I am currently redisgning a multi hop wan backbone that links a number of offices and i thought i had the design but after a previous question it has opened the question of if im doing it right.

When i use a point to point link i normally use a /29 subnet with 4 addresses used, 1 for each P2P device and then 1 for each side of the routers that connect to the WAN devices. After reading another artical it seems that everyone generally uses /31 subnerts for point to point links and i can see why but what i dont see is then how are the routers configured, do they also have a /31 subnet with the interfaces facing eachother?

I have looked at a few articles but none of them are like my setup, i have 1 main offices with 3 sites between then so its not a ring or a mesh, its just a point to point to point to point to point setup where each hop also has a point to multi point for out station telemetry.

The way i have been doing it for years works but i am curious to see if its the right way or if there is a better way, i have added a picture to show in brief how i configure each hop.

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EDIT: i should also note that i dont understand how to access the management interface of the P2P device if its configured with a /31, do i then have to create static routes on the P2P and on the closest router?

Thanks for any help that can be provided.

  • 1
    That is not exactly a point-to-point link, which has exactly two devices. If you have four devices requiring addresses, the network will need to be /29.
    – Ron Maupin
    Sep 2 '19 at 13:13
  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you can provide and accept your own answer.
    – Ron Maupin
    Dec 15 '19 at 18:51

Not all devices support /31 links, but if they do they're functionally equivalent to a /30 subnet with just two usable addresses. Where required, you set up the routes the same way, ie. /31. All devices using the links need to support /31 prefixes. Usually, you don't need to set static routes as the only devices using the addresses are the ones directly connected - unless you need a /31 address as management address from remote but I'd rather use the LAN-facing address, if possible.

/31 is a special prefix where just two interfaces face each other. There's no (need for) broadcast address and the network address itself (host bit zero) is explicitly used, see RFC 3021 for details.

With a main office and multiple remote locations, you might want to consider using a hub-spoke design, without chaining the remote locations. Of course, that depends on resilience requirements and available bandwidth.

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