QoS MPLS Default Behavior E-LSP


Cisco only support E-LSP MPLS QoS forwarding method. By default some rules apply to its behavior in terms EXP and DSCP interaction between labels and IP packets.

Default Behavior

Below are some basic default rules for MPLS QoS behavior is IOS:

  1. When a PE imposes a label, the IP header's IP Precedent or first 3 bits of DSCP are copied into MPLS EXP bit (Imposition). Also referred to as ToS Reflection.
  2. When P routers swap labels, the EXP bits of incoming top label are copied into the outgoing top label (Switching).
  3. When popping a label, the in-coming label's EXP bits are not copied to the lower level outgoing label (Tunneling).
  4. When stripping a label (exposing IP header), the in-coming label's EXP bits are not copied to the IP Precedence or DSCP bits (Disposition)
  • Depending on the DiffServ Tunneling mode, above behavior will change from defaults.

Additional Resources

Configuring MPLS QoS on the PFC - Cisco 7600 Series Cisco IOS Software Configuration Guide, 12.2SR


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