[nsp] Detecting TOS/DSCP changes [was: Re: [nsp] TOS/DSCP marking on 6509 SUP]

From: Simon Leinen (simon@limmat.switch.ch)
Date: Wed May 08 2002 - 13:18:26 EDT


>>>>> On Mon, 22 Apr 2002 18:08:47 -0400, "Hassan, Shehzad" <shehzad.hassan@bell.ca> said:
> I agree with you that the box shud retain its TOS/DSCP value but I
> dont think Cisco 6509 retains the TOS value with QoS turned Off, I
> chatted with a couple of freinds and thye have experienced same, but
> need some documented proof.

A couple of weeks ago I have modified traceroute such that it prints
the TOS byte of the probe packets when it is changed along the path, e.g.

    $ traceroute -t 64 www.surfnet.nl
    traceroute to info2.surfnet.nl (192.87.5.150), 30 hops max, 40 byte packets
     1 swiCS4-G3-13.switch.ch (195.176.255.53) 0 ms 0 ms 0 ms
     2 swiCS3-G3-1.switch.ch (130.59.36.13) 0 ms 0 ms 0 ms
     3 swiEZ2-G3-3.switch.ch (130.59.36.18) 0 ms 0 ms 0 ms
     4 swiCE2-G2-2.switch.ch (130.59.36.41) 5 ms 5 ms 5 ms
     5 switch.ch1.ch.geant.net (62.40.103.17) 5 ms 5 ms 5 ms
     6 ch.fr1.fr.geant.net (62.40.96.30) 14 ms (TOS=0!) 14 ms 14 ms
     7 fr.be1.be.geant.net (62.40.96.17) 25 ms 25 ms 25 ms
     8 [...]

The (TOS=0!) indicates that the TOS byte has been rewritten from 64
(DSCP=16 or PREC=2) to 0 by the router switch.ch1.ch.geant.net - this
shows up at the next hop because the rewriting happens on egress.

My modification is included in the latest "NANOG traceroute" (6.1.4):

    ftp://ftp.login.com/pub/software/traceroute/beta/traceroute.c

Hope this helps,

-- 
Simon Leinen				       simon@babar.switch.ch
SWITCH				   http://www.switch.ch/misc/leinen/

Computers hate being anthropomorphized.



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