Switching to VoIP

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Chapter 9: Quality of Service

VoIP network. The solution is to establish a VLAN strictly for VoIP devices and then

enforce a policy of not using NetBEUI, IPX/SPX, or other non-TCP/IP hosts on that

VLAN.

Layer 3 Switching

Sometimes Ethernet switches can be used to groom, inspect, or route traffic. This

practice is called layer 3 switching, because, like a router, the switch must have some

knowledge of the network-layer protocol. Layer 3 switching accomplishes some

router-like activities: queuing, routing, and packet-inspection.

Queuing and packet inspection are of great interest to people concerned with QoS,

because they can be used to shape the traffic on the data link based on each packet’s

characteristics. For example, it’s possible to drop all non-voice traffic by filtering pro-

tocol types (UDP, TCP, etc.) and port numbers. This isn’t the preferred way of giv-

ing VoIP traffic precedence on the Ethernet segment—just one way. 802.1p is

probably a better way of handling prioritization, and just about all layer 3 switches

support it.

But layer 3 switching can also be used to establish priorities for backbone traffic

among different VLANs, and this is an important technique, especially when voice

endpoints or trunk connections can’t otherwise support a QoS standard. In one such

Figure ...