/31 subnet allocation

[Vasileios Bouloukos MSc, ITIL, CCIE]

Hi all,

I was wondering if there is any practical drawback/issue by allocating /31 subnets to the point to point links between the ISP and the customer.

This could be very useful and could save public IP addresses.

Can you see any limitation that could cause a problem? I do not think that limiting the broacast packets to the p2p link could cause a problem.

Have anyone  seen or used /31 in PE-CE real scenarios?

I was inspired by the next doc

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

Thank you

Vasilis

[Peter Paluch]

Hello Vasilis,

Although I am not working for an ISP and did not deploy /31 netmasks in real life networks, I have been routinely teaching about using the /31 netmasks in my CCNP BSCI/ROUTE courses. I do not see any problems with using these prefixes for p2p links. The /31 netmask is not just a particular vendor's quirk - rather, it is an accepted solution described in RFC 3021. I do not personally see any reasonable limitation with using /31 netmasks.

Best regards,

Peter

[fb_webuser]

Hello,

As mentioned by Mr. Peter, there is no harm at all...Moreover you should also consider using IP Unnumbered.....which is common practice of ISPs to operate when it comes to PPPoE/Broadband customers.

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Posted by WebUser Musab Kiani from Cisco Support Community App

[Nandan Mathure]

Hi Vasileios,

Check this blog:

http://mellowd.co.uk/ccie/?p=630

This guy has tested all the protocols and in comments section one of the guys has mentioned that he has seen a ISP with /31 implementation.

Thanks,

Nandan Mathure.