handling of unreachable routes with gateway direct

[Ondrej Zajicek]

On Thu, Oct 01, 2020 at 11:19:24AM +0200, Alexander Zubkov wrote:
> Hello,
> 
> It seems reasonable, of course, but it breaks things for our case. So
> I want to suggest some changes to make our setup possible. At the
> moment, I consider the possibility of adding something like "gateway
> <direct_with_recursive_fallback>" so that it would behave like
> "gateway direct", but in the case of a non-reachable neighbour, it
> would switch to recursive logic. What do you think? Whether this is a
> good idea or maybe you can suggest some better variants?

Hello

Sorry for late answer. Perhaps it would make sense to have such hybrid
mode (i would imagine it like 'if bgp_next_hop is from same network as
BGP neighbor, use gateway direct, otherwise use gateway recursive'), but
it seems like just another hack to avoid limited recursive handling
(not supporting double recursion, which is a valid configuration).

Also, it is something that could be done in import filter. Setting 'gw'
attribyte removes recursive behavior for a route, so if you set BGP to
'gateway recursive', you can use:

  if bgp_next_hop = from then gw = bgp_next_hop;

To accomplish what you need.


> We have a couple of reasons why we want "gateway direct":
> We want to avoid double recursion, which is not allowed - we might
> have some other recursive routes that use direct routes received in
> this session. We could make separate sessions for direct and transit
> routes, but I'm not sure that the whole thing would be simpler.
> And there is an issue with next-hop resolution when we have the same
> ips on different interfaces. As I understand, bird daemon currently
> does not support that case.

-- 
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."