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I have a super rudimentary understanding of what BGP is (it allows you to advertise addresses something something), but I don't really understand why it's useful and thus I'm finding TFA to be a little inaccessible. Could you provide some context about what you're using BGP for (what is your use case; what does it enable for you; etc) such that it might fill in some gaps for me?

BGP is what "the Internet" uses to say what IP addresses can be reached over the various network links that connect different providers. If you had links to two different providers, you would talk BGP to them and say "You can reach this set of IP addresses via me." They then propagate that announcement on to other providers that they connect to.

This is done via ASNs (Autonomous Systems Numbers), like 32150 (my old ASN). Each hop along the way gets their ASN tacked on. It's like a flood-fill to all the BGP routers in the world.

When you "traceroute", you see IP hops along the way, but the ISPs routers choose links based on this list of autonomous paths (among other things). Basically, a AS-level traceroute is built into the BGP routing information. At it's most basic, a path is selected based on the shortest AS path (traceroute, but at the organization level rather than IP level); the fewest number of networks the traffic has to traverse.

If a link goes down (either because of a network failing, or because of administrative reasons), traffic will eventually switch over to another way of reaching the destination. You can also do things like pad the AS path with multiple copies of your AS to cause traffic to switch to another link more frequently, as a kind of primitive load balancing.

>"If you had links to two different providers, you would talk BGP to them and say "You can reach this set of IP addresses via me."

There is really need to do this is many cases though. If all you are looking for is connectivity redundancy you can just take the default routes from two ISPs and configure floating static routes on your router and not worry about BGP at all.

>"This is done via ASNs (Autonomous Systems Numbers), like 32150 (my old ASN). Each hop along the way gets their ASN tacked on. It's like a flood-fill to all the BGP routers in the world."

Each hop does not get its own ASN tacked on. This only happens when crossing an AS boundary i.e at eBGP speaker. There are generally many hops inside an AS.

> There is really need to do this is many cases though. If all you are looking for is connectivity redundancy you can just take the default routes from two ISPs and configure floating static routes on your router and not worry about BGP at all.

That only gives you outgoing redundancy. You can talk to the world with one of the links down, but the world can't necessarily talk to your IPs with the link down.

BGP lets you advertise IPs to the world and what links they're reachable on. You can announce the same IPs on multiple links to multiple ISPs. This is what makes the internet "route around damage".

Generally ISPs will only let you announce the same prefixes to multiple upstream ISP if you have PI address space. If you have PA address space you are probably out of luck. An ISP won't accept your prefixes if they are PA space from another ISP. At any rate you can can still use floating static routes and use a GSLB provider like Dyn to get your incoming redundancy and still not need to run BGP. In fact if you have PA address space this is your only option.

>Each hop does not get its own ASN tacked on

Correct if we are talking about IP hops (router 1 to router 2 to router 3), the AS does get tacked on (possibly multiple times) for each AS hop (Level-3 to Cogent to Zayo). Thanks for clarifying that.

The internet is a decentralized collection of autonomous systems (AS) and the primary benefit of this setup isn’t exactly realized afaict, which is multiple paths via different carriers. Your BGP AS is advertised via N paths, so your IP networks are thus reachable via multiple paths. Systems will take the shortest/best path adding to redundancy and throughput. Compared that to “business” service without routing options, when your “dedicated” connection drops, your IP addresses are no longer reachable from the greater internet.

>"The internet is a decentralized collection of autonomous systems (AS) and the primary benefit of this setup isn’t exactly realized afaict, which is multiple paths via different carriers."

The primary benefit if fully-realized and is not about "is multiple paths via different carriers."

An AS is an administrative entity. Route prefixes belong to an AS. All routes with an AS have a common routing policy that is managed by a single entity. I can look up a prefix to find it's AS and then I can look up that AS's routing policies. Here is an example for Tier 1 ISP Spring:

https://www.sprint.net/policies/bgp

I could have single ISP that I buy transit from and I might have multiple links with them. If I want to understand how to take advantage of that for example with traffic engineering then I would look the policies for ISPs AS. I would then use those BGP community attributes on my BGP links to them in order to accomplish my own routing goals.

> I have a super rudimentary understanding of what BGP is (it allows you to advertise addresses something something)

The TL;DR:

Yes, BGP allows you to advertise addresses.

Ergo, it is allows you to build resilience, i.e. you can connect to multiple independent ISPs and advertise the same addresses. Hence traffic will always be able to reach you even if one ISP goes FUBAR.

By the same token, it makes pretty much zero sense to have BGP on a home connection (or, in technical speak, a "single-homed connection"). Since really BGP is a zero-sum game on single-homed connections (no real benefit, much added complexity).

> It's frustrating that some ISPs make it artificially difficult for home users to obtain a BGP session

Don't worry, it can be frustrating for businesses too !

Cogent famously nickle-and-dime their customers and consider BGP to be a chargeable extra, even on their IP Transit product which is somewhere where you would expect BGP to be a given.

Of course whether you should do business with Cogent is another matter, especially given their spammy cold-calling tactics, seemingly once you're on their list you can never get off it.

You're lucky. There are basically no small ISPs around here anymore. It's the cable company (DOCSIS) or telco (fiber, or DSL if you're unlucky.) Without paying for an enterprise grade connection, it would be impossible to get a BGP session through either.

I also have a /24 for myself. I've had it since the mid-90's, from the InterNIC days, predating ARIN. I've gone years without using any of the addresses. Currently it's routed to my home lab. I'm doing BGP through a couple VPSes, then tunneling it back.

I have a /24 as well, can you point me to any links or tutorials of how I can actually use it with my home connections

This person has a good overview: https://varunpriolkar.com/2018/12/bgp-on-a-shoestring-the-de...

Basically, you get an ASN, find a BGP-friendly VPS or colo, announce your /24, and tunnel it back to your home.

> This practice of requiring /24 subnets may have made sense in the past to preserve router memory, but with today's technology, it's nonsensical and only serves to artificially deplete available IPv4 addresses.

You should be surprised about how much of the internet still runs on gear from late 1990s

There seems to be a lot of misconceptions in your post.

>"Additionally, running one more BGP session makes their network appear larger, so they benefit as well."

This doesn't make any sense. The number of "BPG sessions" is not a metric any ISP thinks about. A BPG session is just TCP connection. The number of prefixes an ISP is advertising might be something they care about however but for smaller ISPs that doesn't really matter as they are just customers of the Tier 1 ISPs and that just requires paying the Tier 1 ISP. It has nothing to do with the number of sessions or prefixes. I could have a single /22 prefix for my company and the Tier 1 ISP will still sell me the same service.

>"There's no rule that says ISPs should only accept /24 or shorter prefixes, and this is contributing to the depletion of IPv4 addresses. I would happily announce a /25 and return the other /25 to the world, but I must have a /24 to do it."

Well yes there is. Many ISPs will generally not accept routes smaller than a /24. They configure filter-lists that prevent those routes from being accepted. The reason for this is to reduce the size of the global routing table. Each prefix has to be stored in TCAM(memory) on a router. There have been many incidents where older routers couldn't handle the number of routes in the global routing table and caused them outages. See:

https://arstechnica.com/information-technology/2014/08/inter... and https://www.inap.com/blog/growing-pains-internet-global-rout...

Further if you have a /24 you can certainly advertise 2 /25s to the same upstream ISP if you have more than one transit connection with them. In fact this is exactly how do traffic engineering.

>"It's frustrating that some ISPs make it artificially difficult for home users to obtain a BGP session, even when they're not asking for SLAs or dedicated connections."

This because home users absolutely don't need to run BGP! You just take a default route from your ISP and you're done with it. There is not point in running BGP if you are single-homed! You can't do any traffic engineering on it or influence the way upstream ISP route traffic to you. From the hobbyist point view it would be extremely boring. What can you actually do with it? View the global routing table? You can do this without ever running BGP by using a public looking glass server[1]. There is no incentive for ISP whose business is selling internet connectivity to cultivate a hobbyist community.

[1] https://www.bgp4.as/looking-glasses