Technical Documentation

Supported Platforms
EX Series

Understanding Using CoS with MPLS Networks on EX Series Switches

You can use class of service (CoS) within MPLS networks to prioritize certain types of traffic during periods of congestion.

Juniper Networks EX Series Ethernet Switches support Differentiated Service Code Point (DSCP) or IP precedence and IEEE 802.1p CoS classifiers on the customer-edge interfaces of the ingress provider edge (PE) switch. DSCP or IP precedence classifiers are used for Layer 3 packets. IEEE 802.1p is used for Layer 2 packets.

When a packet enters a customer-edge interface of the ingress PE switch, the switch associates the packet with a particular CoS servicing level prior to putting 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 DSCP, IP precedence, or IEEE 802.1 p classifier is translated and encoded in the MPLS header by means of the EXP or experimental bits.

EX Series switches enable a default EXP classifier and a default EXP rewrite rule. You can configure a custom EXP classifier and a custom EXP rewrite rule if you prefer. However, the switch supports only one type of EXP classifier (default or custom) and only one EXP rewrite rule (default or custom).

You do not bind the EXP classifier or the EXP rewrite rule to individual interfaces. The switch automatically and implicitly applies the default or the custom EXP classifier and the default or the custom EXP rewrite rule to the appropriate MPLS-enabled interfaces. Because rewrite rules affect only egress interfaces, the switch applies the EXP rewrite rule only to those MPLS interfaces that are transmitting MPLS packets (not to the MPLS interfaces that are receiving the packets).

This topic includes:

Guidelines for Using CoS Classifiers on CCCs

When you are configuring CoS for MPLS over circuit cross-connect (CCC), there are some additional guidelines, as follows:

You must explicitly bind a CoS classifier to the CCC interface on the ingress PE switch.
You cannot use more than one type of DSCP/IP precedence and not more than one type of IEEE 802.1p classifier on the CCC interfaces. Thus, if you configure one CCC interface to use DSCP1, you cannot configure another CCC interface to use DSCP2. Likewise, if you configure one CCC interface to use IEEE1, you cannot configure another CCC interface on the same switch to use IEEE2. All the CCC interfaces on the switch must use the same DSCP classifier and the same type of IEEE 802.1p classifier.
You cannot configure one CCC interface as DSCP and another CCC interface as IP precedence, because these classifier types overlap.
You can configure one CCC interface as DSCP and another CCC interface as IEEE 802.1p.
You can configure one CCC interface as both DSCP and IEEE 802.1p. If you configure a CCC interface with both these classifiers, the DSCP classifier is used for routing Layer 3 packets and the IEEE 802.1p classifier is used for routing Layer 2 packets.

Note: You can define multiple types of DSCP, IP precedence, and IEEE 802.1p on the switch and use the different classifier types for the non-CCC interfaces on the switch.

Using CoS Classifiers with IP over MPLS

When you are configuring CoS for IP over MPLS, the customer-edge interface uses the CoS configuration that has been set up for the switch as the default. You do not have to bind a classifier to the customer-edge interface in this case. There are no restrictions regarding using multiple types of DSCP, IP precedence, and IEEE 802.1p on the same switch.

You can modify the CoS classifier for a particular interface, but it is not required.
You can configure one interface as DSCP1 and another as DSCP2 and another and IP precedence, and so forth.

Default Classifiers and Default Rewrite Rules

The default classifiers support only two forwarding classes, best-effort and network-control, and use only two queues, 0 and 7. However, EX Series switches support up to sixteen forwarding classes and eight queues. To use the additional forwarding classes and queues, create a custom classifier. To modify the code point and loss priority for a specific forwarding class, configure a rewrite rule on the switch. The default rewrite rule for EXP is enabled in the default configuration. However, the default rewrite rules for the other classifiers are not enabled in the default configuration. You can display the default classifier mappings and default rewrite mappings by entering the show class-of-service command on the switch.

EXP Rewrite Rules

When traffic passes from the customer-edge interface to an MPLS interface, the DSCP, IP precedence, or IEEE 802.1p CoS classifier is translated into the EXP bits within the MPLS header. You cannot disable the default EXP rewrite rule, but you can configure your own custom EXP classifier and a custom EXP rewrite rule. You cannot bind the EXP classifier to individual MPLS interfaces; the switch applies it globally to all the MPLS-enabled interfaces on the switch.

Only one EXP rewrite rule (either default or custom) is supported on a switch. The switch applies it to all the MPLS-enabled egress interfaces.

Policer

Policing helps to ensure that the amount of traffic forwarded through an LSP never exceeds the requested bandwidth allocation. During periods of congestion (when the total rate of queuing packets exceeds the rate of transmission), any new packets being sent to an interface can be dropped because there is no place to store them. You should configure a policer on the ingress PE switch:

If you are using MPLS with CCC, you bind the policer to the LSP. You cannot bind a policer to a CCC interface.
If you are using IP over MPLS, you bind the policer to the inet-family customer-edge interface. You cannot bind a policer to the LSP when you are using IP over MPLS.

Schedulers

The schedulers for using CoS with MPLS are the same as for the other CoS configurations on EX Series switches. Default schedulers are provided for best-effort and network-control forwarding classes. If you are using assured-forwarding, expedited-forwarding, or other custom forwarding classes, we recommend that you configure a scheduler to support that forwarding class. See Understanding CoS Schedulers.