Switch operations over long distances

Some SAN switch products provide features that allow the users to tune the performance of I/O traffic in the fabric in a way that can affect Metro Mirror and Global Mirror performance. The two most significant features are ISL trunking and extended fabric.

The following table provides a description of the ISL trunking and the extended fabric features:

Feature	Description
ISL trunking	Trunking enables the switch to use two links in parallel and still maintain frame ordering. It does this by routing all traffic for a given destination over the same route even when there might be more than one route available. Often trunking is limited to certain ports or port groups within a switch. For example, in the IBM® 2109-F16 switch, trunking can only be enabled between ports in the same quad (for example, same group of four ports). For more information on trunking with the MDS, refer to "Configuring Trunking" on the Cisco Systems Web site. Some switch types can impose limitations on concurrent use of trunking and extended fabric operation. For example, with the IBM 2109-F16 switch, it is not possible to enable extended fabric for two ports in the same quad. Thus, extended fabric and trunking cannot be used together. Although it is possible to enable extended fabric operation to one link of a trunked pair, this does not offer any performance advantages and adds complexity to the configuration setup. Therefore, do not use mixed mode operations.
Extended fabric	Extended fabric operation allocates extra buffer credits to a port. This is important over long links that are usually found in intercluster Metro Mirror operation and Global Mirror operations. Because of the time that it takes for a frame to traverse the link, it is possible to have more frames in transmission at any instant in time than is possible over a short link. The additional buffering is required to allow for the extra frames. For example, the default license for the IBM 2109-F16 switch has two extended fabric options: Normal and Extended Normal. The Normal option is suitable for short links. The Extended Normal option provides significantly better performance for the links up to 10 km long. Note: The extended fabric license provides two extra options: Medium, 10 - 50 km and Long, 50 - 100 km.

Feature

Description

ISL trunking

Trunking enables the switch to use two links in parallel and still maintain frame ordering. It does this by routing all traffic for a given destination over the same route even when there might be more than one route available. Often trunking is limited to certain ports or port groups within a switch. For example, in the IBM® 2109-F16 switch, trunking can only be enabled between ports in the same quad (for example, same group of four ports). For more information on trunking with the MDS, refer to "Configuring Trunking" on the Cisco Systems Web site.

Some switch types can impose limitations on concurrent use of trunking and extended fabric operation. For example, with the IBM 2109-F16 switch, it is not possible to enable extended fabric for two ports in the same quad. Thus, extended fabric and trunking cannot be used together. Although it is possible to enable extended fabric operation to one link of a trunked pair, this does not offer any performance advantages and adds complexity to the configuration setup. Therefore, do not use mixed mode operations.

Extended fabric

Extended fabric operation allocates extra buffer credits to a port. This is important over long links that are usually found in intercluster Metro Mirror operation and Global Mirror operations. Because of the time that it takes for a frame to traverse the link, it is possible to have more frames in transmission at any instant in time than is possible over a short link. The additional buffering is required to allow for the extra frames.

For example, the default license for the IBM 2109-F16 switch has two extended fabric options: Normal and Extended Normal.

The Normal option is suitable for short links.
The Extended Normal option provides significantly better performance for the links up to 10 km long.

Note: The extended fabric license provides two extra options: Medium, 10 - 50 km and Long, 50 - 100 km.