Using /126 for IPv6 router links

Igor Gashinsky igor at
Wed Jan 27 09:12:49 UTC 2010

On Wed, 27 Jan 2010, Pekka Savola wrote:

:: On Tue, 26 Jan 2010, Igor Gashinsky wrote:
:: > Matt meant "reserve/assign a /64 for each PtP link, but only configure the
:: > first */127* of the link", as that's the only way to fully mitigate the
:: > scanning-type attacks (with a /126, there is still the possibility of
:: > ping-pong on a p-t-p interface) w/o using extensive ACLs..
:: >
:: > Anyways, that's what worked for us, and, as always, YMMV...
:: That's still relying on the fact that your vendor won't implement
:: subnet-router anycast address and turn it on by default.  That would mess up
:: the first address of the link.  But I suppose those would be pretty big ifs.

Or, relying on the fact that (most) vendors are smart enough not to 
enable subnet-router anycast on any interface configured as a /127 (and 
those that are not, well, why are you buying their gear?)..

If a worst-case situation arises, and you have to peer with a device that 
doesn't properly support /127's, you can always fall back to using /126's 
or even /64's on those few links (this is why we reserved a /64 for every 
link from the begining)..


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