From: John Caronilli (jcaronilli@yahoo.com)
Date: Thu Dec 01 2005 - 19:51:47 GMT-3
But doesn't the DSCP bits occupy the same field as the
IP Precedence bits? The precedence bits are bits 765
of the ToS field and the DSCP bits are 765432. So how
does the switch know if the ToS field of 00100000 is
an IP Precedence of 1 of a DSCP code of 8.
Also, with IP Precedence, the higher value is better
then the lower value. So a packet with Precedence of
5 is more preferred then a packet with a Precedence of
3.
With DSCP, a packet classified as AF11 (DSCP 10) will
be dropped less often then a packet classified as AF13
(DSCP 14). So it seems as if lower is better for
DSCP.
But the Cisco default mapping from Precedence to DSCP
do not make any sense at all.
Ip Precedence-dscp map:
ipprec: 0 1 2 3 4 5 6 7
--------------------------------
dscp: 0 8 16 24 32 40 48 56
Notice that the DSCP values increase with the IP
Precedence value. If bigger is worse, shouldn't the
DSCP values decrease with increasing IP Precedence?
I hate to be so dense about this.
Thank you for all of your help!!!!!
John
--- Anthony Sequeira <terry.francona@gmail.com> wrote:
> 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/swqos.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
> > > >
> > > >
> > > >
> > > >
> > > >
> > > > __________________________________
> > > > Start your day with Yahoo! - Make it your home
> > > page!
> > > > http://www.yahoo.com/r/hs
> > > >
> > > >
> > >
> >
>
This archive was generated by hypermail 2.1.4 : Mon Jan 09 2006 - 07:07:50 GMT-3