Everything I read says that EIGRP summarizes on classful boundaries. When I look at the routing table (with auto-summary turned on), I see classless routes (ex. 10.210.0.0).
Can someone explain this to me why this is so. Thanks.
Also, if I implement "no auto-summary", what advantage will I have by utilizing the "ip summary-address eigrp" command under an interface, other than minimizing the routing table (and perhaps memory usage).
Auto-summary occurs across major networks ie.
(172.16.10.1/24) R1 (172.16.11.1/24) ->(172.16.11.2/24) R2 (192.168.5.1/24) ->(192.168.5.2/24) R3 (192.168.6.1/24) ->(192.168.6.2) R4 (172.16.12.1/24)
You have auto-summary turned on for R2 & R4.
In R1's routing table there will still be an entries for 172.16.10.0/24 and 172.16.11.0/24. And the same for R2.
But because R2 connects to R3 on an interface in the 192.168.5.0 network and it has auto-summary turned on it will advertise 172.16.0.0 to R3.
R4 is in the same postion. Because it has an interface in the 192,168.6.0 network and it has auto-summary turned on it advertises 172.16.0.0 to R3.
This is a problem because R3 is now receiving 2 routes for 172.16.0.0 goiing different ways.
The solution is to turn off auto-summary on either R2 or R4 so that the more specific routes are received from one of the routers. In reality you generally turn off auto-summary on both routers unless you need it.
The "ip summary-address eigrp" is used for something quite different. This is used to summarise a group of subnets within the same major network but not necessarily across major network boundaries.
The advantage of using it is as you say you minimize the routing table but also when you summarise with this command all more specific routes are suppressed. So if a change occurs in the network and the detail changes for one of the more specific routes it will not be propogated past the "ip summary-address eigrp". This is what the EIGRP stub network concept is for.