User manual

Configuring the HRE-X Router Port
Page 9-29
Configuring and Displaying the HRE-X Hash Table
The HRE-Xs use a hardware implemented hash table to route packets for transmission. The
switch employs a default hash function that works well in a broad range of data environ-
ments. In rare cases, you may want to change the hash table configuration to optimize it for
your particular data flow. This should be done with care because the data population will
change over time. A hash function that works well for one set of data may not work as well
for another. Also, note that optimizing the hash function will cause all of the current entries in
the HRE-X to be cleared and then relearned; therefore, this should be done with extreme
caution.
Two
HRE-X commands are used to optimize the hash function. They are the hrexutil and the
hrexhashopt commands. The hrexutil command displays the current utilization of the hash
table. To view the HRE-X Utilization table, enter
hrexutil
at the system prompt. A screen similar to the following is displayed.
HRE-X Utilization
- - - - - - - - - - - - - - - -
Hash - Total: 65536 Free: 65528
Collisions - Total: 131072 Free: 131069
Cache - Total: 40960 Free: 40949
Collision Length - Max: 3 Avg: 1
The fields displayed by the hrexutil command are described below:
Hash. The number of entries in the hash table.
Total. The total number of units available.
Free. The number of units that are not yet used.
Collisions. The number of entries that have hashed to the same index in the hash table.
Cache. The number of modifications required to route a packet.
Collision Length. The length of the longest (Max) collision list and the average length (Avg) of
the collision lists.
The
hrexhashopt command causes the switch to compute an optimized hash function based
on the data currently in the HRE-X. This function is saved in the configuration file so it will be
present after a reboot.
To use the
hrexhashopt command, enter
hrexhashopt
at the system prompt. The screen does not display a confirmation message after this
command. You can verify optimization by observing the changes in the HRE-X Utilization.
After using
hrexhashopt, the maximum and average collision lengths should be reduced as
shown in the HRE-X Utilization table shown above. If they are not, you should consider
returning to the default hash function by using the hrexhashdflt command.
To use the
hrexhashdflt command, enter
hrexhashdflt
at the system prompt. The screen does not display a confirmation message after this
command. The hrexhashdflt command will return the hash function back to the default value.