The following steps should help to configure QoS for prioritising voice traffic :
Define an access group (105) which matches any VOIP traffic :
access-list 105 remark VOIP (SIP/IAX/IAX2) traffic gets top priority (5)
access-list 105 permit udp any any eq 4569
access-list 105 permit udp any any eq 5004
access-list 105 permit udp any any eq 5036
access-list 105 permit udp any any eq 5060
access-list 105 permit ip host OTHER.VOIP.HOST.HERE any
access-list 105 permit ip any host OTHER.VOIP.HOST.HERE
The access-group is matching UDP 4569 (IAX2), 5004 (RTP), 5036 (IAX1) and 5060 (SIP), also allowing
any traffic from OTHER.VOIP.HOST.HERE to be included.
Now, to prevent the upstream from plugging up the link, set up two input rate-limit rules to police
the incoming traffic. The first will allow 128k of VOIP traffic in no matter what, set its precedence
to high priority (5) and transmit it. The other part of the first rule will allow any excess VOIP
traffic (above 128k), but its precedence set to best-effort (0) and the remaining police rule will be
evaluated as well. The second rate-limit command will allow no more than 1408kbps through; any excess
will be dropped.
Again, this only works for TCP traffic, since dropped packets will cause the sender to back off and
try again slower. If your link is full of other protocols without this particular feature, this won't
do anything to help:
in s0/0
rate-limit input access-group 105 128000 65536 65536 conform-action set-prec-transmit 5
exceed-action set-prec-continue 0
rate-limit input 1408000 8000 8000 conform-action transmit exceed-action drop
At any rate, that takes care of the incoming traffic.
Now to handle outgoing traffic :
You can use service polices and LLQ (Low Latency Queueing) this queueing discipline was _designed_
for VOIP.
Setting it up is a snap:
class-map voice
match access-group 105
!
policy-map policy1
class voice
priority 96
class class-default
fair-queue
!
The first pair of commands sets up a class called 'voice', which matches any traffic which matches
access group 105 (the VOIP ACL I have defined above). The second group actually sets up the police
map; it guarantees 96kbps of traffic for the voice class, and everything else is queued using the
fair-queue discipline. Now you have to do is attach this to an interface :
in s0/0
service-policy output policy1
in e0/0
service-policy output policy1
