CoS for MPLS Overview
When IP traffic enters a label-switched path (LSP) tunnel, the ingress routing device marks all packets with a class-of-service (CoS) value, which is used to place the traffic into a transmission priority queue. On the routing device, each interface has up to eight transmit queues. The CoS value is encoded as part of the Multiprotocol Label Switching (MPLS) header and remains in the packets until the MPLS header is removed when the packets exit from the egress routing device. The routing devices within the LSP utilize the CoS value set at the ingress routing device. The CoS value is encoded by means of the CoS bits (also known as the EXP or experimental bits).
MPLS class of service works in conjunction with the routing device’s general CoS functionality. If you do not configure any CoS features, the default general CoS settings are used. For MPLS class of service, you might want to prioritize how the transmit queues are serviced by configuring weighted round-robin, and to configure congestion avoidance using random early detection (RED).
The next-hop label-switching router (LSR) uses the default classification shown in Table 1.
Table 1: LSR Default Classification
Code Point | Forwarding Class | Loss Priority |
|---|---|---|
000 | best-effort | low |
001 | best-effort | high |
010 | expedited-forwarding | low |
011 | expedited-forwarding | high |
100 | assured-forwarding | low |
101 | assured-forwarding | high |
110 | network-control | low |
111 | network-control | high |
