MPLS Fundamentals: 2 – Forwarding Labeled Packets

CEF switching is the only IP switching mode that you can use to label packets. Other IP switching modes, such as fast switching, cannot be used, because the fast switching cache does not hold information on labels. Because CEF switching is the only IP switching mode that is supported in conjunction with MPLS, you must turn on CEF when you enable MPLS on the router.

show mpls forwarding-table
show mpls forwarding-table

The local label (or tag) is the label that this LSR assigns and distributes to the other LSRs.
If this LSR receives a packet with top label 16, it removes all labels and forwards the packet as an IP packet, because the outgoing label (tag) is Untagged.

show mpls forwarding-table 2
show mpls forwarding-table

The incoming label 23 is swapped with label 20, and label 16 is pushed onto label 20.

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MPLS Building Blocks

IP routers make forwarding decision based on IP packet header, and local CEF and FIB table.
MPLS routers make forwarding decision based on the MPLS label and the LFIB .
MPLS is great technology these days not because it forwards the packets faster , but because of applications and solutions we can provide for our customers like MPLS VPN’s .

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MPLS uses the IP routing information to determine the direction and next hop to forward a labeled packet .

Does this before the first packet even arrives.

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