little help with mpls labels

MACattackMACattack Posts: 121Member
Hi just need a little help from those mpls geeks.
Recently I am reading the mpls fundamentals and also the iscw mpls chapters.

So here is my inquiry, is it possible that to 2 fec can have the same label or what I mean is that
a two routes sharing the same labels (don't want to make complicated) let say and both are different subnet but using the label let say 16 as the starting.

I would appreciate if you could give me some scenario just a little help to clarify.

base on the book of iscw press.. "When MPLS has a path from point A to point B, a label-switched path (LSP) is created. The LSP is essentially a tunnel between source and destination for a particular FEC. Zooming out, it would look more like a tunnel with many forks in the road, because it is feasible for two FECs to share a label at one point, then diverge at another. For a given destination or FEC, however, the end-to-end path is built. LSRs from edge to edge will contain labels for a particular FEC. This allows the implementation of a penultimate hop popping (PHP) of the label. "[/b]


  • WRKNonCCNPWRKNonCCNP Posts: 38Member ■■□□□□□□□□
    The only time i can think of when two FECs would share a label would be when MPLS VPNs are implemented. Say the two subnets you mentioned are connected to different CE routers, but the same PE router. Now say that at a CE connected to a different PE, you send traffic to either of those two subnets. On that traffic's ingress PE, a VPN label and an IGP label will be attached to each of the packets destined for the two subnets.

    Both subnets represent a different FEC, but the IGP labels used to get from the ingress PE to the egress PE could be the same labels (the same LSP). Once the packets get to the P router before the egress PE, the IGP label is popped (PHP) and the VPN label is still there for the egress PE to deal with. The VPN label tells the egress PE how to forward the packets, in this case, to different CEs that have a route to the different subnets.

    I remember reading the paragraph that you quoted in the OP and not really understanding how it worked, but i think if you continue to read MPLS fundamentals (good book) and get to the MPLS VPNs chapter, it will make more sense. Thanks for asking the question. Through helping you (hopefully) i have furthered my own understanding.
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