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EIGRP neighbors do not form over the secondary network

Core Issue

The Enhanced Interior Gateway Routing Protocol (EIGRP) supports secondary addresses. However, since EIGRP sources data packets from the primary address, all the routers should be configured with the primary addresses that belong on the same subnet.

Resolution

EIGRP only forms neighbors over the primary network. Be sure that the primary address on the interface is configured for EIGRP by issuing the network command under the router eigrp configuration command. This network should include the primary address of the interface which runs EIGRP.

This is an example:

interface e0
ip address 172.16.3.1 255.255.255.0
ip address 131.102.3.1 255.255.255.0 secondary

router eigrp 7
network 172.16.0.0
!-- This network covers the address assigned to interface e0.
network 131.102.0.0

The 131.102.0.0 network covers the secondary IP address assigned to interface e0.  Although the 131.102.0.0 network is also included in EIGRP, EIGRP will not form a neighbor over this network. However, the 131.102.0.0 network is advertised through EIGRP.If there is a mismatch between the primary address configured on the interfaces there occurs "not on common subnet for Ethernet0" error message.

For more information about EIGRP neighbors, refer to the Neighbor Discovery and Maintenance section of Enhanced Interior Gateway Routing Protocol.

For more information about Not On Common Subnet error message, refer to the Troubleshoot section of What Do EIGRP "Not On Common Subnet" Messages Mean?

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