Routing
Routing

Does MPLS preserve DSCP of internal packet ?

‎12-19-2018 03:47 AM

I am bit confused how the DSCP value of original ip packet changes during MPLS transit.

 

I understant that reqrite rules on ingress router can - and will - control what value the EXP bit assumes.

But what happens when MPLS packet is decapsulated on egress routers ? Will the originally encapsulated packet retain the original DSCP value if I did not specify any exp rewrite rules ? How can I preserve it and what is the reason by the way to change the original DSCP value ?

1 REPLY 1
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Routing

Re: Does MPLS preserve DSCP of internal packet ?

‎12-19-2018 03:56 AM

Hello,

DSCP is preserved by default.

if You wih to remark DSCP simultaneously with EXP, then:

- on MX, use a separate DSCP rule and EXP rule attached to the core-facing interface

- on other mdels, use one of the knobs described here

https://www.juniper.net/documentation/en_US/junos/topics/usage-guidelines/cos-rewriting-mpls-and-ipv...

  • 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.

   

HTH

Thx

Alex

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