Using /31 subnet mask - Can I use .0?

[lucas.shelton1]

We are redoing our distribution switches and now's a good time to re-IP them using a /31 subnet mask.  Can I use the .0 address?  Seems like I should be able to.

[Jon Marshall · Accepted solution]

Yes you can.

Jon

[lucas.shelton1]

Our distro switches are used for point to point links.  Basically we use routed ports on the connection between the 3750's and 5596's.

I'm in a campus environment.  Each distro switch supports VLAN's for each set of buildings.

So it goes:

Building switch stack trunks to 3750 -> Port XX on 3750 is layer three point to point, dual homed to two 5596's.

[Karsten Iwen]

As Jon answered, the .0 can be used with a /31 mask. Same is true for .255. But I'm not really sure if that is valid for your environment. /31 is for point-to-point links typically found in the WAN. When you are talking about distribution-switches, you probably mean the LAN connections (transfer-networks) between the switches. And there, /30 is still used.

[Joseph W. Doherty]

When you are talking about distribution-switches, you probably mean the LAN connections (transfer-networks) between the switches. And there, /30 is still used.

Perhaps not everybody.  ;)

We use /31s for Ethernet p2p, within our LANs.

[Karsten Iwen]

What equipment are you using? I've only seen it once on Nexus, but failed with that on some Catalyst. And is it already standardized for Ethernet?

[Joseph W. Doherty]

I think we have /31s on Catalysts 3550s, 3560/3750s (original series, -Es, Xs), 4500s (sup 4,5,7), 6500s (sup720, VS, 2T), 6800s, Nexus 5000s and 6000s, (Ethernet) ISRs 2800s/2900s/3800s/3900s and possibly 800 and 7200 routers too.  (We also use on brand "J", which interconnect with Cisco devices.)

Most Cisco devices will throw a warning message, when you use a /31.

If fact, just this morning, I brought up a /31 routed link between a 3750G and a 3550 which took the place of a /30 link between the same 3750G and a 2811.

NB: older p2p is usually a /30, but newer deployments, even on old equipment, is usually a /31.

[Karsten Iwen]

Whow, that's shocking to me ... I think I'll go directly to my lab and do some tests again.

Shows how important it is to regularly question what you believe to be the truth or what was learned some time ago. Thanks for pointing this out!