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New Member

QoS end-to-end ( cos / dscp ) and traffic generated by equipements

I’m currently working on configuring QoS over a gigaethernet backbone provided by some Cisco 3750 series switchs and many 2950 series switchs.

The adopted solution uses 802.1p (also called cos (Class of Service)) layer2 mechanism for LAN traffic and DSCP layer3 mechanism for MAN traffic.

It works almost perfectly except few troubles with management traffic. Indeed traffic generated by equipments and outgoing from them is not tagged as expected.

I did some tests with 2 Cisco 3750:

Cisco 3750 num1 <-- gi1/0/1 -------------gi1/0/1--> Cisco 3750 num2

When I sniff traffic coming from 3750 num1 to 3750 num2 flows are tagged like that:


CDP cos7

DTP cos7

LOOP cos7

STP cos7


RIPv2 cos7 / dscp48

NTP cos6 / dscp48

ICMP request cos0 / dscp0

ICMP reply cos / dscp are the same as ICMP request

SNMP cos2 / dscp16

TELNET cos2 / dscp16

I’d like to tag RIPv2 with cos6 / dscp48 and NTP with cos2 / dscp16 but I don’t know how to change them.

I incurred similar problem with SNMP and TELNET but I solved it using “ip telnet tos 40” and “snmp-server ip dscp 16”.

People advise me to use ACLs on the 3750 num2 but I can’t use it (cpu utilization is already too high :-P).

If someone has an idea … ;-)



Re: QoS end-to-end ( cos / dscp ) and traffic generated by equip

Hi Yann -

Typically , user has no control over changing the cos / dscp for control protocols. They use what is called as a protocol bypass mechanism where they ignore user configured policies.


Salman Z.

New Member

Re: QoS end-to-end ( cos / dscp ) and traffic generated by equip

Hi Yann/Salman ,

It is possible and I am doing it. I have got my config verfied by TAC before I deployed it in production.

This is how I have done it for telnet and BGP on 3560/3750 switches and it works fine.

I have used Prec+ToS for setting DSCP value.

ip local policy route-map set-dscp-local

ip access-list extended LM

remark **Locally originating traffic tcp 23 and 179 from the device,used in route-map set-dscp-local**

permit tcp any any eq telnet

permit tcp any eq telnet any

permit tcp any any eq bgp

permit tcp any eq bgp any

route-map set-dscp-local permit 10

description **used for traffic originating locally, in "ip local policy command"**

match ip address LM

set ip precedence flash

set ip tos 6



New Member

Re: QoS end-to-end ( cos / dscp ) and traffic generated by equip

Thank you very much for your help ! :D



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