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If you use an ID number outside the range of recommended numbers, you may find certain VLAN ID numbers cause "collisions" and are unavailable when you try to create a new VLAN.
The reason for this has to do with the way that the switch hashes VLAN numbers into internal tables. The 'hash' algorithm used by the switch takes VLAN numbers (which have a range from 1 to 4094) and 'hashes' them to a range of 1 to 1024 by picking 10 bits from the VLAN number. This smaller range is used as an index into tables within the switch, and is referred to as the 'index'. Because the algorithm maps 4096 VLAN numbers to 1024 indexes, it is easy to see that more then one VLAN number may map to the same index. This is known as a 'hash collision'.
When a collision occurs, the switch attempts to resolve the hash collision by using 10 other bits from the VLAN ID. When the switch cannot find 10 bits that uniquely hash the VLAN number, an error is returned to the user that the VLAN number cannot be added. In order to avoid this situation, it is suggested that the user pick VLAN numbers only from the range of 1 to 1000, as when this is the case, it is guaranteed that the switch can hash this set of VLAN numbers without having any collisions that cannot be resolved. Specifically, the 10 least significant bits are picked, which hash each VLAN number (in the range of 1 to 1000) to an index that is the same as the VLAN number. In this situation there will never be any collisions.
See the Maximum Number of VLANs Supported section for valid ranges.
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