I’ve just received the following for a student who failed his first CCNA attempt:

“I had a problem with the EIGRP routing simulation. Another person who sat this exam & passed had the same simulation and neither of us could get it working. I am sure I know what the problem was, but just couldn’t get it working”

So for him, and anyone else sitting a CCNA exam with a routing simulation, here is RedNectar’s foolproof method of getting the simulation done correctly.
1. Issue the command:

 show ip protocols

Note the AS number (EIGRP) or process number (OSPF). Lets assume it is EIGRP on AS 10
2. Issue the command:

 show ip interfaces brief

Write down each interface IP (checking of course that they are correct)
3. Remove the old configuration, and replace it with a brand new one. That way you don’t have to figure out what was wrong. Example:

config t 
no router eigrp 10 
router eigrp 10 
network x.x.x.x 0.0.0.0 !where x.x.x.x is the first IP address you noted from show ip interface brief above
network y.y.y.y 0.0.0.0 !where y.y.y.y is the next IP address you noted from show ip interface brief above 
!add as many network statements as needed to exhaust the list
no auto-summary !You will probably need this too
end 
copy run start !don't forget to save

4. Repeat step 3 for the other router, but this time MAKE SURE the AS number is the same – if the 2nd router has AS 100, then:

config t 
no router eigrp 100 
router eigrp 10 ! - make sure both routers have the same AS number
!etc

Finally, check the configuration on all routers with:

show ip protocols
show ip eigrp neighbors !(or show ip ospf neighbors)
show ip route

Footnote:

A quicker method for step 3 above would be:

config 10
no router eigrp 10
router eigrp 10
network 0.0.0.0
end
copy run start

However, I don’t know how these questions are marked – I’m not sure if they parse your saved configs or check to see if the output of show ip route is correct. Safer to go with the method above I reckon.