FW: /8s and filtering

David Schwartz davids at webmaster.com
Tue Dec 10 20:40:58 UTC 2002

On Tue, 10 Dec 2002 11:32:03 -0800 (PST), Harsha Narayan wrote:

>Now I am confused because I have got two sets of contradicting answers.

	Because there are two questions, one about provider independent space and 
one about space allocated by an ISP to its customer.

>Some say that anyone can multihome, some say that you need to be of a
>certain minimum size to multihome. May I know what is the right answer?

	Anyone can multihome. There is no size requirement (except perhaps of your 

>I agree that allowing anyone to multihome would increase the size of the
>routing table. So does this mean that someone has to be of a certain size
>to multihome?

	No, because multihoming does not require that anyone but your direct 
providers see your route. Since you pay them, you aren't imposing any costs 
on anyone else.

	The people you don't pay can choose to accept your small route or not. 
Either way, you can reach them and they can reach you. Their route to you may
not be optimum, but that's their choice. If you want them to hear your route,
you can pay them to do so. It will work either way.

	To repeat:

	1) Small multihomers should get IP address space from their most reliable 
provider. This is the one they trust the most and intend to remain with the 

	2) Small multihomers must get the ISP that assigns them address space to 
allocate them at least a /24 (with multihoming as the justification if 
needed). The ISP must agree to allow them to advertise their allocation 
through other providers and must agree to hear and announce the block from 
the customer *and* *other* *ISPs*.

	3) The ISP that assigns you IPs must prefer the more specific route from 
their peers if they don't hear it directly from the multihomer.

	4) Small multhomers must get their other provider(s) to agree to hear their 
announcement and readvertise it.

	5) Multihoming should be done only by those reasonably competent and 
experienced. You probably can't get enough help from a mailing list and may 
wind up signing contracts or purchasing equipment that harms you.


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