Can OSPF establish a neighbor relationship by using the secondary IP address

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In OSPF, a neighbor relationship cannot be established by using the secondary IP address; instead, a neighbor relationship can be established only by using the primary address of an interface. If the network to which the secondary IP address belongs is added to the OSPF configuration, however, the corresponding route can be advertised.

Other related questions:
Can CE series switches use their secondary IP addresses to establish an OSPF neighbor relationship
CE series switches cannot use secondary IP addresses to establish an OSPF neighbor relationship.

Relationship between a secondary IP address and an OSPF IP address
They are actually the same address. A secondary address is usually referred to as an OSFP IP address on the NMS.

Can an OSPF neighbor relationship be established between devices that are on different subnets
A neighbor relationship can be established between two routers that are not on the same subnet only when the devices are connected through point-to-point (P2P) links. On a Point-to-Multipoint (P2MP) network, you can determine whether adjacencies can be formed between neighbors that are not on the same subnet. In all other cases, the devices must be on the same subnet.

Are the MTUs in the DD packets exchanged to establish an OSPF neighbor relationship checked
When the interfaces are configured with different MTUs, OSPF can normally set up the neighbor relationship. By default, OSPF does not check the MTUs in DD packets. When the ospf mtu-enable command is run on an interface, the OSPF neighbor relationship cannot be set up when the MTUs in the DD packets are inconsistent with the one set on the interface. In this case, the MTUs in the DD packets need to be checked. If the MTU in a DD packet is greater than the MTU set on the interface, the interface discards the DD packet.

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