We are planning to have 3 x 1G bgp connections (full tables) eg: Path A, B, C
Can I say that we have 3G output totally?
From my understanding, the bgp chooses the best path to route automatically
It doesn't. It typically chooses the path with the least number of
autonomous systems for a given destination. That can actually result
in longer physical paths in many cases.
Let's say provider C buys bandwidth from A and B (and nobody else).
If that's the case, you will only use C for things directly connected
to C's network (typically only things that pay C), but every other
internet destination would use A or B. (unless you adjust things to
not do this).
If the path A is best route and that path 1G bandwidth is used up,
will bgp try to use path B and path C automatically?
No, with one caveat. If you fill up the pipe enough that routing
messages don't get through, those routes will eventually time out and
the path won't be used at all.
How can I use up those 3G?
You will need to manually adjust routes, preferences, etc. You'll
still have one path that is hotter than the others (although hopefully
not too much hotter).
Are you worried about incoming or outgoing bandwidth, or both? For
incoming, you will need to do things like:
1) Announce all of your prefixes aggregated out all 3 links
2) Announce parts of your prefixes out ONLY ONE link. So announce /24
#1 out A, /24 #2 out B, /24 #3 out C. This means you're forcing
incoming traffic to generally come in one link per /24. The problem
with this is that a really active /24 will get more traffic still. It
also requires you to have at least 3 /24s (you can't route longer
prefixes, which means you can't route PART of a /24).
For outbound, the easy and obvious way would be for your providers to
just announce 0/0 to you and for you to do some sort of flow-based
load balancing. But if one provider had reachability problems, you'd
go down. So without that, you'll have to adjust the preferences of
Alternatively use BGP multipath and buy from one provider (and connect
to the same router on the provider side). Bandwidth from one provider
isn't necessarily a horrible thing, if you pick a good one provider.
Even with multiple BGP feeds, unless you are really, really careful
(and, most likely, spend tons of money for things like fiber
redundancy so the different fibers don't all end up on one pole or
going into the same telco building) you'll still have single points of