Using /31 for router links
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, On Fri, 22 Jan 2010, Seth Mattinen wrote:
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. +------------------------------------------------------------------------+ | Jay Nugent jjn@nuge.com (734)484-5105 (734)649-0850/Cell | | Nugent Telecommunications [www.nuge.com] | | Internet Consulting/Linux SysAdmin/Engineering & Design/ISP Reseller | | ISP Monitoring [www.ispmonitor.org] ISP & Modem Performance Monitoring | | Web-Pegasus [www.webpegasus.com] Web Hosting/DNS Hosting/Shell Accts| +------------------------------------------------------------------------+ 7:01pm up 43 days, 18:42, 3 users, load average: 1.10, 0.96, 0.63
On 23/01/2010, at 1:31 PM, Jay Nugent wrote:
Greetings,
On Fri, 22 Jan 2010, Seth Mattinen wrote:
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.
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
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.
On Fri, Jan 22, 2010 at 04:08:28PM -0800, Seth Mattinen wrote:
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.
rfc3021 is over 9 years old, so should be no suprise that it works well. :-) -- RSUC / GweepNet / Spunk / FnB / Usenix / SAGE
Joe Provo wrote:
On Fri, Jan 22, 2010 at 04:08:28PM -0800, Seth Mattinen wrote:
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.
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. ;) ~Seth
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! On Jan 22, 2010, at 4:53 PM, Seth Mattinen wrote:
Joe Provo wrote:
On Fri, Jan 22, 2010 at 04:08:28PM -0800, Seth Mattinen wrote:
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. 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. ;)
~Seth
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 -- ------------------------------------------------------------------------- | Foelderichstr. 40 | 13595 Berlin, Germany | +49-151-18721264 | | http://www.quux.de | http://blog.quux.de | jabber: jenslink@guug.de | -------------------------------------------------------------------------
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" ;)
On Jan 22, 2010, at 4:41 PM, Joe Provo wrote:
On Fri, Jan 22, 2010 at 04:08:28PM -0800, Seth Mattinen wrote:
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.
rfc3021 is over 9 years old, so should be no suprise that it works well. :-)
Works well if supported. Vendor b (nee f) apparently dropped it off their roadmap. -- kris
On Fri, Jan 22, 2010 at 07:41:24PM -0500, Joe Provo wrote:
rfc3021 is over 9 years old, so should be no suprise that it works well. :-)
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. -- Richard A Steenbergen <ras@e-gerbil.net> http://www.e-gerbil.net/ras GPG Key ID: 0xF8B12CBC (7535 7F59 8204 ED1F CC1C 53AF 4C41 5ECA F8B1 2CBC)
Shouldn't be any issues...it's 2010 :) And, your IP allocation utilization will love you. tv ----- Original Message ----- From: "Seth Mattinen" <sethm@rollernet.us> To: "nanOG list" <nanog@nanog.org> Sent: Friday, January 22, 2010 6:08 PM Subject: Using /31 for router links
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
* 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 ----- Original Message ----- From: "Florian Weimer" <fw@deneb.enyo.de> To: "Seth Mattinen" <sethm@rollernet.us> Cc: "nanOG list" <nanog@nanog.org> Sent: Saturday, January 23, 2010 4:06 AM Subject: Re: Using /31 for router links
* 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.
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 -- ------------------------------------------------------------------------- | Foelderichstr. 40 | 13595 Berlin, Germany | +49-151-18721264 | | http://www.quux.de | http://blog.quux.de | jabber: jenslink@guug.de | -------------------------------------------------------------------------
participants (12)
-
Chris Costa -
Erik L -
Florian Weimer -
George Bonser -
Jay Nugent -
Jens Link -
Joe Provo -
kris foster -
Nathan Ward -
Richard A Steenbergen -
Seth Mattinen -
Tony Varriale