Filtering ULA?

Fred Baker fred at
Sat Sep 6 00:30:35 CEST 2008

On Sep 5, 2008, at 3:09 PM, david.freedman at wrote:

> Is there any good reason why I shouldn't be filtering the ULA  
> (RFC4193)
> range at the edge (FC00::/7) ?

My understanding:

You should *absolutely* be filtering the ULA range of prefixes from  
you customers unless you have an agreement to do otherwise, and then  
you should implement that agreement. There are some strictures on the  
form of the agreement; if a network agrees to share a ULA with another  
network, it should be shared between those networks and not with  
others. In general - and if I were writing a contract that permitted  
ULA exchange I would include this in the contract - I would expect an  
ISP to refuse all ULA prefixes from customers and expect customers of  
ISPs to share ULAs over some form of VPN or private connectivity.

If you don't do that, a ULA is a random prefix that a network decided  
to use, and may conflict with some other network that randomly picked  
the same prefix. Think about it...

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