Dell POWEREDGE M1000E - Manual (Page 112)

112 Fabric OS Administrator’s Guide
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Routing overview
4
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 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.
Figure 5 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.