Priority on frames on a 7200

[signet]

We are running a few 7206VXR's where one of the interfaces is ATM (PA-A3) and the other is Gig Ethernet to our core. On the ATM we have several subinterfaces to customers. On the ethernet network, we use COS to mark frames with a p-bit and all is good. I'd like to prioritize frames coming in on the ethernet interface, going out on the ATM subinterface. So, if i get a frame with a p-bit value 5, it gets priority on the output side on the ATM subinterface.

All subinterfaces are configured like this:

interface ATM1/0.1641 point-to-point

description PPPoA customer #1

no ip redirects

no ip proxy-arp

pvc 16/41

  vbr-nrt 4096 4096

  encapsulation aal5mux ppp Virtual-Template1

!

So practically, i don't want to reserve bandwidth, but just prioritize the frames over regular traffic.

[paolo bevilacqua]

You can use the VC level command "priority" for the purpose. It is normally auto assigned based on the class of service, but you can override it.

This is a document that explains it, I wrote it a long time ago while working at Cisco.

http://www.cisco.com/en/US/tech/tk39/tk51/technologies_tech_note09186a00800c69bb.shtml

[signet]

So what you're saying is that this is actually already working without any configuration? So if a frame enters the ethernet port on a 7200, it automatically "rewrites" to a priority per-VC?

[paolo bevilacqua]

So what you're saying is that this is actually already working without any configuration? So if a frame enters the ethernet port on a 7200, it automatically "rewrites" to a priority per-VC?

No. Default VC transmit priority is based exclusively on the ATM Class of Service configured.

To override it, you can use the technique described in the document above, or others that also exists.

[signet]

Thanks for your answer.

Is there any way to automate this behaviour with a class-map and a service-policy on the VC?

So that if a frame enters the ethernet interface with p-bit 5, it gets prio 5 on the ATM subinterface?

[paolo bevilacqua · Accepted solution]

You can use

http://www.cisco.com/en/US/docs/ios/qos/configuration/guide/ip_to_atm_cos.html

I remember there was something like match layer-2 prececende, but I don'ìt have the details.