/31 Point-to-Point connection across multiple devices

[alanbolster]

Hi,

I'm hoping some of you clever folks can help me here. I'm planning a new network for a small business. There are two sites in the same town and the company has a 1Gbps leased line connecting the two sites. I am planning to deploy a new two-shelf SG550XG-24T stack as a core L3 switch at each site. My plan is that each of these devices will have a route to the other across the site to site link.

I know that if you are directly connecting two Cisco routers to each other with a point-to-point link, you can use a /31 subnet and give each device's interface one of the two addresses. What I'm wondering is whether I will be able to configure the same thing on my SG550XG-24T devices. The physical topology will be something like this:

Switch A and Switch B are connected to the CPEs at each location by a 1Gbps fibre port, configured as a VLAN trunk. From the company's perspective, it's as if they have a really long Ethernet cable between the two sites. If I were to configure a new VLAN on that trunk, could I give each SG550XG-24T device an interface in a /31 subnet and plug it into an access port in that VLAN on the switch?

I can't think of any particular reason why this setup would behave any differently than if the two devices were directly connected. Although, perhaps it's only dedicated routers which allow the use of /31 subnets for p2p links? Do the SG550XG-24T L3 swtiches even support this functionality. Maybe there are best-practice recommendations which discourage /31 subnets on anything but direct connections? I suppose, if it's not supported or recommended, I can just use a /30 instead.

Any help or advice would be greatly appreciated. Thanks.

[Georg Pauwen]

Hello,

Cisco apparently even recommends the use of /31 masks as defined in RFC 3021. In the link below, scroll down to the section 

Reducing the Number of IP Addresses Required to Establish IP Connectivity by Using IP addresses with 31-Bit Prefixes on Point-to-Point WAN Interfaces

somewhere down the middle of the document.

I think accepting 31 masks is software specific though, I am using 12.x versions, and it won't accept the 31 mask...

http://www.cisco.com/c/en/us/td/docs/ios/ios_xe/ipaddr/configuration/guide/xe_3s/iad_xe_3s_book/iad_config_ipadd_xe.html

[alanbolster]

Interesting stuff. Thanks.

Really, I just want to know whether /31 masks are supported on the SG550XG-24T devices and whether the configuration will work when carried on a VLAN across multiple devices; i.e. the link isn't a direct connection.

[Georg Pauwen]

Alan,

you need to check your switch if it supports /31 masks. The CLI reference for firmware release 2.1.0.x (which you are likely running) doesn't mention if it does or not. As stated, I get an error message when I try to do this on a 12.x release:

"Bad mask /31 for 192.168.1.1"

I have labbed this with a /30 address, and it works fine. A /31 address shouldn't make a difference, since you are basically simulating a WAN link that only needs two addresses anyway.