Routing for VoIP Networks

The Routing portion provides a discussion on the different methods available and options used in routing between voice and data networks. InterVLAN routing is discussed with advantages and disadvantages of each method discussed.

Important Point: In order to route between two networks, the networks must be on separate IP networks. For example, the data network can be on and the voice network can be on These are two separate networks. The networks can be on separate VLANs. In order to route data between the VLANs, a routing device must be used - it can be a router or a layer 3 switch.

This section is divided into the following pages:

Single Port Routing - This refers to LAN side routing between voice and data networks using a single port between the router and the switch. The router has a WAN port and no VLANs have been assigned to the switch.

Separate VLAN Ports - On the switch separate VLAN'd ports are configured for voice and data and the router has two LAN ports in addition to the WAN port

Router on a Stick - This uses only one LAN port to connect to the router. The port is configured as a trunk to the switch that allows multiple VLANs and their networks to pass through. The router must be able to do multiVLAN trunking using IEEE802.1Q.

Layer 3 Switch - this method uses the routing capabilities of a layer 3 switch to route between VLANs. It is simple and the preferred method to use for inter-VLAN routing.

NAT and PAT - Network Address Translation and Port Address Translation can be a huge problem for VoIP, especially SIP traffic. Here's what NAT is and the problems associated with it.

WAN Traffic - the WAN link is the connection to the rest of the world and the bottleneck for traffic coming into and out of your network. Here's what you can do to improve the quality of service for your VoIP traffic.

Subnetting - The classes of networks (A, B and C) do not provide much flexibility in designing a network. Each class of network only provides for a fixed number of networks (125, 16,382 or 2,097,150) and a fixed number of hosts (16,777,214, 65,534 or 254). Using the class system is referred to as having a classful network. In the real world, pretty much all networks do not fit the class system. The solution is to divide the class network into smaller subnetworks or subnets for short. The term for dividing networks into smaller subnets is called subnetting.

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Copyright July 2013 Eugene Blanchard