I dont like auto-summarization, especially since you can always manual summarize networks as you please in EIGRP. But it seems to be enabled by default in all the IOS versions I use and it's probably a good idea to know how it works for the lab.
EIGRP auto-summarizes at classful network boundaries. This means that if one interface is in 1.1.0.0/16 and another interface is in 2.2.2.0/24 then EIGRP will summarize the networks before advertising them out of the opposite interface. This is because each subnet belongs to a separate major classful network: 1.0.0.0/8 and 2.0.0.0/8 in this case.
However if you have one interface in 2.2.2.0/24 and another interface in 2.3.0.0 then auto-summarization will not take affect because both interfaces belong to the major network 2.0.0.0/8. EIGRP may auto-summarize for some networks and not others in the same EIGRP process. Here we take a look:
[R1]----[R2]
R1 and R2 are connected to the same LAN 12.0.0.0/16
R1 has two loopbacks:
R1#show run | section interface Loopback
interface Loopback0
ip address 1.1.1.1 255.255.255.255
interface Loopback1
ip address 12.1.1.1 255.255.255.0
R2 also has two loopbacks:
R2#show run | section interface Loopback
interface Loopback0
ip address 2.2.2.2 255.255.255.255
interface Loopback1
ip address 12.2.2.2 255.255.255.0
All interface have EIGRP enabled with auto-summary on (default).
R1's route table:
R1#show ip route | begin Gate
Gateway of last resort is not set
1.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C 1.1.1.1/32 is directly connected, Loopback0
D 1.0.0.0/8 is a summary, 00:09:40, Null0
D 2.0.0.0/8 [90/156160] via 12.0.0.2, 00:09:34, FastEthernet0/0
12.0.0.0/8 is variably subnetted, 4 subnets, 3 masks
D 12.2.2.0/24 [90/156160] via 12.0.0.2, 00:09:43, FastEthernet0/0
C 12.1.1.0/24 is directly connected, Loopback1
C 12.0.0.0/16 is directly connected, FastEthernet0/0
D 12.0.0.0/8 is a summary, 00:09:40, Null0
R2's route table:
R2#show ip route | begin Gate
Gateway of last resort is not set
D 1.0.0.0/8 [90/156160] via 12.0.0.1, 00:19:01, FastEthernet0/0
2.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C 2.2.2.2/32 is directly connected, Loopback0
D 2.0.0.0/8 is a summary, 00:18:55, Null0
12.0.0.0/8 is variably subnetted, 4 subnets, 3 masks
C 12.2.2.0/24 is directly connected, Loopback1
D 12.1.1.0/24 [90/156160] via 12.0.0.1, 00:00:02, FastEthernet0/0
C 12.0.0.0/16 is directly connected, FastEthernet0/0
D 12.0.0.0/8 is a summary, 00:18:55, Null0
R2#
Analysis:
Looking at R1's route table you can see that R2 has summarized the route 2.2.2.2/32 into 2.0.0.0/8. This is because R2 sits in the boundary between two classful networks 12.0.0.0 and 2.0.0.0. It does not summarize loopback 1 which is 12.2.2.0/24. This shows up on R1 with the 24 bit mask.
On R2 the same can be said for routes from R1. R1 summarizes it's loopback0 interface into 1.0.0.0/8 because it sits on the boundary between 1.0.0.0 and 12.0.0.0. It does not summarize 12.1.1.1/24.
RIP version 2 auto-summarization works pretty much the same way as far as I can tell. If you know any differences please let me know.
