HP StorageWorks 1606 Brocade Fabric OS Administrator's Guide v6.3.0 (53-100133 - Page 102

4 Routing overview FSPF Fabric Shortest Path First (FSPF) is a link state path selection protocol that directs traffic along the shortest path between the source and destination based upon the link cost. FSPF detects link failures, determines the shortest route for traffic, updates the routing table, provides fixed routing paths within a fabric, and maintains correct ordering of frames. FSPF keeps track of the state of the links on all switches in the fabric and associates a cost with each link. The protocol computes paths from a switch to all the other switches in the fabric by adding the cost of all links traversed by the path, and chooses the path that minimizes the costs. This collection of the link states, including costs, of all the switches in the fabric constitutes the topology database or link state database. Once established, FSPF programs the hardware routing tables for all active ports on the switch. FSPF is not involved in frame switching. FSPF uses several frames to perform its functions. Since it may run before fabric routing is set up, FSPF does not use the routing tables to propagate the frames, but floods the frames throughout the fabric hop-by-hop. Frames are first flooded on all the Inter-Switch Links (ISLs); as the protocol progresses, it builds a spanning tree rooted on the Principal Switch. Frames are only sent on the Principal ISLs that belong to the spanning tree. When there are multiple ISLs between switches, the first ISL to respond to connection requests becomes the Principal ISL. Only one ISL from each switch will be used as the Principal ISL. FSPF makes minimal use of the ISL bandwidth, leaving virtually all of it available for traffic. In a stable fabric, a switch transmits 64 bytes every 20 seconds in each direction. FSPF frames have the highest priority in the fabric. This guarantees that a control frame is not delayed by user data and that FSPF routing decisions occur very quickly during convergence. FSPF guarantees a routing loop free topology at all times. It is essential for a fabric to include many physical loops because without loops there would be no multiple paths between switches, and therefore no redundancy. If a link went down, part of the fabric becomes isolated. FSPF ensures that the topology is loop free and that the frame is never forwarded over the same ISL more than once. FSPF calculates paths based on the destination domain ID. The fabric protocol must complete domain ID assignments before routing can begin. ISLs provide the physical pathway when the Source ID (SID) address has a frame destined to a port on a remote switch Destination ID (DID). When an ISL is attached or removed from a switch, the FSPF updates the route tables to reflect the addition or deletion of the new routes. As each host transmits a frame to the switch, the switch reads the SID and DID in the frame header. If the domain ID of the destination address is the same as the switch (intra-switch communications), the frame buffer is copied to the destination port and a credit R_RDY is sent to the host. The switch only needs to read word zero and word one of the Fibre Channel frame to perform what is known as cut-through routing. A frame may begin to emerge from the output port before it has been entirely received by the input port. The entire frame does not need to be buffered in the switch. If the destination domain ID is different than the source domain ID, then the switch consults the FSPF route table to identify which local E_Port provides the Fabric Shortest Path First to the remote domain. Fibre Channel NAT Within an edge fabric or across a backbone fabric, the standard Fibre Channel fabric shortest path first (FSPF) protocol determines how frames are routed from the source Fibre Channel device to the destination FC device. The source or destination device can be a proxy device. 60 Fabric OS Administrator's Guide 53-1001336-01