Black Box Mixed-Media Fiber Hubs
Installation and User Guide (4/99)
LH8050A-SST, LH8050A-SSC, LH8050A-SSCSM, or LH8050A-SRJ) with an SPM installed
in the Bonus Port, accessible from the rear.
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Figure 2.5a: Inside view of LH8050A with SPM installed. (Rear view)
For the specifications of LH8050A models with Switch Port Modules (SPMs)
installed, see Section 1.3. The functions of an SPM are described below.
1.
Filtering and Forwarding / Address Learning
The SPM has two ports - one where media attaches, and one connecting into the
inside of the mixed-media fiber hub in which the SPM is mounted. For each packet received by
either port of the SPM, a decision is made to either filter the packet or to forward the packet to
the other port. Error packets are always filtered. For good packets, the filter or forward decision
is made based on the destination address contained in each packet. If the destination address is
on the segment from which the packet originated, then it is filtered and not forwarded to the
other segment. If the destination address didn’t find a match in the address table, then is it
forwarded to the other segment. If it is a new address which the SPM did not previously know
about, it “learns” the new address and puts it in the port’s address table. The address tables have
a capacity of 1K addresses for each port. With this large address table, SPM port modules can
serve the needs of medium-sized to large networks. The address tables are flushed periodically
to update the network status and to purge any inactive stations from the tables. See the figure
and table that follows:
MAIN BOARD
SPM
MODULE
FAN
STACKING
CONNECTORS
POWER
SUPPLY
