[cisco-bba] How to reset DSCP/TOS on LNS?

Andy Saykao andy.saykao at staff.netspace.net.au
Mon Sep 6 01:36:34 EDT 2010

Hi Tony,

Took me ages to work it out, but the TOS I'm talking about or what we used in our Billing System is defined in RFC 1349 as being bits 3-6. It's a fairly old implementation and has been deprecated and replaced by RFC 2747


                0     1     2     3     4     5     6     7
             |                 |                       |     |
             |   PRECEDENCE    |          TOS          | MBZ |
             |                 |                       |     |

Thanks for looking into it.



-----Original Message-----
From: Tony [mailto:td_miles at yahoo.com] 
Sent: Monday, 6 September 2010 3:27 PM
To: cisco-bba at puck.nether.net; Andy Saykao
Subject: Re: [cisco-bba] How to reset DSCP/TOS on LNS?

Hi Andy,

>From my reading of it that is correct. Bits 0-5 are used for TOS and bits 6-7 are used for Explicit Congestion Notification (ECN) or are UNUSED.

Some reading material:

The router is correctly setting the TOS bits to all zero as the TOS bits are only the first 6 and don't include the last two in this 8-bit header field. If your billing system is looking at all 8 bits as a TOS value then it could be suggested that it might not be an entirely correct implementation (or very old ?).

I've spent a few minutes googling and I can't find any way to explicitly changes the two ECN bits in IOS QoS. Doesn't mean it's not possible, just that nothing jumped out at me.


--- On Mon, 6/9/10, Andy Saykao <andy.saykao at staff.netspace.net.au> wrote:

> Sorry to re-open this again but is
> there a way to reset the entire TOS
> byte. Our Billing System uses the TOS value (bits 3-6) of the TOS byte 
> for billing purposes to classify different traffic types (yes I know 
> TOS value is outdated and we should move to DSCP but that's another 
> story).
> If we reset the DSCP value (bits 0-5) or IP Precedence field, this 
> doesn't really solve our problem of resetting the TOS value to zero.
> Take for example where a customer sets TOS Byte to 3.
> TOS Byte = 0000 0011
> The policy-map applied on the virtual-template will reset the DSCP 
> fields to 0.
> policy-map QOS-POLICY-RESET-0
>  class CLASS-IP-ANY
>   set ip dscp 0
> The TOS value will now be xxx0 001x because the policy-map only resets 
> bits 0-5 (a 1 remains in bit 6). We are left with a TOS value of 1.
> We can probably get the Billing System to ignore any TOS value of 1 it 
> sees but I was just wondering if there was a way to reset the entire 
> TOS byte or TOS value to zero on the LNS???
> Thanks.
> Andy


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