It seems adequate documentation giving explanation on how command "trunk-group name preference" under serial interface configuration works is unavailable. Common belief has provided misleading information on how the "preference" value is related to trunk group's configuration, namely "hunt-scheme sequential both down".
Many believe that by configuration "hunt-scheme sequential both down", router will begin channel hunting from the member port with highest preference value, go from channel 23 to 1 (with T1), and then move on to the member port with next-to-highest preference value. In reality, this command does not provide this behavior, but rather begin from the member port with lowest preference value, go from channel 23 to 1, and then move on to the member with next-to-lowest preference, and so on. Lower number indicates higher preference value, e.g. 1 is more preferable than 2.
This is because once several PRIs are grouped together into a trunk group, the router will treat them as one big collection of channels, ordered by its preference value:
_____PRI 1____|_____PRI 2_____|_____PRI 3_____
1 2 3 4 5...22 23 | 1 2 3 4 5... 22 23 | 1 2 3 4 5... 22 23
PRI 1 will have a preference value of 1, PRI 2 will have 2, and PRI 3 will have 3. If "hunt-scheme sequential both down" is configured, then the router will begin from the bottom of the *entire* list of channels, which is channel 23 of PRI 3, and work its way up:
_____PRI 1____|_____PRI 2_____|_____PRI 3_____
1 2 3 4 5...22 23 | 1 2 3 4 5... 22 23 | 1 2 3 4 5... 22 23
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On the other hand, if "hunt-scheme sequential both up" is configured, router will begin hunting from the very top of the list, which channel 1 of PRI 1, and work its way down:
_____PRI 1____|_____PRI 2_____|_____PRI 3_____
1 2 3 4 5...22 23 | 1 2 3 4 5... 22 23 | 1 2 3 4 5... 22 23
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