What's the point of prepend communities?

Mike Hammett nanog at ics-il.net
Sun Oct 29 12:01:13 CST 2017

If I understand the OP correctly, I will use this real world example: 


174:3001 through 174:3003 as compared to doing the prepending yourself. What is the functional difference? 

BGP neighbors of 174 will see just as many AS hops either way, but non-BGP customers of 174 would see you just one hop away. It's just another method of traffic engineering. 

Mike Hammett 
Intelligent Computing Solutions 


----- Original Message -----

From: "Jason Lixfeld" <jason+nanog at lixfeld.ca> 
To: "William Herrin" <bill at herrin.us> 
Cc: "NANOG" <nanog at nanog.org> 
Sent: Thursday, October 26, 2017 1:47:44 PM 
Subject: Re: What's the point of prepend communities? 

Hi Bill, 

> On Oct 26, 2017, at 2:37 PM, William Herrin <bill at herrin.us> wrote: 
> BGP routing is based on "distance". Distance in BGP is primarily calculated as the number of ASNs in the AS Path. Prepends make a path more distance, 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. 

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