Bridging private and public Fibre Channel loops

Posted on November 01, 1999

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Bridging private and public Fibre Channel loops

To preserve existing investments, it`s sometimes necessary to link hub-based loops with switched fabrics.

Larry Olson

Although Fibre Channel storage area networks (SANs) are relatively new to enterprise storage, Fibre Channel devices have been shipping for several years. Most of these devices (servers, clients, disk arrays, hubs, and switches) use different modes of Fibre Channel communication. Often, existing installations have to be considered when building today`s SAN fabrics. This article describes Fibre

Channel communication modes and explains how they can coexist on a single fabric.

Private loops

The first to market and to be installed in many applications from many vendors, private loops are the simplest and least expensive form of Fibre Channel. They are typically built using hubs. Communication is handled by passing messages around the loop from one device to the next in order to gain control of the loop and to communicate through it.

However, because each device depends on the next device for communication, private loops can be unstable. Adding, removing, or rebooting a device, for example, can cause other devices on the loop to pause. This pause forces every device to examine the change in the environment and to adjust to that change. This can be disruptive when data is being transferred at the same time a change of state takes place on an arbitrated loop.

In a controlled, stable environment, private loops are adequate for many applications. However, in a dynamic environment, when performance, stability, and advanced management are required, a more robust mode of communications may be necessary.

Public loops

Public loops typically consist of a Fibre Channel arbitrated loop with one or more switches. Public loops can have the following advantages over private loops.

•Name server. As described in the Fibre Channel standard, adding a simple name server improves control and stability on the loop because each device registers with the name server at login. The name server eliminates the need for devices to pause and pass messages to each other each time an event happens on the loop. The switch internally controls the name server.

•Management. With a name server and switch management software, devices on a switch can be queried, examined, and brought online and offline. Errors can be logged and trapped with messages sent to an administrator, who can then take appropriate action.

•Zoning. Switch-based zoning can make a Fibre Channel SAN more flexible and secure by separating and segregating data and devices. Zoning can be created via individual switch ports, name server entries, or by each device`s unique World Wide name.

•Device addressability. Public loops use a 24-bit address, compared to a private loop`s 8-bit address, which makes it possible to use more than 126 devices.

While public loops have advantages, a private device cannot always be upgraded to public loop due to design issues or because public loop firmware is not available. For ease of management in these situations, private devices must be able to communicate with fabric devices on a switch.

For a switch bridge public and private loops, it must be able to support three types of translation. Generally speaking, an initiator is a server and a target is a disk or tape device.

•Public initiator to/from private target. In this type of translation, the switch port that connects to the private loop is denoted the translative (TL) port, which tells the switch that a private device is attached. The private loop targets, then log into, this port.

When a public initiator sends a message to a private target, it first looks up the address of the private target. Then the upper 16 bits of its address are stripped off. This is necessary because a private target expects an 8-bit address.

•Private initiator to/from public target. In this type of translation, the opposite occurs. The private initiator keeps track of public addresses via tables. When sending data to a public target, the upper 16 bits are added by performing a lookup in the tables. This is necessary because a public target expects to receive a 24-bit address. To the private initiator, the public target appears to be on its private loop (see figure).

There are some limitations to the number of devices that can be addressed when translating between public and private loops. In general, these methods are adequate for most implementations. For maximum performance, translating should be done in hardware.

•Private initiator through fabric to/from private target on a different loop, or segmented loop mode. Here, multiple physical loops on a switch comprise one large private loop. Segmented loop mode is advantageous because each loop segment can move frames simultaneously, resulting in shorter loop arbitration times and more frame throughput. However, because each device must have a unique address, this mode is limited to 126 devices.

Flexible, powerful SANs can grow to include legacy devices, provided that the switch can communicate between fabric and private loop. An intelligent SAN switching fabric incorporates private loop devices, allowing for the seamless translation of data among all Fibre Channel communication modes.

Click here to enlarge image

In a translation from private initiator to or from a public target, the private initiator keeps track of public addresses via tables.

Larry Olson is senior systems engineer at Ancor Communications Inc. (www.ancor.com), in Eden Prairie, MN. He can be reached at larryo@ancor.com.


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