OSPF for IPv4 over IPv6 only?

[Nico Schottelius]

Good morning Bernd,

Bernd Naumann <bernd at kr217.de> writes:>
> Hi Nico,
>
> If every router has "at least" a /32 IPv4 address on loopback[1], you
> could do "OSPF unnumbered". It's not IPv4 over IPv6 but you do not need
> a bunch of IPv4 peer addresses on each and every interface.
>
> [1] IIRC I had some issues with that setup a few years ago, _BUT_ you
> can /just/ assign the /32 IPv4 loopback address on each OSPF interface,
> too.

Interesting hack, I love to the creativity. We actually do something
similar, albeit a different motivation already: For active-semiactive
routers we use

  /sbin/ip addr add 2a0a:e5c0::a:b::c/64 dev $IFACE nodad preferred_lft 0

With multiple routers having the same address, clients can/will switch
over after the neighbor cache expires, but as long as you can keep the
routing stateless, it doesn't need any external coordination software
such as keepalived.

> Otherwise: If you already have and can use BGP MP, why would you want
> IPv4 in OSPF, too? (Curious questions...)

We use BGP MP a lot, but not everyone we talk to does, resulting in:

    [ peer A ]
      |     |
     IPv4 IPv6
     BGP  BGP
      |
    [ router1 ]--- iBGP MP -------- [ routerX ]
      |
     BGP
     MP
      |
    [ peer X ]


Thus the nexthop for the routes received from peer A are using an IPv4
nexthop to which routerX needs access to, thus the requirement for using
OSPF supporting IPv4.

Interestingly we have a lot of connections as the one depicted with peer
X, where even from outside all routes we receive are always having an
IPv6 nexthop, making the whole setup significantly easier.

BR,

Nico

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