Chapter 2. Loop Configuration With the Fibre Channel Hub

This chapter explains

Fibre Channel Hub Configurations and Applications

Fibre Channel Hub configurations are as follows:

  • single loop

    Two Fibre Channel Hubs can be connected (cascaded) to create a larger single loop.

  • dual-loop: two Fibre Channel Hubs

    One Fibre Channel Hub can be used for two separate loops.

This section gives configuration overviews and planning considerations, in these subsections:

Single-Loop Configuration

A single Fibre Channel Hub accommodates up to ten devices. Note the following:

  • All ports function the same way.

  • Devices need not be cabled to adjacent ports; that is, you can skip ports when attaching cables.

A factor in cabling the devices is the location of the devices to be cabled. Attach cables following conventions at your site; other cable order possibilities include the following:

  • hosts to ports on the front of the hub, storage to ports on the rear

  • hosts to ports 1 through 4, storage to ports 6 through 9

It is a good idea to label the ends of the cables that are to be connected to the hub with the device names, or some appropriate indication of them.

Figure 2-1 shows port numbering.

Figure 2-1. Hub Port Numbering

Figure 2-2 shows a typical single-loop configuration with one disk drive enclosure (Fibre Channel RAID).

Figure 2-2. Single-Loop Configuration Example, Fibre Channel RAID Storage

Dual-Loop Configuration

In a dual-loop configuration, each host or storage enclosure is connected to each of two hubs, or to the separate segments of a segmented hub.

Figure 2-3 diagrams examples that use Fibre Channel RAID storage; each loop is connected to a separate storage processor (SP).

Figure 2-3. Dual-Loop Configurations With Fibre Channel RAID Storage

You can connect the Fibre Channel Hub to either port in an SP. Both SP ports in a Fibre Channel RAID DPE cannot be connected to the same hub.

Figure 2-4 shows an example of Fibre Channel Hub cabling with one storage unit and several servers.

Figure 2-4. Dual-Loop Configuration Example, Fibre Channel RAID Storage

Figure 2-5 diagrams dual-loop configurations with JBOD storage.

Figure 2-5. Dual-Loop Configurations With JBOD Storage

You can connect the Fibre Channel Hub to either LCC. Both LCCs in a Fibre Channel RAID DAE cannot be connected to the same hub.

Figure 2-6 diagrams an example dual-loop configuration with JBOD storage.

Figure 2-6. Dual-Loop Configuration Example, JBOD Storage

Cascaded Hubs

The optional cascade cable can connect two Fibre Channel Hub units to increase connectivity, creating a single logical loop. Note the following:

  • Only one cascaded hub is supported (two connected Fibre Channel Hub units). Although the FC-AL limit is 127 ports (126 NL_Ports), the supported limit of two cascaded hubs yields 18 ports.

  • All ports are identical; each can be used for an NL_port connection or for a cascaded hub connection. For convenience, generally, ports on the front of the two hubs are connected with the cascade cable.

  • Cascading hubs does not affect hub address, or port numbering in the management interfaces. That is, to get information on ports on cascaded hubs, you log in to each hub the same as if it were not cascaded; each hub still requires its own IP address.

Hub Segmentation

Under normal operation, the Fibre Channel Hub acts as a single loop. For dual-loop configurations, two hubs are recommended. The Fibre Channel Hub is designed as a precise half-rack chassis so that two hubs can be installed in a single 1U-to-3U rack space, providing full redundance. Two hubs used together for dual-loop applications provide dual power supplies and independent loop ports, and allow for field repairs without system downtime.

You can also achieve a dual-loop configuration by segmenting a single hub into two smaller loops via the Fibre Channel Hub configuration software. The Fibre Channel Hub contains two controller boards: one board controls ports 1 through 5; the second board controls ports 6 through 10.

Note: Segmenting hubs does not affect port numbering in the management interfaces. A segmented hub requires only one IP address for management purposes.

“Segmenting and Unsegmenting the Hub” in Chapter 4 contains instructions for segmenting a hub and for restoring it to normal operation.

Planning Loop Configuration

Follow these guidelines when you plan your fibre channel loop:

  1. Determine the devices (storage and servers) in the loop and their physical location.

  2. Make sure that the necessary cables and MIAs are on hand. Make sure that the cabling is the correct length and type (copper or fiber optic) for the storage and servers. Note the following:

    • If you plan to cascade two hubs, make sure you have the cascade cable.

    • It is a good idea to label the ends of the cables that are to be connected to the hub with the device names, or an appropriate indication of them.

    • For the connection between the hub and the console terminal or PC, use only the serial cable and adapters included with the hub.

  3. Determine the order in which you want to cable devices to the hub. You can skip ports when you attach cables to the hub.

    Fibre Channel Hub ports are numbered as shown on the front panel, with ports 5 and 10 on the rear; see Figure 2-1.

    LEDs on the front panel show activity for the eight ports on the front panel and the two ports on the rear panel; see Figure 2-7.

    Figure 2-7. Port LEDs

Determining Hub Management

Follow these guidelines for planning hub management:

  1. Determine how the hub is to be managed:

    • locally: ANSI console terminal, or a PC with a serial port

      If you use a PC, its serial port must support 9600 bps, 8 data bits, Xon/Xoff flow control, no parity, and 1 stop bit.

      For the connection between the hub and the console terminal or PC, use only the serial cable and adapters included with the hub.

    • remotely: Internet browser, Telnet console, or SNMP MIB browser

      You can use the 10-Base-T Ethernet port on the hub's front panel for this connection.

      Obtain a unique IP address for the hub from the system administrator. For this, you need the hub's media access control (MAC) address, which is located on the far left of the hub's rear panel, as shown in Figure 2-8.

      Figure 2-8. Hub MAC Address Location

      For initial installation, it is recommended that the hub be on the same subnet as the server or workstation you are using—the same subnet as the network to which it will be initially connected.

  2. Make sure you have the cabling required for the management option you select.

Selecting a Spot for the Hub

Select a spot for the FIbre Channel Hub that is close to an electrical outlet and to the servers and storage that are to be cabled to the hub. Follow these guidelines:

  • Ensure that sufficient airflow exists to cool the hub beneath its maximum operating temperature of 45˚ C (113˚ F).

  • Stay within the hub power ratings (90-240 VAC, 65 W, 50-60 Hz) to prevent overload of supply circuits or damage to overcurrent protection and supply wiring.

  • Maintain reliable earth ground, especially if connection is to a power strip instead of directly to a branch circuit.

  • Protect the hub from exposure to sunlight and to very intense electrical or magnetic fields.

Note: Appendix C contains a checklist for setting up the hub, with references to specific parts of this guide.