QoS MPLS Forwarding


MPLS uses DiffServ as its QoS mechanism. It can't see the IP DSCP values (in IP header), so it relies on the MPLS EXP bits for PHB of DiffServ. The problem is that DSCP has 64 defined per hop behaviors, while MPLS EXP only has 8. There are two solution to figure out the forwarding based on QoS: E-LSP and L-LSP.


  • E-LSP stands for EXP-inferred LSP.
  • To use this method, the network can only supports up to 8 PHBs.
  • Each MPLS switching node has to have a policy configured to match EXP to Per Hop Behavior (scheduling and drop priority).
  • Each DSCP is equivalent to a particular MPLS EXP behavior for scheduling and drop priority.
  • Label determines the where to forward the packet and EXP bits determine the PHB, no signaling for PHB is required as in L-LSP.
  • For more info on the default E-LSP QoS behavior see QoS-MPLS-Default-Behavior-E-LSP


  • L-LSP stands for Label-inferred LSP.
  • To use this method, the network would need to support more than 8 PHBs up to 64..
  • This methods might not be implemented in Cisco.
  • There is a label signaled using LDP that conveys information about the scheduling behavior and path information. Drop priority is determined from EXP bits.

Additional Resources

Q&A - Quality of Service for Multi-Protocol Label Switching Networks


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