How Not to Multihome

Tue Oct 9 01:46:11 UTC 2007

On Mon, 8 Oct 2007, Patrick W. Gilmore wrote:

>> 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.

Internally or externally?  A /24 would exist in the provider's IGP to 
point traffic to that customer.

Off the top of my head, I don't see why the provider who holds the parent 
block would do this externally.  If the provider has, say, a /18 and they 
assign a /24 of that to this customer, there would be no legitimate 
reason to originate that /24 and propagate it out to the rest of the 
Internet.  Note that I don't consider breaking that /18 up into 64 /24s 
and announcing them all separately to accomplish some sort of poor-man's 
traffic engineering to be a legitimate reason :)

jms