OSPF: Route attribute "ospf_tag" lost between different OSPF instances

[Ondrej Zajicek]

On Tue, Dec 27, 2016 at 07:38:35PM +0100, markus/grundmann wrote:
> Hello!
> 
> I believe that I'm a user with BIRD experiences but current I have no
> idea why the attribute "ospf_tag" from one OSPF instance to another is
> unavailable.

Hello

The issue here is that route export to OSPF is a bit counterintuitive.
Instead of keeping the attributes (like e.g. when route is reexported
between BGP protocols), OSPF attributes are reset before export filters
are executed. So both tag and metric is lost when an OSPF route is
propagated between OSPF instances. There is a hardcoded exception for
static routes, so you could set OSPF attributes for them directly for
static protocols.

The origin of this behavior is probably intentional (i am not sure), but
it seems to me that it is confusing, i would assume that a route
attribute would keep existing unless explicitly modified. It is one
behavior i would like to change in the future.

Question is what is a good default behavior when route is propagated
between OSPF instances. Should routes keep their tags and metrics? That
might be confusing as the metric may have different interpretations. Also
received Ext2 route has both metric2 and metric1 part, but the second one
have to be forgotten during reexport. Or should we have two different
classes of OSPF route attributes (say ospf_tag and ospf_rcv_tag) to avoid
such automatic attribute reexport without attribute modification magic?

-- 
Elen sila lumenn' omentielvo

Ondrej 'Santiago' Zajicek (email: santiago at crfreenet.org)
OpenPGP encrypted e-mails preferred (KeyID 0x11DEADC3, wwwkeys.pgp.net)
"To err is human -- to blame it on a computer is even more so."
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