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Page 31 | AlliedWare™ OS How To Note: VPNs for Corporate Networks
How to prioritise outgoing VoIP traffic from the
headquarters router
Add the following steps after step 9 on page 14.
First, classify the VoIP traffic. In many deployments of VoIP, the originating VoIP appliance
marks VoIP packets with a DSCP value. In this example, it marks both VoIP traffic and VoIP
signalling packets with DSCP 48.
create classifier=48 ipds=48
VoIP data packets are small. They can be significantly delayed by big packets on the WAN
port, especially on slow links. Therefore, you may find it helpful to reduce the MTU for all
packets on the WAN port, for example, to 256 bytes.
set int=eth0 mtu=256
You also need to make sure that all large packets are fragmented, even if they were
previously set to not be fragmented.
set int=eth0 frag=yes
Enable software QoS (SQoS) and create an SQoS traffic class. This traffic class tags the
classified traffic as high priority on the interface queue. Also define a small queue size, which
is optimal for VoIP traffic.
ena sqos
cre sqos tr=1 prio=15 maxq=10
Create an SQoS policy and assign the traffic class to this policy. To make SQoS prioritisation
effective, define a suitable virtual bandwidth for the interface being used. As the bandwidth
limit is approached, SQoS can drop packets in a controlled manner and let high priority
packets pass first.
cre sqos policy=1 virt=120kbps
add sqos policy=1 tr=1
add sqos tr=1 classifier=48
Note that this step has not yet applied the policy to interfaces. For site-to-site VPNs, the
next step applies the policy directly to the tunnels. For roaming clients, the interfaces are
dynamically-created for incoming connections, so step 5 on page 32 defines triggers to
automatically apply the policy when connections establish.
1. Create classifiers
2. Reduce the MTU
3. Set up software QoS to ensure VoIP traffic has high priority