Consider the following statement from Cisco.com : “Routers do not form EIGRP neighbors over secondary networks.”
A Routing-BitsHandbook candidate queried this last week, claiming the statement is misleading and that EIGRP will indeed form an adjacency using a secondary IP address under specific conditions.
Consider the following configuration. R1 connects to R2 using a back-to-back serial connection. Both S1/1 interfaces have a primary and a secondary IP address defined. The EIGRP processes only matches the secondary IP addresses.
R1# interface Serial1/1 ip address 10.1.1.1 255.255.255.0 secondary ip address 10.5.1.1 255.255.255.0 ! router eigrp 1 network 10.1.1.1 0.0.0.0 no auto-summary R2# interface Serial1/1 ip address 10.0.1.2 255.255.255.0 secondary ip address 10.5.1.2 255.255.255.0 ! router eigrp 1 network 10.0.1.2 0.0.0.0 no auto-summary
So what do you think will happen in this scenario? Will R1 and R2 become adjacent? Cisco explicitly mentions that a secondary IP address is not used in the EIGRP hello packets, therefore EIGRP neighbors will not become adjacent using secondary IP addresses.
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