MPLS-TE Local and Outgoing Labels in LSP Tunnels

In Multi-Protocol Label Switching (MPLS) networks, understanding the assignment and use of labels is crucial for efficient routing and troubleshooting. When discussing MPLS labels in Label Switched Path (LSP) tunnels, it's important to recognize the distinction between local labels and outgoing labels.

  • Local Label: This is the label assigned by the local router to a specific prefix. For example, a local label might be 24.
  • Outgoing Label: This refers to the label that will be assigned to the packet for the next hop, typically the tail-end router or the endpoint of the tunnel. For instance, an outgoing label might be 26.

In practical terms, when examining the output of commands like show mpls forwarding-table [IP] detail, one can observe both labels being referenced:

PE1#show mpls forwarding-table 7.7.7.7 detail 
Local      Outgoing   Prefix           Bytes Label   Outgoing   Next Hop    
Label      Label      or Tunnel Id     Switched      interface              
24         Pop Label  7.7.7.7/32       0             Tu1        point2point 
        MAC/Encaps=14/18, MRU=1500, Label Stack{26}, via Gi0/1
        5254001A70435254001F16328847 0001A000
        No output feature configured
  • The local label (e.g., 24) acts as an identifier within the local device.
  • The outgoing label (such as 26) is placed in the label stack and is the one visible during operations like a traceroute.

This distinction helps in understanding why only specific labels may appear in diagnostic commands and assists in setting up and troubleshooting MPLS networks more effectively.

https://networklessons.com/mpls/introduction-to-mpls-traffic-engineering-te

https://networklessons.com/mpls/mpls-traffic-engineering-te-ospf-configuration

https://networklessons.com/mpls/mpls-te-static-routes