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I am investingating the option of connecting our business network to 2 separate ISPs for redundancy.

Since we have a couple of B2B services which need to be accessible from outside, in order to have automatic failover not only from inside out, but also from outside in, as I understand, we should implement BGP and have our own (optimally provider independent [PI]) Autonomous System (ASN).

It seems that IPv4 ASNs are depleted and -- especially global ones -- are comparatively expensive and hard to obtain.

A couple of ISPs are offering IPv4 ASNs, however, they are giving provider aggregated ones, which would mean we would be stuck with the provider as long as we use them.

The question is:

  • is there any other way to provide reasonable failover connection to my network beside ASN+BGP?
  • alternatively, is it feasible and recommended to get IPv6 PI ASN -- as I understand those should be available easier and cheaper, but I haven't been able to find the info yet. Would a pure IPv6 network be reacable from the rest of the Internet nowadays anyway?
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You're mixing up terminology a bit here. There's no such thing as an IPv4 ASN or IPv6 ASN. There's IP space, which is either IPv4 or IPv6, and there are ASN's. An ASN can announce IPv4 prefixes, IPv6 prefixes, or both.

As you concluded, PI space (at least in the RIPE region, which roughly covers EMEA, but I think the same goes for other RIRs) is depleted. This leaves you two choices to obtain your own IP addresses:

  1. become a LIR (Local Internet Registry), you get a /22 of IPv4 space
  2. buy IP space. There's a number of IP brokers where you can buy any number of IP addresses

Both these options do cost money. Option 1 has an additional yearly fee, option 2 mostly depends on the number of IP's you're buying.

is there any other way to provide reasonable failover connection to my network beside ASN+BGP?

This depends a bit on what exactly you're trying to achieve. For some services, changing DNS labels when one of the connections may work if the TTL is low enough. Or you can offload the your using for example CloudFlare or Akamai, and switch to another backend IP address. Both these options require manual intervention or maybe some smart scripting.

If you need to have one set of IP addresses available through different networks, you're mostly stuck with BGP.

alternatively, is it feasible and recommended to get IPv6 PI ASN (...) Would a pure IPv6 network be reacable from the rest of the Internet nowadays anyway?

As I said, there's no such thing. You can get an ASN and IPv6 space, but a large part of the internet still only has IPv4 connectivity, so having your services only available on IPv6 will probably not work as well as you'd like it to. You could possibly consider to have a service like CloudFlare or Akamai (or another CDN) to do the IPv4 hosting and connect to your services via IPv6, but I'm not aware if they do support this, and this probably only works for HTTP(S) services.

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  • I am trying to achieve a situation in which two things happen. (A) my organization can use Internet services without interruption, if one ISP link goes down; these services are mostly not IP-based, however, we do have an IPSec tunnel over Internet between 2 of our sites now, and (B) our customers can use our HTTP-based services (both a webapp and REST API) provided from our main site, again -- without interruption, if one ISP link goes down. The customers may have a wide variety of their own software, which we have no control of, trying to connect: SAP, etc. – Gnudiff Jan 13 '17 at 8:45

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