Advantages of Trunking

Trunking provides an incremental increase in backbone performance by simply assigning more ports to the trunk group. The alternative is to use one backbone port with a data rate that is ten-times faster than the station ports. That is a significant increase in performance that may not be needed. A two-times increase in speed may be all that is required which can be accomplished by simply assigning two 100 Mbps ports to a trunk group. If another increment in speed is desired, another port can be added to a trunk group. Therefore, backbone speed can be adjusted by the simple addition or removal of ports within a trunk group.

The other advantage is distance. By using regular ports for trunking, there are no changes to the cabling rules. Although a 100BASE-TX has the same 100 m segment length restrictions as a 1000BASE-T port, that is not the case for fiber optics. Both single-mode and multimode fiber optic maximum segment lengths are much less at Gigabit speed than they are at 100 Mbps.

While single-mode segment lengths of 15 km and greater are quite easy to achieve at 100 Mbps, lengths of only 5 km are possible at Gigabit speed. Segment length for multimode fiber is restricted to only 550 m at Gigabit speed using either longwave or shortwave devices. At 100 Mbps, 2 km can be achieved using longwave devices. Therefore, there is an advantage to using trunking when long distances are involved.

One might say that the cost of a trunk would exceed the cost of a single high-speed connection. Granted, more fiber pairs are required for a trunk, but fiber is generally pulled in bundles to ensure that spare fibers are available. The cost of pulling a fiber cable, regardless of the number of fibers in the bundle, will probably outweigh the cost of the fiber cable used.

There is another advantage to trunking and it is in regard to availability. With a trunk group, transmissions are sent to the first available port within the trunk group. What happens if the cable to one of those ports is defective? With many switches, the port that is no longer available is ignored and therefore bypassed. Traffic will be diverted to the remaining ports within the trunk group. Granted, throughput will be reduced but the network remains functional. With a single high-speed backbone, a lost of the single cable will sever the system.

I like to use the following analogy when discussing availability. Lake Michigan, which Chicago skirts, is a very large lake which many power boaters like to challenge. What is best to have-a boat with a single 150 HP inboard motor or a boat with two, 75 HP outboard motors. The inboard model is nice and sleek, but the outboard model may be more practical. A motor failure on the inboard leaves you "dead in the water;" however, a single motor failure on the dual-outboard model will still get you back to shore with reasonable swiftness. The availability of a second motor ensures continuing operation similar to the additional ports within a trunk group.

Disadvantages of Trunking

The first disadvantage of trunking is that trunk ports must first be configured. This means that the switch cannot be a simple plug-and-play switch. Either a configurable or managed switch must be used with a trunking option. This is not the case with having high-speed backbone ports. With high- speed backbone ports, it is possible to simply have a plug-and-play switch.

The other disadvantage is loss of ports. With a high-speed backbone port, only one port is lost for this function. If it is a mid-span device, two ports are lost. (Figure 4) With trunking, a minimum of two ports are lost for backbone use and with mid-span devices, four ports are lost. If the trunk group is larger than two ports, even more ports are lost. Lost ports mean fewer ports are available on the switch for actual stations.

Take the example of an eight-port switch. If two ports are dedicated for trunking, only six ports are available for stations. If this same switch is located in a mid-span position with other switches connected to its left and right, four-ports would be dedicated for trunking and only four ports remain for stations. If the trunk group were increased to four, no ports would be available for stations. Trunking is best applied to switches that have sixteen or more ports.

Figure 4 — Trunking consumes ports, especially in mid-span positions.