OSPF, BSD: every additional IP on an interface gives another neighbour

[Igor Podlesny]

The docs say: "... secondary switch

On BSD systems, older versions of BIRD supported OSPFv2 only for the
primary IP address of an interface, other IP ranges on the interface
were handled as stub networks. Since v1.4.1, regular operation on
secondary IP addresses is supported, but disabled by default for
compatibility. This option allows to enable it. The option is a
transitional measure, will be removed in the next major release as the
behavior will be changed. On Linux systems, the option is irrelevant,
as operation on non-primary addresses is already the regular behavior.
..."

Let's focus on: "... Since v1.4.1, regular operation on secondary IP
addresses is supported, but disabled by default for compatibility.
This option allows to enable it. ..."

My observations show that what actually happens when whether this
option is On or it's Off is not anything regular. Instead for every
additional IP there's "virtually" additional OSPF neighbour instance.

bird> show ospf
ospf1:
RFC1583 compatibility: disabled
Stub router: No
RT scheduler tick: 1
Number of areas: 1
Number of LSAs in DB:   23
        Area: 0.0.0.0 (0) [BACKBONE]
                Stub:   No
                NSSA:   No
                Transit:        No
                Number of interfaces:   3
                Number of neighbors:    3
                Number of adjacent neighbors:   3

-- every IP-address on interface actually handled as separate
interface with that IP, that's why this weirdness with neighbour count
is seen.

O_o

Is it so on Linux as well, or BSD implementation specific?

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