The question has been posted here once or twice. It goes something like
this. "How do I use BGP to load balance between two ISP's?"
I'm starting to get into BGP in earnest in preparation for the CCIE written.
And I have something of a fascination with design issues. Let me see if I
can sort out my thoughts. Please comment where you can.
1) First of all, the load balancing issue. BGP itself has no mechanism
within it for load balancing of any kind, whether that be per packet or per
destination.
2) If one could use BGP for per packet "load balancing" then one is in the
position of doing suboptimal routing in many cases. For example, if I am
connected to AS101 and AS202, and I want to go to a particular e-commerce
site, and it is 5 hops via AS101 and 20 hops via AS202 then I have created
problems for higher layers due to issues with packets arriving out of
sequence. Potentially I have hurt my performance, maybe even killed it.
3) If one were to use BGP for per destination "load balancing" isn't is
possible that the optimum path for all, or at least most, destinations might
still lie through one AS or the other? I mean, there is no way to predict
this, is there?
4) So from a design perspective, assuming Mr. Pointy-Hair insists on "load
balancing between two ISP's" the setup most likely would be something like
this:
Inside_router-----BGP_router_1--------ISP_1
|------BGP_Router_2-------ISP_2
and doing something like setting up two 0.0.0.0 routes, one to each BGP
router, and letting the inside router to the "load balancing"
5) OR - taking in a full BGP route table, and letting the BGP router
determine the best path to the destination, recognizing that "load
balancing" may or may not occur.
6) Are there a different set of issues if "I" am the e-commerce site? I'm
thinking yes, because then the issue is ability to reach me by the optimum
path. This is not a matter of "load balancing" but of raw reachabilty from
the outside.
Comments welcome. Just trying to clear my thinking.
Chuck
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