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New Member

TOS And DSCP Values

Dear support .

I have a VOIP external Device using Diffserv Feature. I want to match the voice packets out from this device using access-list and class-map features to give them priority..

When enabling the Diffserv feature on the VOIP gateway,The company set The TOS byte to (0100 0000 ) in binary.

what should i write in the access-list and class-map and match commands??

access-list 101 permit ip any any TOS xxx

match ip dscp xx

Best regards,


New Member

Re: TOS And DSCP Values


i hope to understood you right... 0100 0000 is =64 which would be the TOS value in the ACL...

the DSCP contains the 6 leftmost TOS-Bits - 0100 00 is =16 which would be the value in the class-map...



New Member

Re: TOS And DSCP Values

hi , when i write in the ACL

access-list permit ip any any TOS 64 it gives me false command!!

I want ur advice in this, when typing :

class-map voip-traffic

match access-list 102 ( where 102 is the udp ports used by the VOIP gateway )

match ip dscp 16 ( where the VOIP supports the Diffserv with a TOS byte 0100 0000)

and after that i'll give priority to that class " voip-traffic" , and here comes my question :

Will the router give the priority bandwidth to all the matches in that class or it'll give for one of them only ??

Please give me your advice as soos as possible..

New Member

Re: TOS And DSCP Values

Hello again,

what do you mean with "priority bandwidth for one of them only"?

The Policy-Map/Class-Map enforces an aggregate-behaviour - your configuration may be:

policy-map XXXX

class voip

priority 128

All traffic matching the class "voip" will get real-time forwarding (by strict-priority scheduling) and will be limited to 128K!!! The bandwidth-value controlles the operation of the traffic-policing system in the priority-queue.

Best Regards,