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Understanding CoS MPLS EXP Classifiers and Rewrite Rules

You can use class of service (CoS) within MPLS networks to prioritize certain types of traffic during periods of congestion by applying packet classifiers and rewrite rules to the MPLS traffic. (For information about DSCP and IEEE 802.1p classifiers and general information about classifiers, see Understanding CoS Classifiers. For information about DSCP and IEEE 802.1p rewrite rules, see Understanding CoS Rewrite Rules.)

When a packet enters a customer-edge interface on the ingress provider edge (PE) switch, the switch associates the packet with a particular CoS servicing level before placing the packet onto the label-switched path (LSP). The switches within the LSP utilize the CoS value set at the ingress PE switch. The CoS value that was embedded in the classifier is translated and encoded in the MPLS header by means of the experimental (EXP) bits.

EXP classifiers map incoming MPLS packets to a forwarding class and a loss priority, and assign MPLS packets to output queues based on the forwarding class mapping. EXP classifiers are behavior aggregate (BA) classifiers.

EXP rewrite rules change (rewrite) the CoS value of the EXP bits in outgoing packets on the egress queues of the switch so that the new (rewritten) value matches the policies of a targeted peer. Policy matching allows the downstream routing platform or switch in a neighboring network to classify each packet into the appropriate service group.

Note: There is no default EXP classifier. There is no default EXP rewrite rule. If you want to classify incoming MPLS packets using the EXP bits, you must configure a global EXP classifier. If you want to rewrite the EXP bit value at the egress interface, you must configure EXP rewrite rules and apply them to logical interfaces. EXP classifiers and rewrite rules are applied only to interfaces that are configured as family mpls (for example, set interfaces xe-0/0/35 unit 0 family mpls.)

This topic includes:

EXP Classifiers

Unlike DSCP and IEEE 802.1p BA classifiers, EXP classifiers are global to the switch and apply to all switch interfaces that are configured as family mpls. When you configure and apply an EXP classifier, MPLS traffic on all family mpls interfaces uses the EXP classifier, even on interfaces that also have a fixed classifier. If an interface has both an EXP classifier and a fixed classifier, the EXP classifier is applied to MPLS traffic and the fixed classifier is applied to all other traffic.

Also unlike DSCP and IEEE 802.1p BA classifiers, there is no default EXP classifier. If you want to classify MPLS traffic based on the EXP bits, you must explicitly configure an EXP classifier and apply it the switch interfaces. Each EXP classifier has eight entries that correspond to the eight EXP CoS values (0 through 7, which correspond to bits 000 through 111).

You can configure as many EXP classifiers as you want. However, the switch uses only one MPLS EXP classifier as a global classifier on all interfaces. After you configure an MPLS EXP classifier, you can configure it as the global EXP classifier by including the EXP classifier in the [edit class-of-service system-defaults classifiers exp] hierarchy. All switch interfaces use the global EXP classifier to classify MPLS traffic.

Only one EXP classifier can be configured as the global EXP classifier at any time. If you want to change the global EXP classifier, delete the global EXP classifier configuration (use the user@switch# delete class-of-service system-defaults classifiers exp configuration statement), then configure the new global EXP classifier.

If an EXP classifier is not configured, then if a fixed classifier is applied to the interface, the MPLS traffic uses the fixed classifier. If no EXP classifier and no fixed classifier is applied to the interface, MPLS traffic is treated as best-effort traffic. DSCP classifiers are not applied to MPLS traffic.

Because the EXP classifier is global, you cannot configure some ports to use a fixed IEEE 802.1p classifier for MPLS traffic on some interfaces and the global EXP classifier for MPLS traffic on other interfaces. When you configure a global EXP classifier, all MPLS traffic on all interfaces uses the EXP classifier.

Note: The switch uses only the outermost label of incoming EXP packets for classification.

Note: MPLS packets with 802.1Q tags are not supported.

EXP Rewrite Rules

As MPLS packets enter or exit a network, edge switches might be required to alter the class-of-service (CoS) settings of the packets. EXP rewrite rules set the value of the EXP CoS bits within the header of the outgoing MPLS packet on family mpls interfaces. Each rewrite rule reads the current forwarding class and loss priority associated with the packet, locates the chosen CoS value from a table, and writes that CoS value into the packet header, replacing the old CoS value. EXP rewrite rules apply only to MPLS traffic.

EXP rewrite rules apply only to logical interfaces. You cannot apply EXP rewrite rules to physical interfaces.

There are no default EXP rewrite rules. If you want to rewrite the EXP value in MPLS packets, you must configure EXP rewrite rules and apply them to logical interfaces. If no rewrite rules are applied, all MPLS labels that are pushed have a value of zero (0). The EXP value remains unchanged on MPLS labels that are swapped.

You can configure as many EXP rewrite rules as you want, but you can only apply 16 EXP rewrite rules at any time on the switch. On a given logical interface, all pushed MPLS labels have the same EXP rewrite rule applied to them. You can apply different EXP rewrite rules to different logical interfaces on the same physical interface.

You can apply an EXP rewrite rule to an interface that has a DSCP, DSCP IPv6, or IEEE 802.1p rewrite rule. Only MPLS traffic uses the EXP rewrite rule. MPLS traffic does not use DSCP or DSCP IPv6 rewrite rules.

If the switch is performing penultimate hop popping (PHP), EXP rewrite rules do not take effect. If both an EXP classifier and an EXP rewrite rule are configured on the switch, then the EXP value from the last popped label is copied into the inner label. If either an EXP classifier or an EXP rewrite rule (but not both) is configured on the switch, then the inner label EXP value is sent unchanged.

Note: On each physical interface, either all forwarding classes that are being used on the interface must have rewrite rules configured or no forwarding classes that are being used on the interface can have rewrite rules configured. On any physical port, do not mix forwarding classes with rewrite rules and forwarding classes without rewrite rules.

Schedulers

The schedulers for using CoS with MPLS are the same as for the other CoS configurations on the switch. Default schedulers are provided only for the best-effort, fcoe, no-loss, and network-control forwarding classes. If you configure a custom forwarding class for MPLS traffic, you need to configure a scheduler to support that forwarding class and provide bandwidth to that forwarding class. See Understanding CoS Output Queue Schedulers and Example: Configuring Queue Schedulers for more information.