bird adversites a direct route from an interface without carrier

[Ondrej Zajicek]

On Thu, Oct 21, 2021 at 02:47:12PM +0200, Alarig Le Lay wrote:
> Hi,
> 
> On Tue 19 Oct 2021 13:48:53 GMT, Ondrej Zajicek wrote:
> > Hi
> > 
> > You use route from Direct protocol exported to OSPF. Contrary to other
> > protocols, default value of 'check link' option for Direct protocol is
> > 'no'. Just enable it:
> > 
> > https://bird.network.cz/?get_doc&v=20&f=bird-6.html#direct-check-link
> 
> Ah yes, indeed, the route has now disappeared, thanks!
> 
> May I ask why it’s not enabled by default? I don’t see why one would to
> announce an down link without an explicit configuration for it.

At first, i thought it is mostly for historical reasons, as we generally
do not change defaults during minor releases, but there are two arguments
against it:

1) It is the same behavior as Linux kernel, which also does not remove
direct route when link goes down. Although recently they added 'linkdown'
route flag for that purpose.

2) There is one address that is reachable even when link is down - the
local IP address of that iface. If routes from the Direct protocol are
announced to say RIP or Babel, then removing then when link is down would
lead to loss ofvalid connectivity to the router's address. This could be
handled by announcing the IP address as /32 instead of the full prefix
(which IIRC is done by OSPF in such situation), but that is complex and
perhaps unexpected behavior to be done by default.

-- 
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."