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New Member

RSVP and Voice over IP

Here's the test environment setup. Location A's PBX is connected to the FXO ports of 2600 series router. Location A is connected to Location B through 64 Kbps. Location B's PBX is also connected to a 2600 series router. Trying to understand how QoS works, I'm using g.729 r8 codec and have configured two dial-peers on either side and connected two analog phones. What parameters should I use with "ip rsvp bandwidth command"? The serial interface is enabled for fair-queuing. I tried "ip rsvp bandwidth 48 16" command. When I place a call, the routers display that a reservation event 1 occurs. But there is jitter during the call when I am pinging the router B's interface with large packets 1500. The jitter gets less when I stop the pings. When I change to "ip rsvp bandwidth 48 32", the quality gets better but there is no reservation event on the router. My question is why? Also there should be no jitter, the whole point of QoS is that if the resources are not available to satisfy the QoS profile parameters, then don't complete the call. I'm trying to get to the bottom of this. Anyhelp would be appreciated. Thanks in advance.

Cisco Employee

Re: RSVP and Voice over IP

What version of IOS are you using? Earlier versions allowed the call to go through even if the reservation failed. You need at least 12.1(5)T.

I assume your dial-peers have req-qos configured?

RSVP won't guarantee delay or jitter. You need to have LLQ as well--should be enabled by default, but that requires 12.1(5)T or later also.

The second number in the ip rsvp bandwidth command is the maximum bandwidth per reservation. G729 requires 24kpbs. So I would try "ip rsvp bandwidth 48 24"

I find this link useful:

New Member

Re: RSVP and Voice over IP

Thanks for your reply. I'm using 12.1.6. and yes I have the dial peers with req-qos configured. I went back and changed the ip rsvp bandwidth to 48 24. I have another question for you. Can I use priority queue instead of LLQ? I thought of creating a access list to allow udp gt 16384 and get the priority list to use that access list to assign the priority of high leaving all other traffic normal. I was searching for the LLQ commands. Seems like it was no enabled by default or if was it wasn't working. So, I went ahead and tried the PQ with the same scenario. Now my voice quality is definitely clear. Am I on the right track? Thanks for all the help.

Cisco Employee

Re: RSVP and Voice over IP

You're on the right track. The thing is, LLQ *should* automatically do priority queueing for voice and with RSVP and fair-queing turned on the serial interface, LLQ *should* automatically get turned on (although I don't know if the commands are visible in the running config). See if the command "ip rsvp pq-profile voice-like" is in your config. You might try adding it and see if that turns on LLQ.

If that doesn't work, you can try explicitly configuring LLQ (I think there's an example at that same url). It does exactly what you've described--priority queue for voice, wfq for data.

New Member

Re: RSVP and Voice over IP

Thanks for all the help. I added the LLQ config explicitly and it works. Thanks again