We are using 172.20.0.0 internally with /25 mask for local user subnets. This network is spread around 40 locations. Normally we have 'no auto-summary' under our router command like:
router eigrp 1
So if I wanted to know if a particular subnet was in use, ie, 'sh ip route 172.20.100.0', I would see that in a routing table, and felt confident that there truly was a 172.20.100.0 subnet.
Someone inadvertently configured a router with 'auto-summary', and I was trying to troubleshoot a problem on 172.20.100.0. I shut down the interface that is configured for 172.20.100.0, yet it still showed up in the routing table. I track down where that route was advertised, and I found it was coming from a router that has the 'auto-summary' and the 'show ip route 172.20.100.0' on that router showed it coming from interface null-0.
The reason I don't like to use auto-summary is because of just that - I don't get a true picture of what subnets are actually real - everything gets summarized into the major network.
Is that the way auto-summary is supposed to work? If one uses the 'auto-summary on all routers, how does one tell if a particular subnet is in actual use of not?