On Thu, Oct 20, 2022 at 5:13 AM Pirawat WATANAPONGSE via NANOG <nanog@nanog.org> wrote:
I have considered the prepending myself, but dare not implement it yet for the fear that BGP (Human) Community will burn me alive, witch-hunt style, because of the following reasons: 1. I can see from looking glass(es) that my upstreams already practice prepending (some paths) at their level (at least 3 more hops [x4]), supposedly to “balance” their bandwidth. 2. Should I start prepending mine, I might upset their balance, causing them to prepend more, thus starting a “prepend war”. [I imagine that x20+ prepending starts out this way]
The way I see it, prepending (or maybe even the whole BGP-Path thing) is a local-optimization problem: it’s only best for someone, not globally. And the Higher-Tiers (Lower Tier-Numbers) will always “engineer” me in the end.
Worse yet, I might be out-voted by de-aggregation insider “cultists” anyway.
Hi Pirawat, You asked the experts how it's done. It's done with prepends. Do you really want to argue with the answer? De-aggregation is a last resort, the bluntest tool in the toolchest. And it costs other people money so they don't appreciate you doing it unless you absolutely have to. https://bill.herrin.us/network/bgpcost.html As others have said, no one is going to yell at you because you prepended your AS two or three or even five times. If you don't get the desired effect after 5, you're running up against a problem prepends won't solve. The typical problem is that your upstream has used "localprefs" to prefer a particular path to you, overriding AS path length as the deciding factor. Competent upstreams that employ this technique also allow you to set a "BGP community" on your advertisement that overrides this behavior. A "BGP Community" is a 32-bit number often expressed as two 16-bit numbers the first of which is the ISP's AS number. When detected by the router, the number causes it to apply some locally-chosen rule to the route. If you ask them, the ISP will provide you with a list of "BGP Communities" (numbers) they allow you to set on your route advertisement along with what action they will take if they see that number. Regards, Bill Herrin -- For hire. https://bill.herrin.us/resume/