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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 usefu

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?

linsomniac 4y ago
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.
- bogomipz 4y ago
  >"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.
  - candiodari 4y ago
    > 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".
    - bogomipz 4y ago
      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.
  - linsomniac 4y ago
    >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.
traceroute66 4y ago
> 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).
ctime 4y ago
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.
- bogomipz 4y ago
  >"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 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.

linsomniac 4y ago

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.

bogomipz 4y ago
>"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.
- candiodari 4y ago
  > 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".
  - bogomipz 4y ago
    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.
- linsomniac 4y ago
  >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.

traceroute66 4y ago

> 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).

ctime 4y ago

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.

bogomipz 4y ago
>"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 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.