Cisco Router – Legacy ISDN calls fail, Call lasts about 20 seconds

Three main reasons for ISDN calls failing are going to be

Layer 1 Deactivated - Cable not plugged in or possibly a local telephone company line trouble.

  • Ensure the cable you are using is snuggly connected - you may want to try reseating the cord.

SPIDS showing as not valid

  • Need to have the local telephone  company test the lines and verify your SPIDS are correct
  • You can also try clearing your BRI interface (clear int BRI0), occassionally just performing this command can bring up a SPID that appears invalid at first glance
  • Last resort, reload the router, wait a few minutes - then clear the BRI0 card

Authentication issues

  • Tricky because issues with the SPIDS can make it look like an authentication issue.
  • Try removing and readding usernames/passwords on both end routers.  Or try removing and readding the CHAP authentication statement.

You can also run into issues where everything above checks out OK but ISDN still fails.  Cable connected, Telco says lines are good, Authentication appears correct.  There have been some instances where the telephone company changes the bandwidth to 56k rather than the normal 64k - this seems to happen on a whim and can cause troubles for your current configuration.

Here's the configurations required for changing your speed to 56k (Dial on Demand and Dialing Profiles), from Cisco

To resolve speed mismatch issues, configure the call for 56K using one of these methods methods:

  • For dialer profiles:
  maui-soho-01(config)#interface Dialer1
  maui-soho-01(config-if)#dialer string 81560 class 56k
!---Dial 81560 and use the map-class named 56K (defined below).
  maui-soho-01(config)#map-class dialer 56k
!---The map-class named 56K that was used with the dialer string is int Dialer1.
  maui-soho-01(config-map-clas)#dialer isdn speed 56
!---Set the speed of the call to 56K (default is 64k).
  • For Legacy Dial-on-Demand Routing (DDR) (dialer maps):
  maui-soho-01(config)#interface bri 0
  maui-soho-01(config-if)#dialer map ip name maui-nas-08 speed 56 81560
!---The keyword speed 56 sets the outgoing call rate at 56K.

Authentication not valid - check your username/passwords on both ends, or try re-adding them.