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Understanding CoS Rewrite Rules

 

As packets enter or exit a network, edge switches might be required to alter the class-of-service (CoS) settings of the packets. Rewrite rules set the value of the code point bits (Layer 3 DSCP bits, Layer 2 CoS bits, or MPLS EXP bits) within the header of the outgoing packet. Each rewrite rule:

  1. Reads the current forwarding class and loss priority associated with the packet.

  2. Locates the new (rewrite) code point value from a table.

  3. Writes that code point value into the packet header, replacing the old code point value.

Rewrite rules must be assigned to an interface for rewrites to take effect.

You can apply (bind) one DSCP or DSCP IPv6 rewrite rule and one IEEE 802.1p rewrite rule to each interface. You can also bind EXP rewrite rules to family mpls logical interfaces to rewrite the CoS bits of MPLS traffic.

Note

OCX Series switches do not support MPLS and do not support EXP rewrite rules.

You cannot apply both a DSCP and a DSCP IPv6 rewrite rule to the same physical interface. Each physical interface supports only one DSCP rewrite rule. Both IP and IPv6 packets use the same DSCP rewrite rule, regardless if the configured rewrite rule is DSCP or DSCP IPv6. You can apply an EXP rewrite rule on an interface that has DSCP or IEEE rewrite rules. Only MPLS traffic on family mpls interfaces uses the EXP rewrite rule.

You can apply both a DSCP rewrite rule and a DSCP IPv6 rewrite rule to a logical interface. IPv6 packets are rewritten with DSCP-IPv6 rewrite-rules and IPv4 packets are remarked with DSCP rewrite-rules.

Note

There are no default rewrite rules. If you want to apply a rewrite rule to outgoing packets, you must explicitly configure the rewrite rule.

You can look at behavior aggregate (BA) classifiers and rewrite rules as two sides of the same coin. A BA classifier reads the code point bits of incoming packets and classifies the packets into forwarding classes, then the system applies the CoS configured for the forwarding class to those packets. Rewrite rules change (rewrite) the code point bits just before the packets leave the system so that the next switch or router can apply the appropriate level of CoS to the packets. When you apply a rewrite rule to an interface, the rewrite rule is the last CoS action performed on the packet before it is forwarded.

Rewrite rules alter CoS values in outgoing packets on the outbound interfaces of an edge switch to accommodate the policies of a targeted peer. This allows the downstream switch in a neighboring network to classify each packet into the appropriate service group.

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.

Note

Rewrite rules are applied before the egress filter is matched to traffic. Because the code point rewrite occurs before the egress filter is matched to traffic, the egress filter match is based on the rewrite value, not on the original code point value in the packet.

For packets that carry both an inner VLAN tag and an outer VLAN tag, the rewrite rule rewrites only the outer VLAN tag.

MPLS EXP rewrite rules apply only to family mpls logical interfaces. You cannot apply to an EXP rewrite rule to a physical interface. You can configure up to 64 EXP rewrite rules, but you can only use 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.

Note

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.

You can configure enough rewrite rules to handle most, if not all, network scenarios. Table 1 shows how many of each type of rewrite rules you can configure, and how many entries you can configure per rewrite rule.

Table 1: Configuring Rewrite Rules

Rewrite Rule Type

Maximum Number of Rewrite Rules

Maximum Number of Entries per Rewrite Rule

IEEE 802.1p

64

128

DSCP

32

128

DSCP IPv6

32

128

MPLS EXP

64

128

You cannot apply rewrite rules directly to integrated routing and bridging (IRB), also known as routed VLAN interfaces (RVIs), because the members of IRBs/RVIs are VLANs, not ports. However, you can apply rewrite rules to the VLAN port members of an IRB/RVI.

Note

OCX Series witches do not support IRBs/RVIs.