I read quite a few Cisco documentation that EIGRP will not allow adjacencies to form when using secondary addresses. This is wrong, but it is a common misconception that if both addresses can not be secondary. Let's look at R2 and R3 are using secondary addresses to form an EIGRP adjacency.
R2 (config) # interface Ethernet0
R2 (config-if) # ip address 172.12.23.2 255.255.255.0
R2 (config-if) # ip address 23.23.23.2 255.255.255.0secondary
R2 (config) # router eigrp 100
R2 (config-router) # no auto-summary
R2 (config-router) # network 23.23.23.0 0.0.0.255
R3 (config) # interface Ethernet0
R3 (config-if) # ip address 172.12.23.3 255.255.255.0
R3 (config-if) # ip address 23.23.23.3 255.255.255.0 secondary
R3 (config) # router eigrp 100
R3 (config-router) # no auto-summary
R3 (config-router) # network 23.23.23.0 0.0.0.255
Here is the part of the production of IP EIGRP neighbor showR3
R3 # show ip EIGRP neighbors
IP-EIGRP neighbors for process-100
H Address Interface
0 172.12.23.2 ET0
Adjacency has formed! Note: The address is actually the primary IP address for the interface, even if we used the number of secondary network command in EIGRP network. I personally stay away from the numbers of secondary network, if possible, but you should know that the secondary IP addresses can be used to create EIGRPvicinity.
What is the common mistake by using the secondary address has asked? That is, if an address is on the same subnet addressing the main interface to a neighbor and interface secondary to a different address. Suppose we had R2 and R3 configured as follows:
R2 (config) # int e0
R2 (config-if) # ip address 23.23.23.2 255.255.255.0
R3 (config) # interface Ethernet0
R3 (config-if) # ip address 172.12.23.3 255.255.255.0
R3 (config-if) # ip address 23.23.23.3255.255.255.0 secondary
R2, you get this message:
01:54:05: IP-EIGRP: Neighbor 172.12.23.3 not on common subnet for Ethernet0 (23.23.23.2 255.255.255.0)
Since R2 is configured as the primary address 23.23.23.2 interface has the look EIGRP process with the primary interface address potential neighbors. Since R3 is the first Ethernet0 address 172.12.23.2, you get the "not on common subnet" error message - and what he does not, is an adjacency! This is somethingconsciously on your BSCI 642-901 exam as well as working with networks EIGRP in production.
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