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

[Ondrej Zajicek]

On Sat, Jan 26, 2019 at 02:57:53PM +0700, Igor Podlesny wrote:
> The docs say: "... secondary switch
> 
> 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.
> 
> 
> -- 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?

Hi

OSPFv2 does not allow multiple network prefixes per interface/network.
To do so, they must be treated as 'virtual' interfaces for the purposes of
OSPF (OSPFv3 fixed that by separating list of prefixes from network node).
But multiple IP addresses from the same network prefix should not appear
as separate iface. At least that is how it is intended and how it works
on Linux. But it seems that the second part does not work properly on BSD.

So it is useful in this case:

eth0: 10.0.0.1/24, 10.0.1.1/24, 10.0.2.1/24

but not in this case:

eth0: 10.0.0.1/24, 10.0.0.2/24, 10.0.0.3/24

-- 
Elen sila lumenn' omentielvo

Ondrej 'Santiago' Zajicek (email: santiago at crfreenet.org)
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"To err is human -- to blame it on a computer is even more so."