HP P4459A, Installation & Operation Manual

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HP P4459A 8-Port Fibre Loop Switch Installation & Operation Guide
• If node A is free, node C’s data is forwarded directly to node A without
interference. That is, the messages do not circulate through each device on
the loop and so latency savings on this transaction are also gained. Note that
all transactions are directed between the two communicating nodes. No
traffic ever circulates the entire network as it would in a loop topology.
• If node A is busy with another loop tenancy, the open message from node C
is held until that loop tenancy is completed. Holding the open message until
node A is free sounds extreme until one considers the latency around a
normal loop operation with a high number of nodes. The time just for one
open message to circulate around a large loop (greater than 60 nodes) and
get back buffer credit is actually greater than the completion of an entire
loop tenancy transferring a single maximum frame across the switch. The
relative time difference is on the order of a magnitude (tens of microseconds
vs. tens of milliseconds).
• If A is busy and B, C, and D all simultaneously want to send data to A, a
first-come, first-serve algorithm is enforced. This enforcement is realized
due to the prioritized queueing of open messages at the destination. If the
queue is greater than a programmable value, the switch responds with a close
message to all subsequent open requests from other nodes.
Fibre Loop Switch Initialization
The initialization behavior of the fibre loop switch is encompassed within an
intelligent agent referred to as P4459A Name Server. P4459A Name Server
virtualizes the entire initialization process and eliminates the lengthy login
procedure found on other switches.
In a hub-based Fibre Channel network, any device generates Loop Initialization
Protocol (LIP) messages to notify the other devices on the loop that initialization
needs to occur. During the LIP process, each port in turn receives the LIP
sequence and retransmits the LIP. This process continues until the LIP sequence
returns to the initiating port. Since port addresses are assigned during the LIP
process, no “real” work is performed on the loop while it is underway. Therefore,
it is wise to have a measure of visibility and control over LIP generation. This is
the advantage of P4459A Name Server. It limits the effects of a LIP on other
devices in the network.
Generally, a new node requires some period of time to become ready to
participate on the network. The switch does nothing until the end node signals