Link local vs global v6 next hop for BGP rotuer

[Ondrej Zajicek]

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)
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"To err is human -- to blame it on a computer is even more so."