Maybe I can help with this.
BGP doesn't do load balancing in the traditional sense of the word. The
only time you'll see a router have two BGP learned routes to the same
network is when that router has two connections to the same ISP and you
have configured the command "maximum-paths 2" under the BGP routing
process. And I venture to say that you'll never see a BGP router doing
"per-packet" load balancing. Not in the real world. Per-packet load
balancing requires process switching which drags performance of the router
down.
Typically, to make sure that both links get utilized, the edge BGP routers
will advertise a default route into the IGP and from there it is up to the
IGP to make the best routing decision. For instance:
<BGP>------Router1------<OSPF>------Router2------<OSPF>
--------Router3------<OSPF>-------Router4----------<BGP>
Both Router1 and Router4 are injecting 0.0.0.0 into OSPF. Now Router2 will
prefer the 0.0.0.0 coming from Router1 because it has a lower OSPF Metric
than the one coming from Router4. (and the inverse is true of Router3).
Of course, you as the Design Engineer have every option open. You can
influence all the traffic to go one way or another. But you have to know
the network and the traffic patterns very well before you start. You can
set metrics to influence path selection. Or you can use Route Maps (Policy
routing). You can have Router2 Send all HTTP traffic over Router1's
default link, and all other traffic over Router4's link.
You can customize BGP to suit your needs, but the difference between it and
other Routing Protocols is that you have to decide the direction, you can't
leave it up to BGP to pick the best path (like you would with an IGP).
Hope this helps.
Gary Frye
Internetwork Engineer - CCNP,CCDP,CNE,MCSE
Charlotte, NC 28262
Phone: (704) 427-0564
Pager: (800) 504-8567
Cell: (704) 502-7921
Fax: (704) 590-7477
Email: [EMAIL PROTECTED]
WWW: http://www.firstunion.com
"Because we're dealing with Engineers and not Mathematician, 'infinity'
turns out to be 16."
-Radia Perlman on IP RIP
---------------------- Forwarded by Gary Frye/AO/USR/FTU on 09/05/2000
08:18 AM ---------------------------
[EMAIL PROTECTED] on 09/04/2000 10:16:55 PM
To: Gary Frye/AO/USR/FTU@FTU
cc:
Subject: Fw: BGP on the Brain - Design Issue
----- Original Message -----
From: "Chuck Larrieu" <[EMAIL PROTECTED]>
To: "Cisco Mail List" <[EMAIL PROTECTED]>
Sent: Monday, September 04, 2000 12:38 AM
Subject: BGP on the Brain - Design Issue
> 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
>
> Please check out my new footers for a new age
>
> 1) Altruism
> http://www.hungersite.com/
> Please help feed hungry people worldwide. A few seconds a day can make a
> difference to many people
>
> 2) Shameless Commerce
> http://www.certificationzone.com
> An excellent source for information, study materials, practice questions,
> practice exams, and practice labs. Applicable for all levels of
> certification, as well as the attainment of internetworking expertise.
Tell
> them Chuck Larrieu sent you. ( disclaimer - I will receive addition free
> months membership when enough people mention my name upon joining )
>
>
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