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This chapter is from the book

This chapter is from the book


The FC-AL topology allows cost-effective connection of up to 126 devices without the need of an underlying Fabric. However, a daisy chain of devices connected to form a loop makes it difficult to troubleshoot the network. Adding or removing devices from the loop is a highly time-consuming exercise. Also, any cable break or power loss can lead to the temporary shut down of the entire loop and all the nodes attached to the loop until the problem is remedied.

Fibre Channel hubs are used to effectively solve the problems that occur in the FC-AL topology. Analogous to the hubs used in traditional local-area networks (LANs), Fibre Channel hubs form the focal point of the FC-AL topology, as shown in Figure 4-9. As a result, network administrators can centrally monitor and manage the loop. The Port Bypass Circuitry (PBC) used in hubs allows devices to be dynamically added or removed from the loop while the loop is still functional. If a device is added or removed from the loop, these hubs can automatically reconfigure the loop.


A daisy chain is a serial connection of network devices to a single cable.

Figure 4-9Figure 4-9 Hub—The Focal Point of the Loop

Fibre Channel hubs are of the following three types:

  • Unmanaged hubs
  • Managed hubs
  • Switched hubs

The following sections describe each type of hub.

Unmanaged Hubs

Unmanaged hubs are not shipped with built-in software to manage them, thus the name unmanaged hubs. These hubs are used for extending loop distances. A typical unmanaged hub provides eight ports. Some unmanaged hubs also support 12 and 16 ports. Of the available hub ports, one port is used to connect to another hub. The rest of the ports can support loop nodes.

Each loop node is attached to the hub through a GBIC, which plugs into the hub port. When an attached node communicates with another through the hub, the hub port receives the signal, regenerates it, and forwards the signal to the next port. If any part of the loop node is inoperative, the hub detects the missing node with the help of port bypass circuits and automatically excludes the node from the loop. Similarly, any node that is attached to the hub through a GBIC is automatically included in the loop.

Unlike other interconnect devices, such as switches and HBAs, unmanaged hubs used in arbitrated loops are not assigned Fibre Channel addresses. As a result, they cannot take part in protocol-related activities. This makes them merely passive members of the storage network that are used to extend the loop.

Various vendors offer a wide range of unmanaged hubs. These vendors include Vixel, IBM, HP, 3COM, and Veritas. The prices of these hubs can range from $1500 to $8000.

Managed Hubs

Unlike unmanaged hubs, managed hubs are SNMP-controllable and are shipped with integrated hub-management software. As a result, they are highly manageable and to some extent are capable of participating in the activities that are controlled by the protocols.


SNMP is an application layer protocol that facilitates the exchange of management information between network devices. SNMP consists of three components—SNMP manager, agent, and Management Information Base (MIB). The SNMP manager resides on a central management console and retrieves and collates the management information provided by the various SNMP agents. An SNMP agent resides on devices, such as managed hubs, switches, bridges, and routers. The SNMP agent is responsible for storing the local information in a MIB, which in turn is retrieved by the SNMP manager.

Managed hubs are also better at fault isolation. This is because they can isolate Loop Initialization Primitives (LIPs) from general application-related traffic, such as backup and video data that is extremely sensitive to delays. Managed hubs thus help in increasing overall loop stability. Some other functions of managed hubs include the following:

  • Server clustering and storage consolidation


    A server cluster is a group of independent servers interconnected to form a single computing resource.

  • Backups and restorations that do not disturb the normal operations of the attached LAN

  • LAN-free streaming of audio/video applications

  • Remote device monitoring

  • Remote disk mirroring


    Similar to most of the Fibre Channel devices, managed hubs are Field Replaceable Units (FRUs) that must be handled by trained professionals, such as network administrators.

Many SAN vendors, such as IBM, Vixel, HP, Brocade, and Gadzoox, offer a wide range of managed Fibre Channel hubs. Most of these hubs share some common features, including the following:

  • Dedicated port speed—All the individual ports offer 100 Mbps data transfer rates. This data transfer rate is dedicated to the given port and is not affected by simul-taneous transactions being carried out by other hub ports.

  • Hot-pluggable ports—Managed hubs typically provide 8 to 12 hot-pluggable ports. These ports allow dynamic attachment or detachment of nodes (servers, storage devices, and clients) to the loop without the need of shutting down the loop. The nodes are attached to a hub port through a GBIC.

  • Zoning—Most managed hubs allow dynamic zoning of nodes or ports, thus ensuring increased loop security.

  • Management interface—Most of the managed hub vendors, such as IBM and Vixel, offer a graphical user interface (GUI) for management and configuration purposes. Other vendors provide an easy-to-use command-line interface (CLI).

  • Diagnostic capabilities—Managed hubs offer comprehensive online diagnostic capabilities along with Power-On Self Test (POST). POST is run at the time the hub is powered on and helps the hub gather information about the status of all the attached nodes and environmental components such as fans.

    Online diagnostics constantly monitor the status of attached nodes and environmental components. On detecting a failed node or a non-critical component, the corres-ponding port is excluded from the loop with the help of port bypass circuitry. If a critical component of the hub fails, an appropriate warning is issued to all ports before shutdown.

Switched hubs belong to the third category of Fibre Channel hubs. You will now learn about this category of Fibre Channel hubs.

Switched Hubs

Switched hubs are the third and most recent category of Fibre Channel hubs. Unlike the previous categories of Fibre Channel hubs, switched hubs actively participate in protocol-related activities, such as discovery, identification, and management of other devices on the storage network, event logging, and diagnostics. Similar to managed hubs, each individual port of a switched hub is allocated a dedicated bandwidth of 100 Mbps and higher. Switched hubs function well with switches and they actively improve the speed of transactions by performing some of the tasks of switches. Also, they are extremely scalable and adaptable to future growth.

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