Allied Telesis AR440S manual Create classifiers, Reduce the MTU

Headquarters

How to prioritise outgoing VoIP traffic from the headquarters router

Add the following steps after step 9 on page 14.

1.Create classifiers

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

2. Reduce the MTU

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

3. Set up software QoS to ensure VoIP traffic has high priority

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.

Page 31 AlliedWare™ OS How To Note: VPNs for Corporate Networks