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Understanding How Class of Service Works with MX Series Router Cloud CPE Services

 

To prioritize subscriber traffic, you can use the CoS features of the MX Series router along with cCPE services. cCPE services require only a Layer 2 CPE device at the subscriber site. If the subscriber is currently using a Layer 3 CPE device, replace it with a Layer 2 CPE that supports CoS, such as the Juniper Networks EX Ethernet switch. To ensure the proper classification and management of upstream traffic in a subscriber's access link, CoS should be supported from CPE to CPE across the provider network. A Layer 2 CPE that supports CoS can classify traffic based on Layer 1 or Layer 2 information, such as physical port, VLAN, or MAC address. It can subsequently mark the traffic with Ethernet 802.1p, then prioritize the traffic through the CPE before passing it to the access link. If there are aggregation switches in the network path located before the provider edge (PE) router, the switches must also support Layer 2 CoS based on 802.1p bits in the traffic.

When the traffic reaches the PE router, the router uses CoS behavior aggregate (BA) classifiers to map 802.1p bits in the traffic to different forwarding classes and loss priorities. The traffic is prioritized to pass through the PE router.

You configure rewrite rules to change CoS values in outgoing packets on the outbound interfaces of the PE router to meet the policies of the core routers. If the remote Layer 2 CPE requires it, the remote PE router can use rewrite rules to change CoS values back to 802.1p on the outbound interface. By default, the MX Series router uses the BA classifier for 802.1p. For more information see, https://www.juniper.net/documentation/en_US/junos13.1/topics/reference/general/default-802-1p-table-cos-config-guide.html.

If the Layer 2 CPE does not support CoS marking, the MX Series router can classify incoming packets by using a multifield classifier. The multifield classifier is a firewall filter that classifies packets to a forwarding class by examining fields such as source and destination address, port, and IP protocol. CoS rewrite rules are applied to packets on outbound interfaces to set or change CoS markings to meet the policies of the core network.