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BGP Inbound - Getting conflicting info...

 
 
RJParth
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      06-03-2007
I'm hearing two different things on how my inbound traffic will route
with two ISP's. I'd like to use all provider assigned address space
from ISP-A and not use address space from ISP-B at all. ISP-A and ISP-
B are connected to two separate routers on my network. I'd like to
establish an iBGP session between the two of them and then figure out
some form of load balancing between the two for outbound traffic.
That part I think I have under control, for now. The inbound side is
a different story, however.

I've heard that as long as ISP-A is willing to advertise my block
separately from their large aggregate block, then I should be ok.
Otherwise, all inbound traffic will likely flow through ISP-B because
they will be advertising the my specific block. (i.e. ISP-A only
announces my few /24's included within their Class A whereas ISP-B
announces just my few /24's making ISP-B the more specific route).
I've also heard, in conflict to the above, that inbound traffic will
almost always flow back through ISP-A (unless the connection is down)
because they "own" the block.

Can anyone break the tie?

 
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T. Cam
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      06-03-2007
RJParth wrote:
> I'm hearing two different things on how my inbound traffic will route
> with two ISP's. I'd like to use all provider assigned address space
> from ISP-A and not use address space from ISP-B at all. ISP-A and ISP-
> B are connected to two separate routers on my network. I'd like to
> establish an iBGP session between the two of them and then figure out
> some form of load balancing between the two for outbound traffic.
> That part I think I have under control, for now. The inbound side is
> a different story, however.
>
> I've heard that as long as ISP-A is willing to advertise my block
> separately from their large aggregate block, then I should be ok.


If you advertise /24 routes to ISP-A, and they refuse to pass-on those
/24 routes to their peers, then they're not providing typical BGP
service to you.

> I've also heard, in conflict to the above, that inbound traffic will
> almost always flow back through ISP-A (unless the connection is down)
> because they "own" the block.


Many carriers use localpref, to force traffic to go directly to you:
once the packet enters ISP A's network, ISP-A will give it directly to
you, provided that your BGP session to ISP-A is up. This is actually a
Good Thing. If ISP A's network is sufficiently large, then this may
cause (say) 20% of your total traffic to come via ISP-A, regardless of
how you announce your routes.

BGP is really not designed to support load balancing, and for most
enterprises, as long as you receive traffic from ISP-A's customers via
ISP-A, and traffic from ISP-B's customers via ISP-B, then there is
probably little benefit to balancing traffic from the rest of the
Internet across the two links.
 
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