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Chapter 1 Introduction and Planning
1-7
network. The penalty is increased latency caused by the delay while waiting for the
entire frame (up to 1518 bytes) to be read and the CRC checked. Adaptive cut-
through mode allows faster forwarding (lower latency). The penalty here is
increased bandwidth consumed by fragments and some error frames. The selection
between the two modes depends on network priorities.
Back-Pressure Flow Control
If an FEM receives a sustained high volume of traffic, the FEM packet memory can
become full. If this happens, the FEM issues a jam pattern on the upstream link.
Then, an upstream sending adapter, sensing the collision, will stop sending and wait
a random period of time before re-sending a frame. If back-pressure flow control is
not selected, the 8237 will discard any frames it receives during a buffer-full
condition.
Generally, in this case, the sending user application will retry the transmission. In
most cases, the adapter retransmission will cause less delay than an application
retrying the transmission. Thus back-pressure flow control is recommended.
Transmission mode, forwarding mode, and flow control are selectable under
network management control via the 8237 private MIB.
Understanding Managed Stacks
This section describes how 8237s in a stack communicate with one another, the
effects of segmenting 8237s within a stack, and how to link segmented 8237s.
Inter-8237 Communications in Managed Stacks
In an 8237 stack there are three 10-Mbps backplanes and one management
backplane (or bus). This management backplane is used by the management agent
in Model 002 and Model 003 to retrieve management information from all units in the
stack.
Regardless of the segmentation status of any 8237 or its position in the backplane,
the hub retains its full management capability and the ability to be managed by a
single Model 002 or Model 003.
Using any of the management methods mentioned in the previous section, you can
perform any of the following actions on any 8237 in a stack while attached to the
Model 002 or Model 003 that is currently the primary management hub as indicated
by the Prim LED:
•
Display system information
•
Issue management commands to any hub in the stack
- SNMP configuration
- Port security control
- Segment configuration
- Hub configuration
- Port configuration
- User passwords
- Backup ports
- Management Port lockout
•
Collect statistics at the port, hub, or segment level.
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