Today I ran into some issues with a lab I was doing involving an eigrp default route and summarisation.
Firstly the default route, basically the task was to inject the loopback interface IP into eigrp as the default route. Since the loopback is a directly connected interface a static route would not work so I used the ip default-network [address] command.
However after I entered this command I checked the RIB and found the route was not marked as the candidate default route. I checked and double checked the config (which looked fine to me) and eventually had to resort to cisco.com. It was there I found the simple mistake I made.
When I entered the command, I used the IP address of the loopback interface but the ip default-network command is classful… meaning I needed to enter the class C (in this case) address of the loopback interface, once I corrected this I checked the RIB again and there I could see the correct route marked as the candidate default.
On to the summarisation issue. The task was to summarise four loopback networks which were emulating a LAN network connected to the router. I worked out the correct summary address/mask and entered the ip summary-address eigrp 1 [summary address] [metric] command. When I checked the RIB there was no summary route, so again I double checked my summary address/mask and the command before I hit google.
Here I found that when you enter a summary route you need to enter the interface IP address that is part of the summary route. If you just enter the summary subnet and mask, and there is no interface with that IP address configured the command is allowed but IOS does not put the summary into the routing table. Once I entered the interface IP (which was part of the summary route) and the summarised mask the summary route was added to the RIB and all was well.
Here is a link to the ciscopress.com chapter from Troubleshooting EIGRP CCIE Professional Development series that explains the summary route issue I described above.