Rewriting MPLS and IPv4 Packet Headers

You can apply a rewrite rule to MPLS and IPv4 packet headers simultaneously. This allows you to initialize MPLS EXP and IP precedence bits at LSP ingress. You can configure different rewrite rules depending on whether the traffic is VPN or non-VPN.

Devices running Junos OS Evolved do not support different rewrite rules for VPN and non-VPN traffic.

The default MPLS EXP rewrite rules are shown in Table 1.

Table 1: Default MPLS EXP Rewrite Rules

Forwarding Class	Loss Priority	MPLS EXP Rewrite Value
best-effort	low	000
best-effort	high	001
expedited-forwarding	low	010
expedited-forwarding	high	011
assured-forwarding	low	100
assured-forwarding	high	101
network-control	low	110
network-control	high	111

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 applies to rewrite rules you configure for MPLS packets with IPv4 payloads.

To override the default MPLS EXP rewrite table and rewrite MPLS and IPv4 packet headers simultaneously, include the protocol statement at the [edit class-of-service interfaces interface-name unit logical-unit-number rewrite-rules exp rewrite-rule-name] hierarchy level:

The protocol statement defines the types of MPLS packets and packet headers to which the specified rewrite rule is applied. The MPLS packet can be a standard MPLS packet or an MPLS packet with an IPv4 payload. Specify the type of MPLS packet using the following options:

• mpls-any—Applies the rewrite rule to MPLS packets and writes the CoS value to MPLS headers.

• mpls-inet-both—Applies the rewrite rule to VPN MPLS packets with IPv4 payloads. On Juniper Networks M120 Multiservice Edge Routers, M320 Multiservice Edge Routers, and T Series Core Routers (except T4000 routers), writes the CoS value to the MPLS and IPv4 headers. On other M Series Multiservice Edge Router routers, causes all ingress MPLS LSP packets with IPv4 payloads to be initialized with 000 code points for the MPLS EXP value, and the configured rewrite code point for IP precedence.

• mpls-inet-both-non-vpn—Applies the rewrite rule to non-VPN MPLS packets with IPv4 payloads. On Juniper Networks M120 Multiservice Edge Routers, M320 Multiservice Edge Routers, and T Series Core Routers, writes the CoS value to the MPLS and IPv4 headers. On other M Series Multiservice Edge Routers, causes all ingress MPLS LSP packets with IPv4 payloads to be initialized with 000 code points for the MPLS EXP value, and the configured rewrite code point for IP precedence.

Junos OS Evolved supports only the mpls-any option.

On M120 routers, M320 routers with Enhanced-III FPCs, and MX Series routers, you can perform simultaneous DSCP and EXP rewrite by attaching independent DSCP or IPv4 precedence rewrite rules and EXP rewrite rules to the same core interface. Thus, you can rewrite both code points (DSCP and EXP) when the packet is received by the ingress provider edge (PE) router on the MPLS core.

An alternative to overwriting the default with a rewrite-rules mapping is to configure the default packet header rewrite mappings, as discussed in Applying Default Rewrite Rules.

By default, IP precedence rewrite rules alter the first three bits on the 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.