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How does a prefix in the CEF table get the imposed label stack?


How does a prefix in the CEF table get the imposed label stack?


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Re: How does a prefix in the CEF table get the imposed label sta

Hello Wang,

the signalling protocol for MPLS builds other tables.

CEF builds the FIB forwarding information table.

LDP builds LFIB = label forwarding information table.

the routers in the MPLS cloud build LDP sessions between them.

In MPLS framed mode the labels are assigned in unsolicited mode: they are assigned to prefixes in the FIB before traffic for that prefix is seen.

Each router sends out its label assignments towards its LDP neighbors.

On each router each label value is used only once and associated to a prefix or other FEC.

the labels actually travel in the opposite direction of traffic: from nearest to farest node. Thi is called downstream.

unsolicited downstream indipendent

indipendent means that a node is free to associate a label to a prefix in FIB before receiving the label binding from the router that is on the path to the prefix.

note that all prefixes to be in CEF FIB have to be advertised by an IPV4 routing protocol like OSPF, EIGRP or IS-IS.

Also the LDP sessions are built using loopback interfaces so they can be setup only after the routing protocol convergence.

the end result that can be checked with

sh mpls forwarding

is that a local label is associated to the prefix,

the label received by the router upstream (nearest to the destination ) is used to send out MPLS packets

the local label is used by the router downstream (farer from destination) the current node when sending frame to this node

the mpls forwarding table says

if an incoming packet with label local-label is received I will send out a specific interface with label= action or outgoing label.

The edge nodes are the routers with at least one non mpls interface.

These nodes when sending traffic to a prefix perform label imposition by looking at LFIB if an entry is found is used to build the outgoing label.

when the edge routers receive packets destinated to their non mpls connected interaces they do the opposite by extracting the packet.

IOS uses normally penultimate hop popping: actually the last router before the edge node receives a special label (implicit-null) that instructs it to send packets for the prefix untagged to the edge node.

This is an optimization.

To be noted that MPLS support multiple label stacks and these are used for many applications: L3 VPN, L2 VPN, MPLS TE protection, 6PE, 6VPE.

when moving in the MPLS cloud only the more external label is changed at each node hop.

Inner labels travel unchanged end-to-end.

Hope to help



Re: How does a prefix in the CEF table get the imposed label sta


awesome reply.


shivlu jain