Fabric OS Administrator's Guide v7.0.0 (53-1002148-02, June 2011)

62 Fabric OS Administrator’s Guide

53-1002148-02

Routing overview

Paths and route selection

Paths are possible ways to get from one switch to another. Each Inter-Switch Link (ISL) has a metric

cost based on bandwidth. The cumulative cost is based on the sum of all costs of all traversed ISLs.

Route selection is the path that is chosen. Paths that are selected from the routing database are

chosen based on the minimal cost.

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 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. Figure 5 shows the thick red lines as principal ISLs, and thin green lines as regular

ISLs.

FIGURE 5 Principal ISLs