In the past I've always used /30's for PTP connection subnets out of old habit (i.e. Ethernet that won't take unnumbered) but now I'm considering switching to /31's in order to stretch my IPv4 space further. Has anyone else does this? Good? Bad? Based on the bit of testing I've done this shouldn't be a problem since it's only between routers.
~Seth
Greetings,
In the past I've always used /30's for PTP connection subnets out of old
habit (i.e. Ethernet that won't take unnumbered) but now I'm considering
switching to /31's in order to stretch my IPv4 space further. Has anyone
else does this? Good? Bad? Based on the bit of testing I've done this
shouldn't be a problem since it's only between routers.
Yes, this *IS* done *ALL* the time. P-t-P means that there are ONLY
two devices on the wire - hence point to point. It ONLY uses two IP
addresses (one on each end) and there is no reason or need to ARP on this
wire. So no need for a broadcast or network addresses - it is just the
two end points.
--- Jay Nugent
Nugent Telecommunications
Train how you will Operate, and you will Operate how you were Trained.
rfc3021 is over 9 years old, so should be no suprise that it works
well. 
Joe Provo wrote:
We recently did a backbone router upgrade and the vendor surprisingly didn't support /31's. We had to renumber all those interconnects and peering sessions to /30's. That wasn't fun!
> rfc3021 is over 9 years old, so should be no suprise that it works
> well.
>
I'm never surprised anymore by something that should work
turning out to
have some obscure quirk about it, so I figured it was worth asking.
It's not a "quirk", it's an "implementation-specific feature" 
Works well if supported. Vendor b (nee f) apparently dropped it off their roadmap.
Shouldn't be any issues...it's 2010
And, your IP allocation utilization will love you.
tv
Along the same line of logic, it should also be no surprise that Foundry
shits all over itself when you so much as learn a /31 via a routing
protocol (last I looked at any rate). Every other piece of gear seems
to handle it fine, though admittedly it breaks my automatic mental
calculations of "what is the peer IP" something fierce.
ARP is still required on ethernet links, so that the MAC address can be discovered for use in the ethernet frame header. /31 does not change the behavior of ARP at all.
ARP is still required on ethernet links, so that the MAC address can
be
discovered for use in the ethernet frame header. /31 does not change
the behavior of ARP at all.--
Nathan Ward
I often manually configure the MAC addresses in static fashion on
point-to-points to eliminate the ARPing but that is nothing unique to a
/31. It does eliminate the need for ARP, though.
* Seth Mattinen:
In the past I've always used /30's for PTP connection subnets out of
old habit (i.e. Ethernet that won't take unnumbered) but now I'm
considering switching to /31's in order to stretch my IPv4 space
further. Has anyone else does this? Good? Bad?
Bad. For some systems, such tricks work to some degree only due to
lack of input validation, and you get failures down the road (ARP
ceases to work, packet filters are not applied properly and other
fun).
And now is not the time to conserve address space. You really should
do everything you can to justify additional allocations from your RIR.
That's a vendor specific issue. Maybe you could take it up with them and ask what year they think this is?
tv
Florian Weimer <fw@deneb.enyo.de> writes:
Bad. For some systems, such tricks work to some degree only due to
lack of input validation, and you get failures down the road (ARP
ceases to work, packet filters are not applied properly and other
fun).
I never had any problems using Cisco to Cisco, Linux to Linux or Cisco
to Linux using /31. Only problem I encountered was when a Linux based
router was replaced by a Windows box (please don't ask).
cheers
Jens
Chris Costa <ccosta@cenic.org> writes:
We recently did a backbone router upgrade and the vendor surprisingly
didn't support /31's.
Mind dropping a name?
Jens
* Tony Varriale:
That's a vendor specific issue. Maybe you could take it up with them
and ask what year they think this is?
I think they support it on point-to-point media only, which seems
sufficient for RFC 3021 compliance. Ethernet is a different story,
unfortunately.