/31 mask a good idea?

DARYLE DIANIS · ‎07-16-2008

Is anyone using a /31 mask on point to point links or uplinks between layer 3 switches? Is there any downside or gotchas? I'm planning on running layer 3 all the way to my access switches and using routed ports to connect the access to the distribution. A /31 mask would help conserve ip addresses, right?

thanks

keeleym · ‎07-16-2008

Hi There

To the best of my knowledge a /31 netmask is not possible.

The smallest subnet possible is a /30 which yields 2 usable IP addresses along with the broadcast address of the subnet and the subnet number. This is what is recommended for a point to point link.

I think I remember reading somewhere about an RFC that was in the experimental stage which would allow a /31 mask (though I may be dreaming), but I have never heard of it being released for use.

I have seen instances where errors were made in configurations and either the subnet mask or the subnet number was assigned to a host and it appeared to work, but I think that this would be frowned upon in a live production network.

Best Regards,

Michael

Harold Ritter · ‎07-16-2008

Michael,

The draft you are referring to was draft-retana-31bits and made it into RFC3021 and has been supported in IOS since 12.2T.

Please refer to the following document for more information on RFC3021:

http://www.ietf.org/rfc/rfc3021.txt

Regards,

Harold Ritter
Sr Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
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DARYLE DIANIS · ‎07-16-2008

I will admit its new, but here is a link that talks about it. I'm more curious than anything,because I thought /30 was the smallest.

http://www.cisco.com/en/US/docs/ios/12_2t/12_2t2/feature/guide/ft31addr.html

burleyman · ‎07-16-2008

While you are correct it can be done does it make it a "good idea".....Just because I could pierce my tongue 100 times does not mean it is a "good idea" :-)

Unless you have a shortage of IP addresses I would not introduce this.... just my Half-Penny (was two-cents but you know with the economy and gas prices).

Mike

autobot130 · ‎07-16-2008

/31s are awesome. it saves IP space! The problem is its not interroperable with other vendors. Works on newer code.

sirdudesly · ‎07-16-2008

saves IP space... if you're using it internally you more than likely have enough space anyway :P

Joseph W. Doherty · ‎07-16-2008

Possibly another alternative, for saving your address space, would be usage of IP unnumbered. See http://www.cisco.com/en/US/tech/tk648/tk362/technologies_tech_note09186a0080094e8d.shtml for more information.

DARYLE DIANIS · ‎07-18-2008

thanks everyone for letting me know what you think. I'm going to try the /31 in a test environment and see if I like it. I am on a lan and do have plenty of addresses and of course, we're an all Cisco shop, so interoperability won't be a problem.

thanks again