Re: maximum ipv4 bgp prefix length of /24 ?

[Owen DeLong via NANOG <nanog () nanog org>]

First, no, a transient where all route disaggregating disappears from the global table is extraordinarily unlikely. 

Second, as I understand it, each update cycle results in rebuilding the fib from scratch rather than figuring out how 
to splice and dice it, so the computation required to cope with that single /24 change isn’t exactly as described. 

Finally, unless that /24 change ends up changing the exit interface or next-hop on the exit interface (which decreases 
in probability with topological distance), it wouldn’t actually change the fib content. 

The process of downloading a new fib to line cards from the RE is very fast. Break before make may be tolerable risk. 
It won’t lose a statistically significant number of packets in a single event. If events are occurring frequently 
enough to be an issue, then I think the route flapping is a bigger problem than the lost packets. 

YMMV

Owen


> On Oct 1, 2023, at 21:55, Jakob Heitz (jheitz) via NANOG <nanog () nanog org> wrote:
>
> 
> While I did allude to some of the complexity, my main point
> is that FIB compression does not allow you to install a FIB with less memory.
> Because you must be prepared for transients during which the FIB needs to store
> mostly uncompressed anyway.
> All it does is to increase convergence time.
>  
> Kind Regards,
> Jakob
>  
>  
> From: William Herrin <bill () herrin us>
> Date: Sunday, October 1, 2023 at 6:32 PM
> To: Jakob Heitz (jheitz) <jheitz () cisco com>
> Cc: nanog () nanog org <nanog () nanog org>
> Subject: Re: maximum ipv4 bgp prefix length of /24 ?
>
> On Sun, Oct 1, 2023 at 5:40 PM Jakob Heitz (jheitz) via NANOG
> <nanog () nanog org> wrote:
> > Among the issues:
> > Suppose the FIB has all the /24 components to make a /20, so it programs a /20.
> > Then one of the /24's changes nexthop. It now has to undo all that compression
>
> Yeah... all this stuff is on the same level of complexity as
> implementing a B-Tree. Standard task on the road to an undergraduate
> computer science degree. Compared to decoding a BGP update message,
> where nearly everything is variable length and you have to nibble away
> at the current field to find the start of the next field, this is a
> cakewalk.
>
> It doesn't actually get complicated until you want to do more than
> just joining adjacent address blocks.
>
> Regards,
> Bill Herrin
>
>
>
>
> -- 
> William Herrin
> bill () herrin us
> https://bill.herrin.us/