Filtering ULA?

Iljitsch van Beijnum iljitsch at
Mon Sep 22 14:11:32 CEST 2008

On 6 sep 2008, at 0:09, david.freedman at wrote:

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

Are you talking about the prefix or the packets?

Obviously everyone only allows prefixes from customers that are  
actually the right prefixes for those customers so the ULA stuff is  
rejected implicitly.

Towards your own ISPs or peers you could reject the prefix because  
nobody should be advertising it. But if you have a default then you  
can send packets to those locations anyway...

As for the packets: what if someone generates an ICMP too big message  
with a ULA source address? That could happen. It would be really bad  
if people filtered out those packets because that creates PMTUD black  

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