IP-over-MPLS QoS TTL propagation control

MPLS QoS uniform mode

By default, the device use MPLS QoS uniform mode. In the MPLS label header, the TTL field indicates the Time To Live (TTL) value for an MPLS packet. For IP-over-MPLS application at the ingress LER, an IP packet‘s TTL value is decremented by one and the IP checksum is recalculated. Then, the IP packet‘s TTL value is copied to its MPLS TTL field. At each transit LSR hop, the MPLS TTL value is decremented by one. When the MPLS TTL value reaches one or zero, the packet is discarded.

At the MPLS router that pops the label (either the penultimate LSR or the egress LER), the incoming packet‘s MPLS TTL value is copied to the packet‘s IP TTL field, the IP TTL field is decremented by one, and the checksum is re-calculated. The packet is discarded if the TTL value reaches one. The result is that each LSR in the MPLS domain is counted as one hop.

For uniform mode, the QoS mapping table maps an incoming DSCP to EXP. EXP values may change based on QoS map.



Uniform mode is overridden for an LSP when the LSP is specifically configured a COS value. When a COS value is configured for an LSP, the device sets the tunnel as if global pipe mode is set. The outbound packet has an EXP value of the configured COS and the default system wide TTL value of 255.

MPLS QoS pipe mode

Optionally, you can configure MPLS QoS pipe mode. The ingress LER decrements the IP packet‘s TTL value by one and then places a value of 255 in the packet‘s MPLS TTL field. The MPLS TTL value is decremented by one as the MPLS packet passes through each LSR in the MPLS domain. When the label is popped, the value in the MPLS TTL field is discarded and is not copied to the packet‘s IP TTL field. The unlabeled IP packet‘s TTL is decremented by one as it passes through the egress LER. With the packet‘s IP TTL being decremented twice, from the time it enters the ingress LER to the time it exits the egress LER, the MPLS domain appears as two hops.

For pipe mode the TTL value is the default system wide value of 255. The default EXP value for the pipe, if the LSP is not configured, is the COS value configured in the LSP configuration. The LDP COS value is 0.