nanog mailing list archives
Re: What's the point of prepend communities?
From: Steve Dodd <sdodd () salesforce com>
Date: Thu, 26 Oct 2017 13:10:12 -0600
Keep in mind also that when you allow a customer to deprioritize a particular peer you can really blow up your COGS model (assuming you buy transit). Cheers, Steve On Thu, Oct 26, 2017 at 1:05 PM, William Herrin <bill () herrin us> wrote:
On Thu, Oct 26, 2017 at 2:47 PM, Jason Lixfeld <jason+nanog () lixfeld ca> wrote:Hi Bill,On Oct 26, 2017, at 2:37 PM, William Herrin <bill () herrin us> wrote: BGP routing is based on "distance". Distance in BGP is primarilycalculated as the number of ASNs in the AS Path. Prepends make a pathmoredistance, encouraging routers to choose a different path if one is available. I understand how prepends fit in the context of best path selection, but my question was more the difference between a customer signalling the ISP to prepend their AS using a BGP community stamped to a prefix vs. the customer prepending their own AS instead.Hi Jason, You'd only use communities like that if you want to signal the ISP to deprioritize your advertisement on a particular peer or set of peers but not others. That's when you're getting fancy. It's not the norm. The norm is you want to deprioritize one of your paths as a whole. Maybe that link has less capacity or is enough better connected that it would always override your other links unless you detune it a little. I mean, you could tell the ISP to prepend everything based on a community, assuming they support such a community, but why would you? That needlessly makes things more complicated. Regards, Bill Herrin -- William Herrin ................ herrin () dirtside com bill () herrin us Dirtside Systems ......... Web: <http://www.dirtside.com/>
Current thread:
- Re: What's the point of prepend communities?, (continued)
- Re: What's the point of prepend communities? William Herrin (Oct 26)
- Re: What's the point of prepend communities? Jason Lixfeld (Oct 26)
- Re: What's the point of prepend communities? Christopher Morrow (Oct 26)
- Re: What's the point of prepend communities? Job Snijders (Oct 26)
- Re: What's the point of prepend communities? Jason Lixfeld (Oct 26)
- Re: What's the point of prepend communities? Rob Foehl (Oct 28)
- Re: What's the point of prepend communities? Jason Lixfeld (Oct 26)
- Re: What's the point of prepend communities? William Herrin (Oct 26)
- Re: What's the point of prepend communities? William Herrin (Oct 26)
- RE: What's the point of prepend communities? Naslund, Steve (Oct 26)
- Re: What's the point of prepend communities? William Herrin (Oct 26)
- Re: What's the point of prepend communities? Brett Frankenberger (Oct 26)
- Re: What's the point of prepend communities? Steve Dodd (Oct 28)
- Re: What's the point of prepend communities? Leo Bicknell (Oct 26)
- Re: What's the point of prepend communities? Clinton Work (Oct 26)
- RE: What's the point of prepend communities? Naslund, Steve (Oct 26)
- Re: What's the point of prepend communities? Leo Bicknell (Oct 30)
- Re: What's the point of prepend communities? Michael Hallgren (Oct 30)
- Re: What's the point of prepend communities? Mike Hammett (Oct 29)
- Re: What's the point of prepend communities? Brett Frankenberger (Oct 29)