Marking DSCP traffic generated internally in a 3750.

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Jul 24th, 2008
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I'd like to mark DSCP in hardware at the remote sites 3750 (12.2(44)SE1) switches instead of at the remote WAN router. It's working fine except for traffic that is generated in the 3750, like SNMP reply's and IP SLA responder traffic - traffic that is actually generated in the 3750. The 3750 will only let me apply a service policy to the inbound direction of a physical interface in a 3750. Any ideas?

Correct Answer by andrew.butterworth about 8 years 11 months ago

This is the IP SLA configuration I have on a Catalyst 3550 to test VoIP:


ip sla 10

udp-jitter 192.168.255.250 32768 source-ip 192.168.255.254 source-port 32768 codec g711alaw

tos 184

!

The 'tos' setting sets the whole ToS byte, so in this case it sets DSCP EF (101110 + 00).


HTH


Andy

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andrew.butterworth Sat, 07/26/2008 - 05:00
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With the 3750 you can configure DSCP for certain management traffic generated by the switch itself. By default some traffic is already marked with CS6. I looked into this a while ago:


ip telnet tos <0-FF>

ip ssh dscp <0-63>

snmp-server ip dscp <0-63> or

snmp-server ip precedence <0-7>


For IP SLA you can set the ToS on the 'server' and the responder will use the same ToS value.


You can also use a local policy to set IP Precedence for traffic generated by the 3750:


ip access-list extended NTP

permit udp any any eq ntp

!

route-map Set-DSCP-NTP

match ip address NTP

set ip precedence <0-7>

!

ip local policy route-map Set-DSCP-NTP


I have never tested the local policy stuff on a Catalyst 3750 however I have on a 'real' router and it worked on that. The other stuff works fine though.


HTH


Andy

r.solem Sat, 07/26/2008 - 06:28
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That is a very helpful reply. Thank you.

Can you tell me how to set IP SLA ToS on the 'server'? I don't see that option in the CLI help.





Correct Answer
andrew.butterworth Sat, 07/26/2008 - 09:19
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This is the IP SLA configuration I have on a Catalyst 3550 to test VoIP:


ip sla 10

udp-jitter 192.168.255.250 32768 source-ip 192.168.255.254 source-port 32768 codec g711alaw

tos 184

!

The 'tos' setting sets the whole ToS byte, so in this case it sets DSCP EF (101110 + 00).


HTH


Andy

andrew.butterworth Tue, 07/29/2008 - 03:09
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Good, I am glad you found it useful. You can always rate useful posts you know..... :o)


Andy

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