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>"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 t

>"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.

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.