I am trying to understand when is Border Gateway Protocol needed and I stumbled upon this question:
The chosen best answer says:
You could peer with your service provider and advertise the space to them via BGP. In the case where you have more than one ISP, this is the only option.
So meaning when you have more than one ISP, then you always need BGP? Why?
BGP is required when you've got your own, portable address range - portable meaning that it's independent of any ISP. You can advertise that range to any number of providers or peers.
If you just use addresses from your ISP(s) there's no need to advertise anything and no need for BGP.
If you want the multiple ISPs to work as backups to each other, yes.
Say you have two different ISPs, A and B, to connect to and get two different IP networks from each. If you want the ISP A to route the IP network you got from the ISP B to you, and vice versa, which is what you usually want for the enterprise network, then you need BGP because you have to advertise both of the IP addresses through A and B. Otherwise, the ISP A won't advertise the IP addresses from the ISP B for you.
So, when you have a portable address range(as Zac67♦ said) or multiple different address ranges belong to different entities, you need BGP, because ISPs won't advertise the IP address ranges that don't belong to them. And many ISPs don't accept the IP ranges they didn't assign or made a business deal for. This way, no entities advertise the IP address spaces that don't belong to them, hopefully. (I heard that some ISPs don't manage these IP advertisements they accept.) This is more of a policy thing than a technicality.
However, if you just want to use them separately and not to advertise both of the IP addresses to both ISPs, then you're fine without BGP.
