Re: BGP prefix filter list

[Blake Hudson <blake () ispn net>]

Baldur Norddahl wrote on 5/17/2019 11:05 AM:
> On Fri, May 17, 2019 at 3:28 PM Blake Hudson <blake () ispn net 
> <mailto:blake () ispn net>> wrote:
>
>      From my perspective one's ability to intelligently route IP
>     traffic is
>     directly correlated to the data they have available (their routing
>     protocol and table)
>
>
> One point perhaps being missed by some is that routing decisions are 
> not always best made in the very last moment when you have a packet 
> and need to decide on the destination. The culling of routing table I 
> wanted to do is on a full feed from my upstream providers. I am not 
> taking a default, but I may add a default manually.
>
> Think about this way to save at least half the size of the FIB with 
> two transit providers: Find out which provider has the most prefixes 
> going their way. Make a default to them and a route-map that drops 
> every route. For the other provider, keep only the routes where they 
> have better routing. This way you only use FIB space for the smaller 
> provider. Everything else goes by default through the larger provider.
>
> Now doing that in practice is hard because router vendors did 
> generally not make route-map or similar constructs flexible enough for 
> the needed logic.
>
> But we can do other things, some of which have already been proposed 
> in this thread. Like before have a default to the "best" of your 
> transit providers and using culling to drop routes. Are we not all 
> doing something like that already, with route maps to give some routes 
> higher priority instead of always going strict shortest AS-path? Only 
> difference is that you can fully drop the routes from FIB if you 
> install defaults to handle it instead.
>
> Or what if I know that one of my transit providers are really good 
> with Asia? I just want traffic to Asia by default go to them. I can 
> install my own covering routes from the APNIC address space and then 
> save a ton of FIB space by dropping routes within that space. I can 
> have exceptions if needed.
>
> The above does not give you poorer routing decisions and may give you 
> better.
>
> Regards,
>
> Baldur

Baldur, I believe most routing platforms already make use of clever 
shortcuts or techniques to reduce their FIB usage, but I don't think 
anyone has found a good, reliable method of reducing their RIB at zero 
cost. For example, what happens in your above configuration when your 
"better/default" transit provider is down due to maintenance or outage 
and your equipment continues to use its default route to direct traffic 
that direction? What happens if the transit provider that you normally 
only retain the best paths for becomes the best path for all 
destinations (for example if your connection to the better/default 
transit provider is down for maintenance or there is an upsteam peering 
change) and your router that normally only has a few thousand routes in 
RIB suddenly gets tasked with a 768k-1M route RIB?

I would argue that one can generally safely add information to his or 
her router's RIB (such as adding a local preference, weight, or 
advertising with prepends to direct traffic toward a better performing, 
less utilized, or lower cost peer), but that removing information from a 
router's RIB always comes at some cost (and some may find this cost 
perfectly acceptable).