How Not to Multihome

Tue Oct 9 01:23:27 UTC 2007

On Oct 8, 2007, at 6:19 PM, Justin M. Streiner wrote:
> On Mon, 8 Oct 2007, Keegan.Holley at sungard.com wrote:
>
>> That brings up an interesing point.  My biggest fear was that one  
>> of my
>> other customers could possible be closer to me that the ISP that  
>> provides
>> the primary link and it would cause them to favor the backup link  
>> because
>> of AS path.  I think they are going to fight me on this and  
>> telling them
>> to multihome to their original ISP would probably be frowned upon  
>> at this
>> point.  I was hoping that there was an RFC for multihoming that I  
>> could
>> use to bail myself out.
>
> If you went ahead and did this, the more specific route being  
> announced by you on behalf of your customer would be more likely to  
> attract traffic back to you.  Prefix length is checked in the BGP  
> route selection process before AS path length.  This would work in  
> normal "everything works fine" situations, but when things break,  
> troubleshooting the source of the customer's reachabilit woes will  
> get very interesting.

You have made an assumption that the original upstream would not  
originate a prefix equivalent to the one you are originating.

-- 
TTFN,
patrick