Re: What's the point of prepend communities?

2017-10-29 Thread Brett Frankenberger 
On Sun, Oct 29, 2017 at 07:01:13AM -0500, Mike Hammett wrote:
> If I understand the OP correctly, I will use this real world example: 
> 
> https://onestep.net/communities/as174/ 
> 
> 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.

According to the link you provided, 3001..3003 are effective on "ALL
peer[s]" (which is differnet from all BGP neighbors).  So BGP-speaking
customers of 174 will not see the prepending if you use 174:3001..3003,
but peers will; but if you do the prepending yourself, then all 174's
peers and all 174's BGP-speaking customers see it.

 -- Brett

Re: What's the point of prepend communities?

2017-10-29 Thread Mike Hammett 
If I understand the OP correctly, I will use this real world example: 

https://onestep.net/communities/as174/ 


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 
http://www.ics-il.com 

Midwest-IX 
http://www.midwest-ix.com 

- Original Message -

From: "Jason Lixfeld"  
To: "William Herrin"  
Cc: "NANOG"  
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  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.