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  1. Member
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    #1

    Default MPLS reserved labels (explicit null & FEC)

    In the following context can anyone explain me the sentence mark with red ? Label 0 is assigned to multiple FECs?- or what he is trying to teach ?

    The explicit NULL label is the solution to this problem, because the egress LSR signals the IPv4 explicit NULL label (value 0) to the penultimate hop router. The egress LSR then receives labeled packets with a label of value 0 as the top label. The LSR cannot forward the packet by looking up the value 0 in the LFIB because it can be assigned to multiple FECs. The LSR just removes the explicit NULL label.
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    #2
    To me this means, the explicit null zero label is used to signal that the next hop is an IP next hop and not a label switched next hop. The LSR will not forward the packet because when a label value of zero is seen this means the router is the egress LSR, effectively the end of the label switched path. So the zero label value can be assigned to multiple FEC's because it is used to signal the end of the FEC and not to indicate a labelled next hop.

    I assume because PHP is mentioned that this action may occur on the router in the LSP before the edge LSR. YMMV though as I am not an MPLS guru.
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  4. Went to the dark side.... Moderator networker050184's Avatar
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    #3
    The label value 0 is used for explicit null, rather than the default implicit, when you want the packet to arrive at the PE with the top and bottom tag rather than just the bottom tag. An example for this is EXP bits. If the penultimate router pops the label prior to forwarding there are no bits to take action on by the egress router.

    The reason the reserved value of 0 can be reused over multiple FECs is because there is no forwarding action taken on it. It's merely a pop action.
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