3Com SuperStack 4 Configuration Manual page 423

Unit ID Numbering
Mechanism
If you are unable to use link aggregation on multihomed links, then STP/RSTP
should be used as the second option, and the last option would be to use
resilient links.
This implementation increases the level of fault tolerance as it also protects
against loss of the physical interfaces at the core.
If you use the Switch 5500 in a Distributed Fabric further resilience can be
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achieved by utilising a redundant power source.
You always have STP/RSTP enabled on your network to prevent the risk of
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loops occuring if you have links that are multihomed, particularly if you are
using link aggregation.
All multihomed links and alternate paths must carry all VLANs, and packets
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must be tagged.
The Distributed Fabric is the STP root bridge.
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Individual port members of each aggregated link must have VLAN membership
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manually configured before the aggregated link is set up. You must not rely on
port members inheriting VLAN membership configuration from the aggregated
link. (See "VLANs"
If you are using resilient links, these must be configured on the remote unit,
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not on the units within the Distributed Fabric.
If you follow the 3Com recommendations, should there be a unit or interconnect
failure within the Distributed Fabric, traffic flow will be maintained at all times. If
you want to know more detail about how the Distributed Fabric behaves in certain
failure scenarios, see "How a Failure affects the Distributed Fabric"
This section outlines the mechanisms that the Switch 5500G-EI Family uses to
determine the unit IDs for management purposes.
When a Fabric is created using the Switch 5500G-EI the unit numbering can be
determined in two ways.
You can manually assign unit IDs 1 to 8 to specific units using the
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change[self-unit, unit-id] to [1-8, auto-numbering]
command from the System View. If you manually assign unit IDs to a Switch via
the change command the IDs will be retained after a power cycle.
If you add a unit to a Fabric that has previously been manually configured with
a unit ID and this conflicts with an ID already within the Fabric, then the Switch
with the lowest MAC address assumes the ID in question and the other unit
will automatically renumber.
3Com recommends that you manually assign the unit IDs within the Fabric if you
wish to have predictability of knowing which units have which IDs at all times.
Fabric topology is 'discovered' and the units auto-number their IDs.
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Adding and removing units from the Fabric does not cause any renumbering to
occur and the Fabric will continue to work normally. Renumbering only occurs
when the Fabric is next power cycled if the units are configured to auto-number.
on page 438 for more information.)
Unit ID Numbering Mechanism
on
435
page 441.