Using /126 for IPv6 router links
bicknell at ufp.org
Mon Jan 25 16:42:39 UTC 2010
In a message written on Mon, Jan 25, 2010 at 05:14:06PM +0100, Mathias Seiler wrote:
> Ok let's summarize:
> + Sticks to the way IPv6 was designed (64 bits host part)
> + Probability of renumbering very low
> + simpler for ACLs and the like
> + rDNS on a bit boundary
> <> You can give your peers funny names, like 2001:db8::dead:beef ;)
> - Prone to attacks (scans, router CPU load)
> - "Waste" of addresses
> - Peer address needs to be known, impossible to guess with 2^64 addresses
+ 65535 possible addresses, can use a standardized subnet for everything
from a 2 router point to point, to a six address vrrp to vrrp dual
router static setup, and beyond. Becomes the universal "edge
interface" when the far end is routers not hosts.
+ rDNS bit boundary++, since it falls on a :.
+ Limits the effects of scan-like attacks.
+ Can set aside 1 /64 of /112's for, well, forever.
> + Only 4 addresses possible (memorable, not so error-prone at configuration-time and while debugging)
> + Not prone to scan-like attacks
> - Not on a bit boundary, so more complicated for ACLs and ?
> - ? rDNS
> - Perhaps need to renumber into /64 some time.
> - No 64 bits for hosts
> Like /126 but there's an RFC not recommending it and an RFC (draft) which revises that non-recommendation.
> On 25 Jan 2010, at 10:14, Matthew Petach wrote:
> > On Sat, Jan 23, 2010 at 4:52 AM, Mathias Seiler
> > <mathias.seiler at mironet.ch> wrote:
> >> Hi
> >> In reference to the discussion about /31 for router links, I d'like to know what is your experience with IPv6 in this regard.
> >> I use a /126 if possible but have also configured one /64 just for the link between two routers. This works great but when I think that I'm wasting 2^64 - 2 addresses here it feels plain wrong.
> >> So what do you think? Good? Bad? Ugly? /127 ? ;)
> >> Cheers
> >> Mathias Seiler
> >> MiroNet GmbH, Strassburgerallee 86, CH-4055 Basel
> >> T +41 61 201 30 90, F +41 61 201 30 99
> >> mathias.seiler at mironet.ch
> >> www.mironet.ch
> > As I mentioned in my lightning talk at the last NANOG, we reserved a
> > /64 for each
> > PtP link,
> > but configured it as the first /126 out of the /64. That
> > gives us the most
> > flexibility for expanding to the full /64 later if necessary, but
> > prevents us from being
> > victim of the classic v6 neighbor discovery attack that you're prone
> > to if you configure
> > the entire /64 on the link.
> I think I will go this way. Since we've got the usual /32 assignment I have plenty of /64 to "waste".
> If I continue assigning a /48 to every customer I can set apart a /64 for each PtP link and still have room to grow for a very long time (I'm not taking into account the assignment of IPv6 addresses to high amounts of M&Ms so far ;) )
> This way the configuration and addressing plan is simple and understandable to anyone.
> > All someone out on the 'net needs to do
> > is scan up through
> > your address space on the link as quickly as possible, sending single packets at
> > all the non-existent addresses on the link, and watch as your router CPU starts
> > to churn keeping track of all the neighbor discovery messages, state table
> > updates, and incomplete age-outs.
> Well I could filter that in hardware with an interface ACL but a /126 seems much easier to maintain.
> > With the link configured as a /126, there's
> > a very small limit to the number of neighbor discovery messages, and the amount
> > of state table that needs to be maintained and updated for each PtP link.
> > It seemed like a reasonable approach for us--but there's more than one way to
> > skin this particular cat.
> > Hope this helps!
> Yes it does. Thanks!
> Mathias Seiler
> MiroNet GmbH, Strassburgerallee 86, CH-4055 Basel
> T +41 61 201 30 90, F +41 61 201 30 99
> mathias.seiler at mironet.ch
Leo Bicknell - bicknell at ufp.org - CCIE 3440
PGP keys at http://www.ufp.org/~bicknell/
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