Understanding Using CoS with MPLS Networks on EX Series Switches

EXP Classifiers and EXP rewrite Rules

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).

After traversing the MPLS tunnel, the traffic flows out from the egress provider edge (PE) switch. Before the traffic leaves the egress interface, the egress PE switch copies the EXP bits from the MPLS header to the most significant bits in the original IP packet--- that is, to the IP precedence bits.

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 must use the same DSCP, IP precedence, or IEEE 802.1p classifier on CCC interfaces. However, if the CCC interfaces are on the same switch, you cannot configure both a DSCP and an IP precedence classifier on these interfaces. Thus, if you configure one CCC interface to use a DSCP classifier DSCP1, you cannot configure another CCC interface to use another DSCP classifier DSCP2. All the CCC interfaces on the switch must use the same DSCP (or IP precedence) classifier and the same IEEE 802.1p classifier.

• You cannot configure one CCC interface to use a DSCP classifier and another CCC interface to use an IP precedence classifier, because these classifier types overlap.

• You can configure one CCC interface to use a DSCP classifier and another CCC interface to use IEEE 802.1p classifier.

• You can configure one CCC interface to use both a DSCP and an IEEE 802.1p classifier. If you configure a CCC interface to use 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.

• You can configure one CCC interface to use both an IP precedence and an IEEE 802.1p classifier. If you configure a CCC interface to use both these classifiers, the IP precedence classifier is used for routing Layer 3 packets and the IEEE 802.1p classifier is used for routing Layer 2 packets.

You can define multiple DSCP, IP precedence, and IEEE 802.1p classifiers for the non-CCC interfaces on a 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 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 on using multiple DSCP, IP precedence, and IEEE 802.1p classifiers on the same switch.

• You can modify the CoS classifier for a particular interface, but it is not required.

• You can configure a DSCP classifier, DSCP1 on the first interface, another DSCP classifier, DSCP2 on the second interface, and an IP precedence classifier on a third interface, and so forth.

Setting CoS Bits in an MPLS Header

When traffic enters an LSP tunnel, the CoS bits in the MPLS header are set in one of two ways:

• The number of the output queue into which the packet was buffered and the packet loss priority (PLP) bit are written into the MPLS header and are used as the packet’s CoS value. This behavior is the default, and no configuration is required. The Junos OS Class of Service User Guide for Routing Devices explains the IP CoS values, and summarizes how the CoS bits are treated.

• You set a fixed CoS value on all packets entering the LSP tunnel. A fixed CoS value means that all packets entering the LSP receive the same class of service.

The CoS value can be a decimal number from 0 through 7. This number corresponds to a 3-bit binary number. The high-order 2 bits of the CoS value select which transmit queue to use on the outbound interface card.

The low-order bit of the CoS value is treated as the PLP bit and is used to select the RED drop profile to use on the output queue. If the low-order bit is 0, the non-PLP drop profile is used, and if the low-order bit is 1, the PLP drop profile is used. It is generally expected that random early detection (RED) will more aggressively drop packets that have the PLP bit set. For more information about RED and drop profiles, see the Junos OS Class of Service User Guide for Routing Devices.

Note:

Configuring the PLP drop profile to drop packets more aggressively (for example, setting the CoS value from 6 to 7) decreases the likelihood of traffic getting through.

Table 1 summarizes how MPLS CoS values correspond to the transmit queue and PLP bit. Note that in MPLS, the mapping between the CoS bit value and the output queue is hard-coded. You cannot configure the mapping for MPLS; you can configure it only for IPv4 traffic flows, as described in the Junos OS Class of Service User Guide for Routing Devices.

Table 1: MPLS CoS Values

MPLS CoS Value	Bits	Transmit Queue	PLP Bit
0	000	0	Not set
1	001	0	Set
2	010	1	Not set
3	011	1	Set
4	100	2	Not set
5	101	2	Set
6	110	3	Not set
7	111	3	Set

Because the CoS value is part of the MPLS header, the value is associated with the packets only while they travel through the LSP tunnel. The value is not copied back to the IP header when the packets exit from the LSP tunnel.

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 egress interfaces on which MPLS is enabled.

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 can configure a policer on the ingress PE switch to prevent this:

• If you are using MPLS over 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 any custom forwarding class, we recommend that you configure a scheduler to support that forwarding class. See Understanding CoS Schedulers.