In order to simplify QoS management through the network core, some operators aggregate multiple forwarding classes of traffic at the ingress LER or PE and use two or three QoS markings instead of the eight different QoS markings that a customer device might be using to dictate QoS treatment. However, in order to ensure the end-to-end QoS enforcement required by the customer, the aggregated markings must be mapped back to their original forwarding classes at the egress LER (eLER) or PE.
For IP traffic (including IPSec packets) riding over MPLS or GRE tunnels that will be routed to the base router, a VPRN interface, or an IES interface at the tunnel termination point (the eLER), the 7705 SAR can be configured to ignore the EXP/DSCP bits in the tunnel header when the packets arrive at the eLER. Instead, classification is based on the inner IP header, which is essentially the customer IP packet header. This configuration is done using the ler-use-dscp command.
When the command is enabled on an ingress network IP interface, the IP interface will ignore the tunnel’s QoS mapping and will derive the internal forwarding class and associated profile state based on the DSCP values of the IP header ToS field rather than on the network QoS policy defined on the IP interface. This function is useful when the mapping for the tunnel QoS marking does not completely reflect the required QoS handling for the IP packet. The command applies only on the eLER where the tunnel or service is terminated and the next header in the packet is IP.