Link local vs global v6 next hop for BGP rotuer

[Alexander Zubkov]

Hi,

In documentation part it is written that the default for the option is
"disabled". But in the code I see it chooses the value based on
"gw_mode":
> Different default for next_hop_prefer

Am I missing something or there is a typo?

On Mon, Oct 10, 2022 at 5:44 AM Ondrej Zajicek via Bird-users
<bird-users at network.cz> wrote:
>
> On Wed, Sep 21, 2022 at 08:47:03AM +0000, Mazur, Dariusz wrote:
> > Hello Ondrej,
> > Thanks for quick response. Additional option to ignore link-local BGP next hops would help us a lot.
>
> Hello
>
> Done, here is a commit with BGP channel option 'next hop prefer global',
> which makes BIRD to use global next hop instead of link-local:
>
> https://gitlab.nic.cz/labs/bird/-/commit/8f79e6b93e32a4eb7e4dda9bd4a9d04400b79d45
>
>
> > On 9/20/22, 6:11 PM, "Ondrej Zajicek" <santiago at crfreenet.org> wrote:
> >
> >     On Mon, Sep 19, 2022 at 01:23:28PM +0000, Mazur, Dariusz via Bird-users wrote:
> >     > Hello,
> >     > In my setup I have iBGP session with peer , and I learn v6 route. It is iBGP so we have 2 next hops global and link-local. By default bird uses link-local when route is exported to kernel. Is there any command to change this behavior and use global v6 address as BGP next hop when we export routes to kernel ?
> >
> >     Hello
> >
> >     The dual next hop for IPv6 routes is kind of hack and not really
> >     reflected in filter code, which means that bgp_next_hop attribute
> >     represents just global address. Therefore, you can do:
> >
> >       bgp_next_hop = bgp_next_hop;
> >
> >     in remote IBGP export filter to reset bgp_next_hop attribute to just
> >     its global address.
> >
> >     I think it would not work in local IBGP import filter - we assign
> >     recursive immediate next hop based on bgp_next_hop when route is received
> >     before import filter, so later change of bgp_next_hop in IBGP import
> >     filter would not affect existing immediate next hop.
> >
> >     You can also change immediate next hop (route attribute 'gw'), either in
> >     local IBGP import filter, or Kernel protocol export filter:
> >
> >       gw = bgp_next_hop;
> >
> >     but that would work only if you have flat network and there is no
> >     intermediate hop between local system and bgp_next_hop.
> >
> >
> >     We probably should just add option to ignore link-local BGP next hops ...
> >
> >
> >     > Table master6:
> >     > fc00:2001:100:12::/64 unicast [2001:4878:c225::1:3:2__r02.ien.labkrk01.sdn 12:57:29.076] * (100) [i]
> >     >                 via 2001:4878:c225::1:3:2 on eth-1\1\4
> >     >                 Type: BGP univ
> >     >                 BGP.origin: IGP
> >     >                 BGP.as_path:
> >     >                 BGP.next_hop: 2001:4878:c225::1:3:2 fe80::1:3:2
> >     >                 BGP.local_pref: 400
> >     >                 BGP.community: (20940,550)
> >     >
> >     > Thanks,
> >     > Dariusz
>
> --
> 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."
>