[j-nsp] re-write rules - query

[Shiva Shankar]

Dmitry, Thanks for the clarification. In a specific example, on an interface
can I have both exp and dscp rewrite rules, so that

(i)  I can re-write MPLS packets using exp rewrite and
(ii) for ip packets (not using mpls) to re-write dscp using dscp re-write
rules.

I have seen errors in log messages when I use dscp re-write rule in the
above scenario, saying 'invalid rewrite table type 1'. This was in J series
using Junos 8.4.

Cheers
Shiva
On Thu, Jul 30, 2009 at 6:25 PM, Dmitry Perelman <dmitrynyc at gmail.com>wrote:

> 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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>
>