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For your average enterprise network, what are the pros and cons for using a /31 mask on point-to-point links with private addressing over the generally accepted practice of using /30? There's really no need for a broadcast address on a P2P, so what does a /31 offer to an enterprise, if anything, if burning a couple more addresses for the network or broadcast addresses is not a concern? I do understand in public IPv4 space with providers how the /31s are attractive for conservation though I haven't run into one yet that required them.

Am I overlooking making anything more difficult with /31s for administration or in the way any typical protocols work over a P2P? With OSPF, it identifies /30 links as P2P, so I don't see how using /31 makes any difference. OSPF is my largest concern.

There's a similar question on /31s, but the answers about impact to routing protocols like OSPF were mostly speculation, and I would like to have a definitive answer.

Community
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generalnetworkerror
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3 Answers3

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Generally it comes down to interoperability - if you can guarantee that using a /31 is fine for both hosts on the link, and you are 100% sure that if one of the hosts fail, you will not be replacing it with something that can't handle a /31, you should absolutely just use a /31.

In reality, the prime candidate for issues is going to be a routing protocol like OSPF, but as usual, this is implementation dependent - the vast majority of routers out there will behave properly when using a /31 - the adage "test your equipment" holds true.

Olipro
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    Agreed. We tested /31 on all our equipment and been using them exclusively on p2p links for a few years now. Most Junos/IOS/Netiron code has no issues – mellowd Jun 24 '13 at 11:04
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/31 addressing preserves IP addresses, otoh if you find yourself replacing a device on /31 links, make sure the replacement also supports /31 addressing or you'll have to re-number. BTDT.

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I would recommend the use of /31 only when short on IPv4 addresses and have many P2P links to deploy. So, how many links are we talking about?

Marco Marzetti
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    I think this is a bad approach - IPv4 *is* scarce, if there is no sane reason not to use /31, you should do so, irrespective of how many links you have. – Olipro Jun 24 '13 at 10:55
  • @Olipro i would agree only if the amount of "wasted" IP address is considerable during the planning. Who cares if you have a /19 and only 50 P2P links? – Marco Marzetti Jun 24 '13 at 11:05
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    For future growth. If you have a /19 are you ever only going to have 50 p2p links? Why use four when two will do? – mellowd Jun 24 '13 at 11:41
  • @mellowd my answer is not ethical, but pratical. I'm not saying that he MUST not use /31s, but that he SHOULD consider the amount of wasted IP addresses before deploying them and discover vendor-specific interoperability problems. Of course future growth should be kept in mind, but, again, it depends on the network scheme. – Marco Marzetti Jun 24 '13 at 15:52
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    IPv4 is not necessarily scarce. What if he's using internal addresses--like an entire 172.18.0.0/16 block set aside for P2P links? – Smithers Aug 22 '14 at 18:12