Avaya Cajun P882 User Manual page 232

Chapter 6
Reassign VLAN
IDs?
Optimal Bucket
Utilization and
Hash Table size
Maximum
Number of
VLANs
6-4
tables within the switch, and will be 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 num-
ber (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.
It is not recommended to reassign VLAN numbers in a switch that is
currently using VLAN numbers outside the range of 1 to 1000. The
only potential issue is an attempt to add more VLANs, it is possible
that particular VLAN numbers will not be available to use.
To achieve optimal Bucket Utilization, Hash Table size should be ¼
times the number of expected AFT entries (MAC entries) on the
VLAN.
* Note: Each VLAN has 18 AFT entries automatically entered by
the switch that are reserved for internal use. (Refer to
Chapter 1, "Introduction," for a detailed listing of Self
Addresses).The total amount of AFT memory used for
Hash Tables should not exceed 20K.
The following information provides the maximum number of
VLANs that are supported depending on the version of code that is
running and the "Fabric Mode" that the switch is configured with.
* Note: Pre-5.0 code does not support 'fabric modes', and when
a switch is in Fabric Mode 2, only Cajun II hardware is
supported.
Avaya P550R, P580, P880, and P882 Multiservice Switch User Guide, Version v5.3.1