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  1. Senior Member
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    #1

    Default What is the meaning of label space in MPLS?

    Hi,

    Can anyone explain to me what a label space is?
    More specifically what is a platform-wide label space?
    I can only guess what a per-interface label space is.
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  3. lrb
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    #2
    Let's say a router running MPLS (an LSR) has the prefix 100.100.100.0/24 in it's RIB, a label will be created for this prefix, say label 500. If this LSR is an LDP neighbour with three LSRs out three different interfaces, the local LSR will send the label binding (100.100.100.0/24, 500) to all three neighbours.

    Now that all three remote LSRs have the label binding from our local LSR, when one of them sends a labelled packet to our local router with label 500, our local LSR will receive the labelled packet and does one of two things depending on whether per-platform or per-interface label space is being used.

    If the router considers label 500 to have global significance over the router (i.e. the "platform"), it only bases it's forwarding decision based on the incoming label. This is the default for non-ATM interfaces and is what you will mostly see out there. From a certification POV, I don't think Cisco tests on LC-ATM anymore so per-platform label spaces is definately what you will see on the exams.

    If the router considers label 500 to have significance based on which interface it arrived on, it bases it's forwarding decision based on both the incoming interface and the incoming label. Therefore if one remote router sends a lableled packet using label 500 to our local router on one interface, this can be part of a different FEC than if the labelled packet had arrived on a different interface, even though they had the same label.

    Make sense now?
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  4. Senior Member bharvey92's Avatar
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    #3
    Quote Originally Posted by lrb View Post
    Let's say a router running MPLS (an LSR) has the prefix 100.100.100.0/24 in it's RIB, a label will be created for this prefix, say label 500. If this LSR is an LDP neighbour with three LSRs out three different interfaces, the local LSR will send the label binding (100.100.100.0/24, 500) to all three neighbours.

    Now that all three remote LSRs have the label binding from our local LSR, when one of them sends a labelled packet to our local router with label 500, our local LSR will receive the labelled packet and does one of two things depending on whether per-platform or per-interface label space is being used.

    If the router considers label 500 to have global significance over the router (i.e. the "platform"), it only bases it's forwarding decision based on the incoming label. This is the default for non-ATM interfaces and is what you will mostly see out there. From a certification POV, I don't think Cisco tests on LC-ATM anymore so per-platform label spaces is definately what you will see on the exams.

    If the router considers label 500 to have significance based on which interface it arrived on, it bases it's forwarding decision based on both the incoming interface and the incoming label. Therefore if one remote router sends a lableled packet using label 500 to our local router on one interface, this can be part of a different FEC than if the labelled packet had arrived on a different interface, even though they had the same label.

    Make sense now?
    Just finished my CCNP R+S and I want to go deeper into MPLS. Can you recommend any books/videos that will give me a good platform for MPLS?
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  5. lrb
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    #4
    MPLS Fundamentals should have you covered for the most part. MPLS-enabled applications is also not a bad read either.
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  6. Senior Member bharvey92's Avatar
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    #5
    I'll check them out, thanks man!
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  7. Senior Member
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    #6
    MPLS-enabled applications would not be a good introduction to MPLS. It is an extremely dry read, and is geared purely towards theory, with a focus on advanced MPLS solutions... in other words... it's an amazing book just terrible to throw at a beginner.
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