Data moves through a fabric from switch to switch and from storage to server along one or more paths
that make up a route. Routing policies determine the path for each frame of data.

Before the fabric can begin routing traffic, it must discover the route a packet should take to reach the
intended destination. Route tables are lists that indicate the next hop to which packets are directed to
reach a destination. Route tables include network addresses, the next address in the data path, and a
cost to reach the destination network. There are two kinds of routing protocols on intranet networks,
distance vector and link state.

• Distance vector is based on hop count. This is the number of switches that a frame passes through

to get from the source switch to the destination switch.

• Link state is based on a metric value based on a cost. The cost could be based on bandwidth, line

speed, or round-trip time.

With the link state protocol, switches that discover a route identify the networks to which they are
attached, receiving an initial route table from the principal switch. After an initial message is sent out,
the switch only notifies the others when changes occur.

It is recommended that no more than seven hops occur between any two switches. This limit is not
required or enforced by Fabric Shortest Path First (FSPF). Its purpose is to ensure that a frame is not
delivered to a destination after the Resource Allocation TimeOut Value (R_A_TOV) has expired.

Fabric OS supports unicast Class 2 and Class 3 traffic, multicast, and broadcast traffic. Broadcast and
multicast are supported in Class 3 only.

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.

Fabric OS Administrators Guide

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