ospf stub interfaces appears as two entries in routing table (direct + ospf)

[Ondrej Zajicek]

On Thu, Nov 26, 2020 at 08:02:38PM +0000, Kenth Eriksson wrote:
> If I configure an OSPF interface as a stub interface, why does it appear twice the routing table? E.g. setting interface lo as stub interface results in the following;
> 
> 10.210.137.78/32     unicast [direct1 15:59:09.550] * (255)
>         dev lo
>                      unicast [ospfv2_1 20:47:56.864] I (145/0) [10.210.137.78]
>         dev lo
> 
> The source OSPF-I misleads me in thinking that it was learned from the OSPF area.

Hi

OSPF protocol in BIRD offers routes to all destinations in OSPF domain,
including ones that are currently best-reachable by a direct route.

Note that even if a prefix is a local stub/direct network, there may be
(in principle) an indirect route with lower OSPF cost, in that case OSPF
would supply that, while the Direct protocol (direct1) would still supply
the direct route.

It is also useful because direct routes from OSPF protocol have attached
OSPF metric, so in conjunction with BGP recursive next hop processing
proper IGP metric is used (if the route is not shadowed by route from the
Direct protocol).

The OSPF-I just means the route is intra-area route (which is true).

In general, having direct routes from Direct protocol is probably not
much useful, if you already have one from OSPF.

-- 
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."