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Preparing for Installation SmartStack STS16-20D/STS16-20R Token Ring Switches Installation and User Guide

Configuration Guidelines

This section will help you understand the physical configuration restrictions for the
SmartStack STS16-20D/STS16-20R. Read the guidelines in the following sections
before you start installing the switch. In brief, remember the following rules when
planning to install the switch:
•18,192 byte maximum physical frame length.
•For shared-media LAN segments, acceptable distances are defined by the hub
or concentrator attached to the switch port.
•Straight-through cables for all ports.
•If you create parallel paths directly between switches, be sure that you have
enabled the spanning tree (see “Spanning Tree for BRF Screen” on page99).
The default setting for the spanning tree protocol is disabled.
•The spanning tree protocol will not function between different BRFs within
one switch.
The following sections contain more detailed information.

Frame Length Limit

The SmartStack STS16-20D/STS16-20R supports a maximum physical frame
length of 18,192 bytes (from the Frame Control, FC, to the Frame Check Sequence,
FCS, characters). This corresponds to a Maximum Transfer Unit (MTU) of 17,800
bytes.
The default maximum physical frame length of the switch is 4,546 bytes which
corresponds to an MTU of 4,472 bytes. The actual MTU size of a VLAN is
configured in the VLAN Parameter Configuration for BRF screen, see page91.
The switch truncates frames larger than the configured maximum physical frame
length and adds an abort sequence at the end. Characteristically, if frames are sent
longer than the frame length limit, the abort sequences will be reported as frame
errors by other ring stations.
In a stack of switches, all stacker link modules in the stack must support 18 KB
frames for any switch in the stack to support frames sizes longer than 4,546 bytes.
Additionally, if you are using ATM uplinks in any SmartStack STS16-20RM/
STS16-20FRM, all ATM uplinks in the stack must support 18 KB frames for any
switch in the stack to support MTU sizes greater than 4,472 bytes.
To verify that your hardware supports 18 KB frames, view the Module
Information screen, described on page 84. Stacker link modules an d ATM uplinks
that do not support 18 KB frames will have the text (4K) displayed immediately
after the hardware revision level.