Fibre Channel - Fibre Channel Topologies

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.