The CoS architecture for MX-series routing platforms such as the MX960 Ethernet Services Router is similar in concept, but different in particulars, from other routing platforms. The general architecture for the MX-series routing platform is shown in Figure 3.
Figure 3: MX-series Packet Forwarding and Data Flow
The MX-series routing platform can classify incoming packets at the ingress Dense Port Concentrator (DPC). Fixed classification places all packets in the same forwarding class, or the usual MF or BA classifications can be used to treat packets differently. BA classification with firewall filters can be used for classification based on IP precedence, DSCP, IEEE, or other bits in the frame or packet header.
However, the MX-series routing platforms can also employ multiple BA classifiers on the same logical interface. The logical interfaces do not have to employ the same type of BA classifier. For example, a single logical interface can use classifiers based on IP precedence as well as IEEE 802.1p. If the CoS bits of interest are on the inner VLAN tag of a dual-tagged VLAN interface, the classifier can examine either the inner or outer bits. (By default, the classification is done based on the outer VLAN tag.)
Internal fabric scheduling is based on only two queues: high and low priority. Strict-high priority queuing is also supported in the high-priority category.
Egress port scheduling supports up to eight queues per port using a form of round-robin queue servicing. The supported priority levels are strict-high, high, medium-high, medium-low, and low. The MX-series architecture supports both early discard and tail drop on the queues.
All CoS features are supported at line rate. For more information about MX-series CoS support and configuration, see CoS Features of MX-series Platforms.