I have a 128Kbps Leased line link between two of our branch offices. The link is used for both voice and data. I would like to configure QoS on the cisco routers 1841. Pls help. The backup link between the two branches is an ISDN BRI.
The other posters have provided CBWFQ policies that might work for you. Some other considerations and thoughts include:
128 Kbps is tight for sharing data and voice. Assuming your voice is VoIP, bandwidth demand varies based on the codec you'll be using. Even when the codec uses little bandwidth, you'll still need to account for IP header bandwidth expansion. For G711 assume about 92 Kbps, for G729a assume about 33 Kbps (per call).
What's important is insuring VoIP goes first out your serial interface. With CBWFQ, a class defined with "priority" assures this. You need to insure that voice bearer packets, and only those packets, are placed into your priority class. You might find your VoIP devices already pre-mark their voice packets, often with a DSCP EF, if they do you can match against it.
Assuming pre-marked VoIP packets, another variation of CBWFQ, for two G729a flows, might be:
e.g. (syntax might be incorrect)
match ip dscp EF
service-policy output CBWFQ
As a rule-of-thumb, Cisco recommends you don't allocate more than 1/3 of the link for priority traffic. My example violates that with two G729a calls (and so would one G711).
If you're only going to have just one special class, I recommend fair-queue for other traffic. This is especially important since we haven't also treated the call's control traffic as special.
When a policy is in action, check that your VoIP packets are being identified by the priority class and that they aren't being dropped (a priority class can also cap the bandwidth at the value you specific).
Other issues you might run into, again since your overall bandwidth is so small, Cisco recommends using link fragmentation interleaving (LFI) when the bandwidth is less than 768 Kbps. What might also help is using cRTP. I haven't had to use either of these features, but perhaps someone with experience with them might post.
Lastly, I doubt you'll need to configure it, but sometimes reducing the interface's hardware queue with tx-ring-limit is necessary too.
What you might also try is the AutoQoS command, if supported. I recall one of its variants was dedicated just for VoIP support, but with it too, I don't have practical hands-on experience.
You might also consider posting either to the QoS topic and/or Voice forums.
Hi everyone, I would like to thank you in advance for any help you can provide a newcomer like myself!
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