BGP Inbound - Getting conflicting info...

Discussion in 'Cisco' started by RJParth, Jun 3, 2007.

  1. RJParth

    RJParth Guest

    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?
     
    RJParth, Jun 3, 2007
    #1
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  2. RJParth

    T. Cam Guest

    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.
     
    T. Cam, Jun 3, 2007
    #2
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