Fibre Channel Topologies

The SAN fabrics are built based on three physical topologies:

• Point-to-Point
• FC-AL (Fibre Channel Arbitrated Loop)
• Switched fabric (SAN)

Figure 10-12 shows the different SAN topologies.

  

Figure 10-12 FC SAN Topologies

Point-to-Point Topology

The point-to-point topology is the smallest and simplest one. It can be qualified as a DAS, as the target is directly connected to the initiator. The initiator and the target are equipped with Fibre Channel HBAs and use fiber optics with FC transceivers for the physical connectivity.

The obvious drawback of this architecture is that the target is dedicated to a single initiator only, meaning that either the resources can be underutilized or they can’t match the requirements of the server’s workloads. If designed correctly, such a topology might be used for some specific applications requiring more security and control. As this topology lacks scalability, it is not common in the data centers. The topology is shown on Figure 10-13.

   

Figure 10-13 Point-to-Point FC SAN Topology

Arbitrated Loop Topology

The FC-AL (Arbitrated Loop) topology involves physical connectivity in which the initiator and the targets are connected using a ring, or loop, as illustrated in Figure 10-14. In FC-AL, up to 127 devices can participate—a limitation imposed by the FC-AL protocol, which allows up to 127 addresses, as one is reserved for an FC switch in case the loop needs to connect to a SAN fabric. Usually about 12 devices are connected in an FC-AL topology.

  

Figure 10-14 FC-AL SAN Topology

The devices can be physically connected to each other through the HBAs in a loop topology. In this setup, if a device’s HBA malfunctions, the ring is broken and there is no communication. To overcome this limitation, you can use a fiber channel hub. As it is a passive device (unlike an FC switch), it does not process the communication but instead (in case of a failed HBA) preserves the physical connectivity for the rest of the participants in the loop.

In the FC Protocol, the ports are assigned different roles, based on which participant they belong to and what they connect to. The ports, which are used to connect to an FC-AL, are assigned the role of NL_ports, which stands for Node Loop ports.

When there is an FC-AL that connects only NL_ports, meaning it is not connected to an FC switch, this is called a “private arbitrated loop.”

When the FC-AL uses a hub, and one of the ports is connected to an FC switch, which is called an FL_port (Fabric Loop port), this topology is usually referred to as a “public arbitrated loop.”

The performance of the arbitrated loop is affected by the type of communication defined by the physical connectivity in a ring—only two of the participants can speak at a time. This introduces very high latencies.

Another drawback is the lack of redundancy in a private loop topology, as the failure of a node will bring down the whole loop.