From: Anthony Sequeira (terry.francona@gmail.com)
Date: Thu Dec 01 2005 - 19:19:29 GMT-3
The switch does not care if there is a DSCP value already set - it is going
to use the IP Precedence value of the incoming packet and remark it based on
what YOU have instructed in your IP Prec-DSCP map.
I agree - the lack of granularity is pretty painful on the 3550. It is also
painful how dramatically different the QoS defaults and capabilities are as
you move from switch to switch when working with Cisco equipment. Be sure to
always check the documentation for your particular switch and IOS since
there are some pretty big differences from model to model and IOS to IOS.
On 12/1/05, John Caronilli <jcaronilli@yahoo.com> wrote:
>
> Thanks Anthony,
>
> How does the router know the arriving packet is
> configured with a precedence bit and not already
> configured with a DSCP value?
>
> Also, isn't it a little sloppy to force every packet
> through the switch to convert it's precedence to DSCP?
> What if you had a port that you did not want to
> convert the precedence bit?
>
> It just seems the entire operation is not granular
> enough?
>
> John
>
> --- Anthony Sequeira <terry.francona@gmail.com> wrote:
>
> > Hi John!
> >
> > You cannot enter the command below on a 3550 without
> > getting an error
> > message! You need to actually provide the DSCP
> > values that you want to
> > assign based on the IP Precedence values of 0
> > through 7. Here is an example
> > of the full command syntax:
> > *mls qos map ip-prec-dscp 10 15 20 25 30 35 40 45
> > *
> > Now - as far as how you apply it to an interface -
> > the 3550 documentation
> > states:
> >
> > "All the maps, except the DSCP-to-DSCP-mutation map,
> > are globally defined
> > and are applied to all ports.You can have multiple
> > DSCP-to-DSCP-mutation
> > maps and apply them to different Gigabit-capable
> > Ethernet ports. However, on
> > 10/100 Ethernet ports, you can attach only one
> > DSCP-to-DSCP-mutation map to
> > a group of twelve ports. "
> >
> > So, based on this, realize that you ip-prec-dscp map
> > is automatically
> > applied to all interfaces after you create it.
> >
> > Also - do not forget the command mls qos before you
> > do anything with QoS
> > manipulation on the 3550.
> >
> > Here is the documentation link I consulted and
> > quoted:
> >
> >
>
>
http://www.cisco.com/univercd/cc/td/doc/product/lan/c3550/12225sec/3550scg/sw
qos.htm#wp1150175
> >
> >
> > On 12/1/05, John Caronilli <jcaronilli@yahoo.com>
> > wrote:
> > >
> > > On the 3550, I configured the command:
> > >
> > > mls qos map ip-prec-dscp
> > >
> > > How is that command applied to an interface? How
> > do
> > > you tell the switch to convert all packets
> > arriving on
> > > say Fast 0/3 from the IP precedence value to DSCP
> > > values?
> > >
> > > How does the switch know the arriving packet is
> > not
> > > already using DSCP?
> > >
> > > Thanks!
> > >
> > > John
> > >
> > >
> > >
> > >
> > >
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>
>
>
>
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