Preparing the CCNA is being challenging, troubleshooting just guessing is consuming time, besides you get quicker results if you define some steps about how to proceed. This is the procedure I will normally follow if something is not working properly.
1. Checking the interfaces are in UP/UP state
Before starting with routing protocols, verify the interfaces are working properly
R1> show ip int brief
2. Checking L2
Review if there is any problem on the serial link:
- KeepAlive removed from one router, it will appear up/up for that interface, on the other end of the link up/down
- Authentication with either wrong username or wrong password will show down/down on both ends of the link.
- Mismatched Encapsulation will show down/down on both ends of the link.
3. EIGRP neighbors and AS
Confirm that you have the expected neighbors, besides examine the AS is the same in all the routers.
R1> show ip eigrp neighbor
4. EIGRP interfaces
It may occur that either some interfaces are not enabled or there are some interfaces enabled with a wrong network command.
R1> show ip eigrp interface
If there is some interface that is enabled (from the previous step), but neighbors routers do not see that network, review the configuration. Regarding a network command, review the command itself or the wilcard.
i.e: network 10.0.0.0 (but you actually have 10.4.0.0) i.e: network 126.96.36.199 0.0.0.1 (actually you want 0.0.0.3 for /30) R1> show ip protocols
The above command will show if there is any error, with the network definition. If the network command for that interface was not added, it will not appear.
5. K-values and passive interfaces
Actually, we can get the next information from the previous check, however I think is much clear to review it aside.
R1> show ip protocols
This command shows you any passive-interface, what it would actually avoid to establish neighbor relationships. Remember that K-values must match on both routers, you can check the values using the previous command as well.
R1(config-router)# no passive-interface s0/0/0
6. EIGRP Authentication
I’m not sure if this topic is covered for the CCNA, however is for sure one of the issues you may find. In this case, configuring:
- Key chain
- Key ID
- Key String
The previous parameters must agree when setting up EIGRP authentication.
7. Multicast in serial links
If Frame Relay is configured on a physical interface, broadcast will not be supported, the same occurs for point-to-multipoint links. Define subinterfaces or modify the ospf network type.
In multipoint networks add the broadcast keyword at the end. The show frame-relay map shuld show broadcast, otherwise it will not work.
R1(config-if)# frame-relay map ip <IPADDRESS> <DLCI> broadcast
8. Access lists filtering
At this point everything has been configured properly, you can see one router is showing adjacency going up and down. Take a look if there is any access list blocking the IP traffic.
R1# show access-list
Definitely a guess method is not an option when you are trying to narrow down some issues and the clock is not from your side.
In my opinion an analysis through the layers L1, L2, L3 and EIGRP details should point you out the error.