Fibre Channel topologies

There are three major Fibre Channel topologies, describing how a number of ports are connected together. A port in Fibre Channel terminology is any entity that actively communicates over the network, not necessarily a hardware port. This port is usually implemented in a device such as disk storage, an HBA on a server or a Fibre Channel switch.

Point-to-Point (FC-P2P). Two devices are connected directly to each other. This is the simplest topology, with limited connectivity.

Arbitrated loop (FC-AL). In this design, all devices are in a loop or ring, similar to token ring networking. Adding or removing a device from the loop causes all activity on the loop to be interrupted. The failure of one device causes a break in the ring. Fibre Channel hubs exist to connect multiple devices together and may bypass failed ports. A loop may also be made by cabling each port to the next in a ring.

A minimal loop containing only two ports, while appearing to be similar to FC-P2P, differs considerably in terms of the protocol.

Only one pair of ports can communicate concurrently on a loop.

Maximum speed of 8GFC.

Switched fabric (FC-SW). All devices or loops of devices are connected to Fibre Channel switches, similar conceptually to modern Ethernet implementations. Advantages of this topology over FC-P2P or FC-AL include:

The switches manage the state of the fabric, providing optimized interconnections.

The traffic between two ports flows through the switches only, it is not transmitted to any other port.

Failure of a port is isolated and should not affect operation of other ports.

Multiple pairs of ports may communicate simultaneously in a fabric.

June 30, 2011

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