Swapping routes without deletion

[Pim van Pelt]

Hoi,

As a quick followup why I'm asking about versions -- on a Bird2.0.7, I do
see the delete-before-insert:

root at chgtg0:~# ip -6 monitor route | grep 2001:678:d78::6


# Raise OSPFv3 cost to prefer tf-0-0

*Deleted* 2001:678:d78::6 via fe80::21b:21ff:febd:c718 dev xe0-3.3102.20
proto bird metric 32 pref medium

2001:678:d78::6 via fe80::6eb3:11ff:fe20:e0c4 dev tf0-0 proto bird metric
32 pref medium


# Lower OSPFv3 cost to prefer xe0-3.3102.20 again

*Deleted* 2001:678:d78::6 via fe80::6eb3:11ff:fe20:e0c4 dev tf0-0 proto
bird metric 32 pref medium

2001:678:d78::6 via fe80::21b:21ff:febd:c718 dev xe0-3.3102.20 proto bird
metric 32 pref medium

groet,
Pim

On Sat, May 20, 2023 at 10:51 PM Pim van Pelt <pim at ipng.nl> wrote:

> Hoi folks,
>
> At Coloclue AS8283, we upgraded from Bird1.6.8 to Bird2.0.12 this week. We
> use two separate processes, one for IPv4 and one for IPv6 - and 2.0.7 in
> Debian is missing the ability to select 'accept ipv4' and 'accept ipv6' in
> BFD, so we installed backports and version 2.0.12).
>
> I am wondering if Bird2 later than 2.0.7 perhaps has an optimization when
> swapping routes? I would expect a swap to be "delete + add" but I am seeing
> only "add with new nexthop" appear in Netlink.
>
> Considering the following topology with link names and OSPFv3 costs
> associated:
>
>   dcg-1  bond0.130 ---- bond0.130 eun-2
>
>    |               2000             |
>
> enp1s0f3                         enp1s0f2
>
>    |                                |
>
>    | 10                          10 |
>
>    |                                |
>
> enp1s0f3                         enp1s0f3
>
>    |               1000             |
>
>   dcg-2  eno2.3469 ---- eno2.3469 eun-3
>
> If I restart the OSPFv3 protocol, I see that the topology settles in the
> expected way.  What I observed with bird 2.0.12 is that there is a deletion
> of the currently selected route followed by one addition, when the shortest
> path reveales (dcg1 - dcg2 - eun3 - eun2, ospf_metric1 is 1020, this is
> fine):
>
> root at dcg-1:~# birdc -s /run/bird/bird6.ctl restart ospf1
>
> root at dcg-1:~# ip -6 monitor route | grep 2a02:898:0:300::3
>
> Deleted 2a02:898:0:300::3 via fe80::669d:99ff:feb1:31af dev bond0.130
> proto bird metric 32 pref medium
>
> 2a02:898:0:300::3 via fe80::669d:99ff:feb1:3910 dev enp1s0f3 proto bird
> metric 32 pref medium
>
> Now I lower the cost of the dcg-1 -- eun-2 link from 2000 to 100, so that
> it becomes preferred (cost ospf_metric is 120):
>
> root at dcg-1:~# birdc -s /run/bird/bird6.ctl reconfigure ospf1
>
> root at dcg-1:~# ip -6 monitor route | grep 2a02:898:0:300::3
>
> *[[ HERE ]]*
>
> 2a02:898:0:300::3 via fe80::669d:99ff:feb1:31af dev bond0.130 proto bird
> metric 32 pref medium
>
> I would expect this new addition of the installed route on bond0.130 to be *preceded
> by a deletion* of the previous route from enp1s0f3, but this is not the
> case (marked in red with [[ HERE ]]).
>
> To anyone's knowledge: *Has this behavior changed between 2.0.7 and
> 2.0.12 ?*
>
> groet,
> Pim
> --
> Pim van Pelt <pim at ipng.nl>
> PBVP1-RIPE - http://www.ipng.nl/
>


-- 
Pim van Pelt <pim at ipng.nl>
PBVP1-RIPE - http://www.ipng.nl/
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