How Not to Multihome

Patrick W. Gilmore patrick at ianai.net
Tue Oct 9 02:15:45 UTC 2007


On Oct 8, 2007, at 9:46 PM, Justin M. Streiner wrote:
> 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.

Well, "internally" is kinda useless to this discussion, wouldn't you  
think?

I get the feeling that you are trying to ask a clever question there,  
but it didn't come across that way.


> 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 :)

Interesting.  Did you not read the first paragraph in this e-mail?   
In fact, I seem to recall that you wrote it (attribution is missing,  
so I can't be 100% certain).

Personally, I'd call that a "legitimate reason".

To be clear, I am not suggesting de-aggregating every CIDR down to / 
24s.  But the global table doesn't grow any more whether the customer  
announces the /24 from their own ASN, or if you muti-originate it  
from two upstreams - or just one upstream for that matter.  So there  
is no "legitimate reason" to _not_ announce it, but there is a reason  
to announce it.

-- 
TTFN,
patrick




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