11Routing Traffic; Routing Overview; Path Versus Route Selection; Fspf - HP A7533A - Brocade 4Gb SAN Switch Base Administrator's Manual

1 1 Routing traffic

Routing overview

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 to route a packet, it must discover the route that 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 method, 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
notifies the others only when changes occur.
HP recommends that no more than seven hops occur between any two switches. This limit is not required or
enforced by FSPF. Its purpose is to ensure that a frame is not delivered to a destination after R_A_TOV has
expired.
Unicast, multicast, and broadcast traffic are supported. Both Unicast Class 2 and 3 traffic are supported.
Broadcast and multicast are supported in Class 3 only.

Path versus route selection

Paths are possible ways to get from one switch to another. Each 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 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 sent only 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
Fabric OS 6.2 administrator guide 283