Ascending and descending refer to *numerically* ascending or descending
“ascending” numerically (from channel 1 to channel N) means going “top down”.
“descending” numerically (the default; from channel N down to channel 1) means going “bottom up” on the ladder.

descending is 23-1 as is the default

Top Down = Assigned starting at channel 1 and working through the high end (23 on a T1 PRI)
Bottom Up = Assigned starting at channel 23 (T1 PRI) and working through to the low end

int s0/0/0:23
isdn bchann selection-order ?

show isdn status
debug isdn q931
debug isdn q921

To find out which channels are in service, which are out of service, use command,
sh isdn service.
sh isdn service

About Alexis Katsavras

Working as Freelance Cisco Unified Communications Consultant in the UK.