how to prioritize dscp packets entering my network

[kaonthana]

I have a pair of c7200 routers in my network with no QoS policy setup. They are connected with a OC3 line (POS interface).

I have a server outside my network that sends traffic into my network. These packets come in with an IP header value/Diffserv = 51

How can I get my routers to prioritize the packets tagged with diffserv 51 to make sure I don't drop these packets? 

[Carlos Villagran · Accepted solution]

Hi!

Well the use of QoS will be needed, I would suggest the use of LLQ to match DSCP 51 so packets traversing the router with this mark get always served first. 

Please refer to the following link for detailed instructions in LLQ configuration:

http://www.cisco.com/c/en/us/td/docs/ios/12_0s/feature/guide/fsllq26.html

As a brief config you should be doing something like this:

class-map PREFER 

match ip dscp 51

end

policy-map MYMAP

class PREFER

priority [Bandwidth you are willing to reserve for this traffic]

Be aware though, this BW  assigned to the LLQ if exceeded the packets will be dropped.

Also if you have problems with the configuration or any other questions please let me know.

Hope it helps, best regards!

JC

[kaonthana]

I appreciate your help! This seems to be working for me thanks a lot!

One more question:

Assuming the packets are not already tagged with the DSCP value.

Is it possible to apply a similar policy to the ingress interface to tag the packets (based on source IP address) and keep the same egress policy to prioritize the packets?

[Carlos Villagran]

Hi!

Sure, you will have to apply an ingress policy such like this:

class-map MARK

match access-group {Here you have to match a standard ACL to match your source IP address} 

end

Policy-map INGRESS_MARK

class MARK

set dscp 51

end

interface [Ingress interface]  <<< 

service-policy in INGRESS_MARK

service-policy out MYMAP

Configuration Above will mark specified source IP address traffic with DSCP 51.

Yes, you can apply a policy for out and in direction at the same time, although this is platform independent sometimes, you should have no problems applying it in your router.

Hope it helps, best regards!

JC