From: Andrew Lissitz \(alissitz\) (alissitz@cisco.com)
Date: Thu Aug 18 2005 - 13:09:59 GMT-3
Great write up Arun. I am not an expert @ MPLS... and after seeing
Arun's write up, discussed this issue with a colleague.
Basically he added:
With MPLS VPNs there is a second VPN tag that is not popped @ the last
hop, making this not an issue @ the last provider hop. PHP pops the
first label, but the VPN label is still there and preserves your QoS
markings.
The explicit null causes additional overhead since two lookups are
occurring and not just one (as PHP is designed to help with). You would
need to consider what hardware is being used for your PEs and decide if
they can handle multiple lookups for the same packet... this is a
serious concern if you are moving a lot of packets...
HTH
Andrew
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
Arun Arumuganainar
Sent: Thursday, August 18, 2005 3:47 AM
To: Zahid Hassan; C&S GroupStudy; R&S CCIE GroupStudy
Subject: Re: QOS for MPLS VPNs
Hi ,
> What I have not being able to understand is how my set MPLS EXP bits
> will
be
> carried to the other PEs ?
> Will I need to apply any service policy to carry MPLS EXP bits from PE
> to the other ?
>
By default . DSCP or IP Prec or Dot1Q COS will be automatically copied
to MPLS EXP !!! No configuration is needed for doing this .
However if you want to change behavior you will have create policy map
and apply it on the input or output interface using service policy
command .
There are two set command available for use in your policy map !!!
a) set mpls exp topmost : This will set exp for topmost label .
b) set mpls exp imposition : This will set exp bits in the entire label
stack .
Also it will be interesting to understand how various MPLS operations
affects your exp values . Basically there are four variants of MPLS
operations .
1) Label Imposition : This happens in IP-MPLS boundaries . Default :
DSCP /IP Prec is copied . The behavior can be changes using set commands
in a policy map
2) Label Swap : This happens in MPLS to MPLS path . Here incoming label
is swapped with outgoing label . Default : EXP bit of incoming label is
copied to outgoing label's EXP . This behavior can be changed using set
commands discussed above
3)POP operation in Label Stack : Incoming packet has stack of labels and
top most label will be poped( Removed ) . Exp bit on topmost label will
be copied to topmost label( next-level label ) on the outgoing interface
. This behavior can be changed using set commands discussed above .
4) PHP POP operation : Incoming packet has a single label and Label
will be removed as we forward the Packet . Here EXP will be fully lost .
By default it is not copied on to DSCP or IP-Prec back !!!
This is a special case where in end to end QoS policies implementation
based on exp is not possible . However you can work around this
situation via two methods.
a) Signaling explicit null between PHP LSR and Egress LSR . When
explicit nul is done then PHP-POP( case 4) operation will be changed to
Label Swap ( case 2) Hence Problem can be worked around ,
b) Use policy map to mark discard class( one discard class for each exp
values ) in the incoming interface of PHP router and use policies based
on Discard class on the output interface !!!
Based on your need you can employ any of the techniques for achieving
the QoS in MPLS VPN environment .
Hope this helps .
Thanks and Regards
Arun
----- Original Message -----
From: "Zahid Hassan" <zhassan@gmx.net>
To: "C&S GroupStudy" <comserv@groupstudy.com>; "R&S CCIE GroupStudy"
<ccielab@groupstudy.com>
Sent: Thursday, August 18, 2005 3:54 AM
Subject: QOS for MPLS VPNs
> Dear All,
>
>
> I would be really grateful if someone could suggest or propose any
> config for implementing QOS for MPLS VPNs.
>
> I have the following scenario:
>
> CE1 -->PE1-->PE2-->CE2
>
> I understand that it is possible to mark and match DSCP and MPLS EXP
> bits
as
> follows:
>
> DSCP (CE1)-->EXP(PE1)--->EXP(PE2)--->DSCP(CE2)
>
> I am being able to match DSCP bits from CE on PE and apply whatever
policing
> required.
> I can also set MPLS EXP bits on PE according to matched DSCP bits from
CE.
>
> What I have not being able to understand is how my set MPLS EXP bits
> will
be
> carried to the other PEs ?
> Will I need to apply any service policy to carry MPLS EXP bits from PE
> to the other ?
>
>
> Regards,
>
> Zahid
>
> _____________________________________________________________________
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