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Understanding Using CoS with MPLS Networks on EX4500 and EX4550 Switches

You can use class of service (CoS) within MPLS networks to prioritize certain types of traffic during periods of congestion. See EX Series Switch Software Features Overview for a complete list of the Juniper Networks Junos operating system (Junos OS) MPLS features that are supported on specific Juniper Networks EX Series Ethernet Switches..

Juniper Networks EX4500 and EX4550 Ethernet Switches support DiffServ code point (DSCP) or IP precedence and IEEE 802.1p CoS classifiers on the customer-edge (CE) interfaces of the ingress provider edge (PE) switch. DSCP or IP precedence classifiers are used for Layer 3 packets. IEEE 802.1p classifiers are used for Layer 2 packets. EX4500 and EX4550 switches also support DSCP or IP precedence and IEEE 802.1p CoS rewrite rules on the CE interfaces of the egress PE switch.

EX4500 and EX4550 switches enable a default EXP classifier and a default EXP rewrite rule. If you want, you can also configure a custom EXP classifier and a custom EXP rewrite rule. However, the switches support only one type of EXP classifier—default or custom—and only one type of EXP rewrite rule—default or custom. The default and custom EXP classifier or rewrite rules are global to the switch or a virtual chassis and not applied on any interface.

After CoS is configured, each incoming IP packet is processed as follows at each switch in your MPLS network:

Ingress Provider Edge Switch

On the ingress PE switch:

Configure and bind a classifier to the CE interface. The choice of classifier to use—DSCP, IP, or IEEE 802.1p—depends on the type of IP packet. Using these classifiers, the switch assigns a forwarding class and a loss priority value to the incoming IP packet on the basis of the packet’s DSCP, IP precedence, or IEEE 802.1p value. This configuration applies to both the inet and the circuit cross-connect (CCC families and also to VLAN CCC interfaces)..
EXP EXP re-marking in MPLS packets on the core facing interfaces is achieved through the Global CoS profile table, which can be configured by the user. You cannot control EXP rewrite on individual interfaces. The EXP value is set on the basis of the forwarding class and loss priority that were assigned to the packet. The rewrite applies to both outer and inner labels. After a rewrite rule is configured, it is applied to all the MPLS packets entering the tunnel.
The IP precedence bits of DSCP are not copied to the EXP bits. EXP bits are always taken from the Global CoS profile table.

For more details about configure CoS on an MPLS PE switch, see Configuring CoS on an MPLS Provider Edge Switch Using Circuit Cross-Connect (CLI Procedure) or Configuring CoS on an MPLS Provider Edge Switch Using IP Over MPLS (CLI Procedure).

Provider Switch

On the provider switch:

The EXP classification occurs through the Global EXP to CoS profile table, which can be configured by the user. You cannot control EXP classification on individual interfaces. Using this table, a forwarding class and a loss priority value is assigned to the incoming MPLS packet on the basis of the EXP value of the outer MPLS label.
The EXP value of the outer label is not preserved during a SWAP operation at the provider switch. When the outermost label is swapped with a new label, the EXP value in that label is also reconfigured on the basis of the Global CoS profile table.
To retain the EXP value from the outer label, you must configure an EXP classifier to map the label's EXP to a CoS profile and also configure an EXP rewrite rule that maps the CoS profile to the same EXP value.
EXP rewrite is achieved through the Global CoS profile table, which can be configured by the user. You cannot control EXP rewrite on individual interfaces. The EXP value is set on the basis of the forwarding class and loss priority that were assigned to the packet. The rewrite applies to only the outer label. After a rewrite rule is configured, it is applied to all the MPLS packets entering the tunnel.

For more details on configuring CoS on an MPLS provider switch, see Configuring CoS on Provider Switches of an MPLS Network (CLI Procedure).

Provider Switch That Is a Penultimate-Hop Pop Switch

On the provider switch, that is a Penultimate-hop Pop switch:

The EXP classification occurs through the Global EXP to CoS profile table, which can be configured by the user. You cannot control EXP classification on individual interfaces. Using this table, a forwarding class and a loss priority value is assigned to the incoming MPLS packet on the basis of the EXP value of the outer MPLS label.
On popping the outer label, the EXP value is not copied from the outer label to the inner label and you cannot use the Junos OS CLI to rewrite the inner label's EXP value.

Egress Provider Edge Switch

On the egress provider edge switch:

The EXP classification occurs through the Global EXP to CoS profile table, which can be configured by the user. You cannot control EXP classification on individual interfaces. Using this table, a forwarding class and a loss priority value is assigned to the incoming MPLS packet on the basis of the EXP value of the MPLS label.
The EXP value is not copied to the IP precedence bits, but it is possible to overwrite the DSCP, IP precedence, or IEEE 802.1p values by configuring and binding a rewrite rule to the CE interface for Layer 3 VPN. Using these rules, the DSCP, IP precedence, or IEEE 802.1p value is overwritten on the basis of the forwarding class and loss priority that were assigned to the packet.
DSCP, IP precedence, and IEEE 802.1p rewrite rules are not supported on CCC or VLAN CCC interfaces.