Link definitions enable the appliance to prevent congestion and loss on your WAN links and to perform traffic shaping. A link definition specifies which traffic is associated with the defined link, the maximum bandwidth to allow for traffic received on the link, and the maximum bandwidth for traffic sent over the link. The definition also identifies traffic as inbound or outbound and as WAN-side or LAN-side traffic. All traffic flowing through the appliance is compared to your list of link definitions, and the first matching definition identifies the link to which the traffic belongs.
By performing the Quick Installation procedure, you customize the appliance's default link definitions. You have then defined the appliance's link to the WAN and its link to the LAN. For a simple inline deployment, no further configuration of link definitions is necessary. Other types of deployments require additional configuration of link definitions.
Every link has two bandwidth limits, representing the sending speed and the receiving speed. Only when the link speed is known can the appliance inject traffic into the link at exactly the right speed, thus eliminating the congestion and packet loss that result from attempting to send too much, or the loss of performance that results from sending too little. When placed between a fast LAN and a slower WAN and acting as a virtual gateway, the appliance has the ability to receive traffic faster than the WAN can accept it, creating a backlog of traffic. The existence of this backlog enables the appliance to choose which packet to send next, and this choice in turn makes traffic shaping possible. Unless there are packets from multiple streams to choose from, there is no ability to favor one stream over the other. Traffic shaping is therefore dependent on the existence of the virtual gateway and correctly set bandwidth limits.
Note: Link definitions normally apply to connections to the accelerated pair of bridge ports. The two motherboard ports, Primary and Aux1, can also be defined as links, but doing so rarely serves any purpose, because they are used for management and as a back-channel for high-availability and group modes, not for WAN traffic.
For link-definition purposes, a link
is a physical link, with its own bandwidth capacity. It is typically a cable that leaves the building. Remember the following points:
- A VLAN is not a link.
- A virtual link is not a link.
- A tunnel is not a link.