grg2,
The simple answer - most people I know who have taken the exam say - not much!
From the Cisco Exam topics page for the CCNA, here are the topics that have to do with ISDN -
- Choose WAN services to meet customer requirements
Understanding the difference between ISDN, PPP, Frame Relay, HDLC etc.
- Implement simple WAN protocols
Understanding IOS commands used to configure ISDN
- Perform simple WAN troubleshooting
Understanding show and debug commands used with ISDN
- Evaluate key characteristics of WANs
Once again - Understanding the difference between ISDN, PPP, Frame Relay, HDLC etc.
These are the 4 areas where you might see an ISDN question, with about 55 questions on the exam, you would be lucky to see 2-3 questions.
Here is a good link at Cisco that covers the basics of ISDN.
What book did you study for the exam - reread that chapter!
Also, if you have access to the Sybex CCNA Study Guide by Todd L. read it - always felt that Todd's book was one of the best for JUST getting the facts needed for the test.
I hope this helps!
E.A. Broda
CCNA, CCDA, CCAI, Network +