No, they're asking why a single set of routers is in charge of announcing BGP routes for all of facebook. If you have multiple ASes, with independent configuration sources and independent routers broadcasting them, it's a lot harder to break everything at once.
However if even one set of routers were misconfigured and it was announcing incorrect routes for all their ASes as result of the issue then their peers will not typically drop that set alone automatically.
BGP doesn't have a Paxos / Raft style smart consensus algorithms, it runs on trust. Either their peers had to trust what FB published or they won't be peering with FB ASes in the first place.
That's what I was meant when I said it comes down to one network web of trust, and if there is a breakdown of that redundancy cannot typically help
The config database would reject routes for the wrong ASes. The router would reject it. The peers would be told "add filters so you only accept these ASes from these routers".
Maybe they have all that and it somehow broke anyway? But what it looks like from the outside is that all the ASes are controlled by the same system.
> their peers will not typically drop that set alone automatically.
I'm not sure what this sentence means.
> router would reject it.
Any of these hardware could have bugs, if one of them announces wrongly it will be propagated wrongly by all other ASes peering with them and to the next level so on and on. That is the point, at this level it is possible to fuck up globally.
> peers would be told "add filters so you only accept these ASes from these routers".
There are hundreds of ISPs , all of them peer cannot directly with each other. Routes are propagated downstream and upstream it is a web of networks running on trust.
While filtering is built into most implementations ( sadly not the protocol itself), practically ISPs have no easy way to determine which AS can actually originate which other AS'es traffic, so they don't actually implement a lot of filtering. Remember traffic can have more than 2 hops. Effectively that means you would be routing traffic for AS 3/4 hops away. Neither you nor your peer would know anything about it or whether you can trust it etc.
Even if some ISPs do drop/block the announcements, unless every single AS also implements the block there won't be an impact. Traffic would route through ASes which don't have filtering and announce the routes incorrectly . For example say AT&T blocks an incorrectly announced FB route, but British Telecom does not, BGP is designed to assume that FB has lost peering with AT&T and route all traffic for FB via British telecom.
If filtering was robustly possible we wouldn't have periodic BGP hijacking incidents as we do whether accidental or maliciously. The famous Pakistan Telecom Youtube hijacking [2] or as recently as April-2021 [3] or incidents over the last few years usually authoritative governments (such as China/Russia etc) but with impact well beyond their networks.
[1] http://www.bgpexpert.com/article.php?article=145
[2] https://www.ripe.net/publications/news/industry-developments...
[3] https://blog.apnic.net/2021/04/26/a-major-bgp-route-leak-by-...
