if a packet frome net1 to net3,when it pass lsr1,it was encaped by two label,one for vpn and another for mpls forwording;When it pass lsr2,it only left one label because of penutimate hop pop;Then lsr3 receive a packet with on label.Lsr3 how to tell this label is a vpn label or a mpls forwarding label?If It recognize it as a vpn label ,it will forward the packet to net3,contrary,it will forward the packet to lsr4.

Then,the question--how?

I have this problem too.
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swaroop.potdar Sat, 06/02/2007 - 23:26

Thats a good point. But for the good this can never happen.

As per the RFC 3031 for frame mode MPLS we always have to use "Per Platform Label Space"

that means unique labels would exist and be assigned for the available FEC's(net's or LSR id)

So essentially the router can and will keep switching based on label values, irrespective of the hierarchy of the label space, ie: whether its a IGP label or VPN label. As the labels assigned on a platform would be always unique with unique forwarding behaviour.



xiaofeng.tu Sun, 06/03/2007 - 06:20

thanks,I misunderstand "per platform".now you correct it ,thanks!


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