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Configuring Rewrite Rules

You define markers in the rewrite rules section of the CoS configuration hierarchy and reference them in the logical interface configuration. This model supports marking on the DSCP, DSCP IPv6, IP precedence, IPv6 precedence, IEEE 802.1, and MPLS EXP CoS values.

To configure a rewrite-rules mapping and associate it with the appropriate forwarding class and code-point alias or bit set, include the rewrite-rules statement at the [edit class-of-service] hierarchy level:

The rewrite rule sets the code-point aliases and bit patterns for a specific forwarding class and packet loss priority (PLP). The inputs for the map are the forwarding class and the PLP. The output of the map is the code-point alias or bit pattern. For more information about how CoS maps work, see Mapping CoS Component Inputs to Outputs.

By default, IP precedence rewrite rules alter the first three bits on the type-of-service (ToS) byte while leaving the last three bits unchanged. This default behavior is not configurable. The default behavior applies to rules you configure by including the inet-precedence statement at the [edit class-of-service rewrite-rules] hierarchy level. The default behavior also applies to rewrite rules you configure for MPLS packets with IPv4 payloads. You configure these types of rewrite rules by including the mpls-inet-both or mpls-inet-both-non-vpn option at the [edit class-of-service interfaces interface-name unit logical-unit-number rewrite-rules exp rewrite-rule-name protocol] hierarchy level.

Note:

The IRB classifiers and rewrite rules are used only for routed packets; in other words, it is for traffic that originated in the Layer 2 domain and is then routed through IRB into the Layer 3 domain, or vice versa. Only IEEE classifiers and IEEE rewrite rules are allowed for pure Layer 2 interfaces within a bridge domain.
Note:

The forwarding class and loss priority are determined by ingress classification.

Platform-Specific Rewrite Rules Behavior

Use the following table to review platform-specific behaviors for your platforms.

Table 1: Platform-Specific Rewrite Rules Behavior

Platform

Difference

EX Series

  • On EX Series switches, if you configure vlan-vpls encapsulation and add an IEEE 802.1 header on a Gigabit Ethernet or 10 Gigabit Ethernet interface to output traffic, but do not apply an IEEE 802.1 rewrite rule, then the default IEEE 802.1 rewrite rule is ignored and the IEEE 802.1p bits are set to match the forwarding class queue.

MX Series

  • MX Series routers with MPCs support rewrite rules that rewrite the first three bits of the IPv6 DSCP value through the inet6-precedence statement at the [edit class-of-service rewrite-rules] hierarchy level. This allows you to set a 3-bit code point for a particular forwarding class and loss priority for IPv6 traffic. This rewrite rule option can also be applied to packets entering an MPLS LSP by including the protocol mpls statement when applying the rewrite rule to a logical interface.

  • Integrated Bridging and Routing (IRB) interfaces are used to tie together Layer 2 switched and Layer 3 routed domains on MX routers. MX routers support classifiers and rewrite rules on the IRB interface at the [edit class-of-service interfaces irb unit logical-unit-number] level of the hierarchy. All types of classifiers and rewrite rules are allowed, including IEEE 802.1p.

  • On MX960 routers, if you configure vlan-vpls encapsulation and add an IEEE 802.1 header on a Gigabit Ethernet or 10 Gigabit Ethernet interface to output traffic, but do not apply an IEEE 802.1 rewrite rule, then the default IEEE 802.1 rewrite rule is ignored and the IEEE 802.1p bits are set to match the forwarding class queue.

PTX Series

  • The inet-precedence statement is supported on PTX Series routers only when network services is set to enhanced-mode. For more information, see enhanced-mode (Network Services).

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