How to advertise /31 P2P links

[sidneyvogel532685277]

Hi all, i have a question, to save ipv4 addresses we can use /31 on p2p links, but my question is since what routing protocol supports /31 to advertise these networks betwen lets say 2 router to reach other networks? do we use static routs, that would be efficient in large networks, so how do ve dinamically advertise /31's?

[Reza Sharifi · Accepted solution]

Hi,

You advertise /31 the same way you advertise /30. All routing protocols support both /30 and /31.

It is just a different mask

example for a point to point link

192.168.1.0/31

and

192.168.1.1/31

HTH

[Joseph W. Doherty]

"All routing protocols support both /30 and /31."

Laugh, "all" is one of those words that can so easily get us in trouble.  I believe (?) RIP would not advertise a /31 as a /31, as it is a classful protocol.  Basically, network size would depend on the class of the prefix, Reza's example of using 192.168.1.0/31 would be considered a class C (or /24) by RIP.

RIPv2, I'm not sure about, as it's a bit "quirky" with classless addressing.

From first hand experience, current OSPFv2, as least Cisco's implementation, handles /31s just like other size networks.  (Also can pass about /32s, used for loopbacks.)

I suspect, for IPv4, EIGRP, IS-IS and BGP all (there's that word again - laugh) can handle /31s.

I further suspect, for IPv6 OSPFv3, EIGRP, IS-IS and BGP can handle /127s, the two host network size (but as an aside, I recall [?] in a large Enterprise IPv6 using IS-IS, we used /64s for p2p links.  [As we're "never" run out of address space.  Hmm, déjà vu?)

[Reza Sharifi]

Apologies Joe! A better choice of a word would have been "most" but not "All".

Reza

[Joseph W. Doherty]

Reza, no need to apologize.  Been there, done that (too many times myself).

"All" is just one of those words you need to be careful of, unless you're really, really (really) sure.

BTW, notice how often how I waffle with words like "suspect" or "recall (?)", only because I might be a bit further along the "oops" road, especially when someone springs a recent/rare feature.

[Giuseppe Larosa]

Hello @Joseph W. Doherty ,

RIPv2 does not support supernetting that is going beyond the major network mask  for example 192.168.0..0/20 , so it may support /31 prefixes but I didn't tested this and likely few people tried to do.

RIPV1 does not carry subnet masks and supports fixed lenght masks subnettting FLSM so it is out of the game

Hope to help

Giuseppe

[Joseph W. Doherty]

@Giuseppe Larosa., re: RIPv2 - thanks - that's probably one of the "quirks" I found with RIPv2.  I recall (?) trying RIPv2 expecting full classless behavior, but bumped into addressing issues, so didn't "explore" it very much.