Displayed update timestamps vary by 1 second for the exact same update

[Maria Matejka]

Hello!

This is probably due to BIRD updating the "current real time" once per 
IO cycle which is in idle case once per 3 seconds. This may change in 
future versions, anyway I wouldn't trust the time as an indicator of 
route change at all.

The most reliable way is to write your own protocol inside BIRD <^-^> 
and get the updates from the source directly. Anyway, it takes some 
work. (We plan to make this method simple, hopefully in near future.)

Another quite reliable way is to export the routes to kernel and then 
run 'ip monitor' or 'route monitor' command to read them. This anyway 
drops some attributes.

Or you can use the script to compare the MD5 of "show route all" 
outputs, yet you should probably s/\d\d:\d\d:\d\d/<TIME>/ (or something 
like that). The output is also done by walking over a hash table which 
may rehash sometimes and change the overall order of prefixes. Anyway, 
rehashing happens only when a route is changed (no sudden rehashes out 
of nothing).

Maria

On 11/25/21 9:51 PM, bird21 at sensation.net.au wrote:
> Hi all,
> 
> If I do several consecutive "show route" queries, the timestamp keeps 
> changing back and forth, by about a second. Example:
> 
> 0.0.0.0/0          via x.x.x.x on em2.100 [aapt 05:20:28] * (100) [AS2764i]
> 0.0.0.0/0          via x.x.x.x on em2.100 [aapt 05:20:28] * (100) [AS2764i]
> 0.0.0.0/0          via x.x.x.x on em2.100 [aapt 05:20:27] * (100) [AS2764i]
> 0.0.0.0/0          via x.x.x.x on em2.100 [aapt 05:20:28] * (100) [AS2764i]
> 
> I notice (via public route servers) that Quagga and FRRouting seem to do 
> the same thing. Example:
> 
>        Last update: Thu Nov 25 18:20:31 2021
>        Last update: Thu Nov 25 18:20:30 2021
>        Last update: Thu Nov 25 18:20:30 2021
>        Last update: Thu Nov 25 18:20:31 2021
> 
> Does Bird do this in order emulate some convention (like perhaps 
> something Cisco has done for decades), or is there some other reason 
> it's displaying an inconsistent timestamp?
> 
> I have a script that compares the md5 of the current output with the 
> previous minute to decide whether to log the routing table, but because 
> of this quirk the md5 changes almost every time...
> 
> So, is this a bug, or feature? :) And is there some other reliable way 
> to determine that Bird's routing table has changed?
> 
> Thanks.
>