Cisco WS-SUP32-GE-3B Software Configuration Guide - Page 645

Chapter 39 Configuring MPLS QoS Understanding MPLS QoS 3. The PFC3B performs a table lookup to determine the next-hop LSR. 4. An appropriate label is placed (swapped) into the packet and the MPLS EXP bits are copied into the label header. 5. The PFC3B forwards the labeled packets to the appropriate output interface for processing. 6. The PFC3B also forwards the 802.1p bits or the IP ToS bits to the output interface. 7. The outbound packet is differentiated by the MPLS EXP field for marking or policing. 8. The labeled packets (marked with EXP) are sent to another LSR in the core MPLS network or to an LER at the output edge. Note Within the service provider network, there is no IP precedence field for the queueing algorithm to use because the packets are MPLS packets. The packets remain MPLS packets until they arrive at PE2, the provider edge router. LERs at the Output Edge of an MPLS Network At the egress side of an MPLS network, LERs process packets as follows: 1. MPLS-labeled packets (and 802.1p bits or IP ToS bits) from a core LSR arrive at the egress LER (PE2) from the MPLS network backbone. 2. The PFC3B pops the MPLS labels (disposition) from the packets. Aggregate labels are classified using the original 802.1p bits or the IP ToS bits. Nonaggregate labels are classified with the EXP value by default. 3. For aggregate labels, the PFC3B performs a lookup on the IP address to determine the packet's destination; the PFC3B then forwards the packet to the appropriate output interface for processing. For non-aggregate labels, forwarding is based on the label. By default, non-aggregate labels are popped at the penultimate-hop router (next to last), not the egress PE router. 4. The PFC3B also forwards the 802.1p bits or the IP ToS bits to the output interface. 5. The packets are differentiated according to the 802.1p bits or the IP ToS bits and treated accordingly. Note The MPLS EXP bits allow you to specify the QoS for an MPLS packet. The IP precedence and DSCP bits allow you to specify the QoS for an IP packet. Understanding MPLS QoS MPLS QoS supports IP QoS. For MPLS packets, the EXP value is mapped into an internal DSCP so that the PFC3B can apply non-MPLS QoS marking and policing. For both the ingress and egress policies, MPLS QoS marking and policing decisions are made on a per-interface basis at an ingress PFC3B. The ingress interfaces are physical ports, subinterfaces, or VLANs. The QoS policy ACLs are programmed in QoS TCAM separately for ingress and egress lookup. The ternary content addressable memory (TCAM) egress lookup takes place after the IP forwarding table (FIB) and NetFlow lookups are completed. OL-11439-03 Catalyst Supervisor Engine 32 PISA Cisco IOS Software Configuration Guide, Release 12.2ZY 39-7