Swapping routes without deletion

[Pim van Pelt]

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/
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