Deployment Guide

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 is also referred to as
Layer 2 routing. 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 also 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. Because 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 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 is used as the principal ISL.
The following figure shows the thick red lines as principal ISLs, and thin green lines as regular ISLs.
FIGURE 5 Principal ISLs
NOTE
FSPF only supports 16 routes in a zone, including Traffic Isolation Zones.
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
100 Fabric OS Administrators Guide
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