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On our office network (26 people), some users have complained of poor Skype call quality, particularly in the upstream direction. I wanted to ask, how do I identify Skype traffic, considering that it uses a random port, in order that I might prioritise it at the router level?

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2 Answers 2

I think you are right; Skype uses random non-common outbound ports (greater than 1024) for each session which makes QoS tagging problematic

But you can use QoS for SIP and provide voice data priority as long as you have SIP Quality of Service-based hardware.

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How does the SIP QoS hardware prioritise Skype? I can buy whatever hardware is needed! –  chrism2671 Oct 29 '11 at 21:31
@chrism2671, you can find Skype's description of QoS here link. Also, SIP protocol tagging is strait forward. Here link is a good article on how to implement the service on Cisco products. –  Fergus Oct 30 '11 at 3:08
Neither article give any indication of how Skype might be identified as SIP traffic. It looks like Skype's documentation applies to using Skype Connect, where the internal traffic is all SIP (not Skype) and can this have QoS applied easily. In these cases, Skype is only used when placing a call to an external system. E.g. SIP Client -> SIP Server -> Skype Connect -> Internet - so the QoS is being applied internally only on SIP traffic. –  dunxd Jun 25 '12 at 14:14

You can match skype traffic in a class-map using the configuration below on Cisco routers. To do this, the router uses a feature called NBAR (it looks at layer 4 and higher information in the packet to determine the application.)

class-map priority
  match protocol skype

Once matched, you can then give that class higher priority like this:

policy-map outbound
 class priority
  priority 2000 ! Gives a dedicated 2Mbits/sec
interface Gigabit0/1
 description Outside interface
 service-policy output outbound

I'm sure other vendors offer similar functionality, but I can't really say for sure.

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