[j-nsp] re-write rules - query

[Dmitry Perelman]

1. You can rewrite DSCP (all 6 bits) for IPv4 packets only. The restriction
that is referenced (3-bit TOS) in your example exists when the rewrite is
done on the IPv4 payload ingress to an LSP.

2. The link below covers how to modify the EXP and TOS bits at LSP ingress.
To sum it up: You are able to rewrite the EXP bits along with the TOS bits
only at LSP ingress (via exp rewrite rule) which makes sense because of IPv4
payload visibility. Since you have no visibility into the IPv4 payload on
transit nodes you only have the option to rewrite the EXP bits. You do then
have the ability to rewrite the full DSCP field on the egress PE interface.

http://www.juniper.net/techpubs/en_US/junos9.5/information-products/topic-collections/config-guide-cos/cos-rewriting-mpls-and-ipv4-packet-headers.html#id-11667692



On Thu, Jul 30, 2009 at 11:13 AM, Shiva Shankar <shankarks at gmail.com> wrote:

>  Gents, Try to understand in depth re-write rules in M/J series...From
> JUNOS
> 9.5 notes I found the following:
>
> *For every incoming packet, the ingress classifier decodes the ingress CoS
> bits into a forwarding class and packet loss priority (PLP) combination.
> The
> egress CoS information depends on which type of rewrite marker is active,
> as
> follows: *
>
>   - *For Multiprotocol Label Switching (MPLS) EXP and IEEE 802.1 rewrite
>   markers, values are derived from the forwarding class and PLP values in
>   rewrite rules. MPLS EXP and IEEE 802.1 markers are not preserved because
>   they are part of the Layer 2 encapsulation. *
>   - *For IP precedence and DiffServ code point (DSCP) rewrite markers, the
>   marker alters the first three bits on the type-of-service (ToS) byte
> while
>   leaving the last three bits unchanged. *
>
> *Questions:
> *
> *1. Does it mean that on Juniper M series routers, I can never chnage the
> last 3 bits of the TOS byte...meaning never use the full potential of the
> DSCP (6 bits)?
> *
> *2. So, can't I use dscp and exp re-write rules for a MPLS packet entering
> a
> LSP on a specific interface..? It means that again I can't use DSCP in MPLS
> evnvironment?
>
> *Cheers
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