MPLS DiffServ Tunnel Modes

Uniform: Packet priorities are uniformly defined on the IP network and the MPLS network, so the priorities are globally valid. On the ingress node, each packet is assigned a label and its DSCP or 802.1p priority is mapped to an EXP value. A change in the EXP value on the MPLS network determines the PHB used when the packet leaves the MPLS network. The egress node maps the EXP value to the DSCP or 802.1p priority. Figure 3-4 shows priority mapping in a uniform tunnel using an L3VPN network as an example. PE_1 maps the DSCP priority to the outer and inner MPLS EXP values (5). P_2 pops out the outer MPLS label and changes the inner MPLS EXP value to the outer MPLS EXP value. PE_2 changes the DSCP priority to 46.

Figure 3-4 Uniform mode

Pipe: The EXP value can be manually configured, and the ingress node adds this EXP value to MPLS packets. Any change in the EXP value is valid only on the MPLS network. The egress node selects the PHB for MPLS packets according to the EXP value. When the packets leave the MPLS network, their DSCP or 802.1p priority is still valid. Figure 3-5 shows priority mapping in a pipe tunnel using an L2VPN network as an example. PE_1 changes the outer and inner MPLS EXP values to 1. PE_2 retains the DSCP priority of packets and selects a PHB based on the inner MPLS EXP value.

Figure 3-5 Pipe mode

Short pipe: The EXP value can be manually configured, and the ingress node adds this EXP value to MPLS packets. Any change in the EXP value is valid only on the MPLS network. The egress node selects the PHB for MPLS packets according to the DSCP or 802.1p priority. When the packets leave the MPLS network, their DSCP or 802.1p priority is still valid. Figure 3-6 shows priority mapping in a short-pipe tunnel using an L3VPN network as an example. PE_1 changes the outer and inner MPLS EXP values to 1. PE_2 retains the DSCP priority of packets and selects a PHB on the DSCP priority.

Figure 3-6 Short pipe mode