[mrtg] avoid "no response" error on unconnected ISDN router

[scott.list]

 I tried the archives, but am not sure what to look for, and had no sucess.

 I have a customer who connects to me periodically with an ISDN router that
I
 monitor with mrtg.  All is fine except when it's not connected.  I get the
 errors sent to me:

 SNMP Error:
 no response received
 bla, bla....

 Is there a way I can avoid these errors when the router is not
connected/up?

 Thanks for the help,
 Scott