How Cisco verify a RTP packet?

[patrick.zhang]

Hi,I have a question about RTP when optimize our VoIP network.That is how Cisco router verify a packet as a RTP packet?When capture a voice packet,how the router analyze the packet,and how router know it's a RTP packet?

Thanks!

[anup.anand]

Routers consider UDP packets on ports 16384 to 32764 as RTP packets.

Regards,

Anup

[mheusinger]

Hello,

I would slightly disagree with the previous post.

In case you classify RTP through NBAR, then the router checks whther the packet is really RTP by looking at UDP ports and the RTP content like payload type.

An example config would be:

class-map VoIPbearer

match protocol rtp audio

If you use only an access-list specifying UDP ports for classification then RTP will not be investigated.

So finally this depends on configuration.

Hope this helps! Please rate all posts.

Regards, Martin

[anup.anand]

Martin is correct. I should have been more clear.

Routers don't classify RTP until you enable some kind of QoS.

Apart from the methods Martin mentioned, you can also do this using the interface command

ip rtp priority

Low Latency Queuing give you a lot of options to classify RTP.

Check this link for more details on LLQ

http://www.cisco.com/en/US/tech/tk652/tk698/technologies_tech_note09186a0080094660.shtml#stpri

Regards,

Anup

[patrick.zhang]

Thanks you guys very much!