FTD 6.2.3 on ASA5506-X - unable to use /31 mask on ipv4 data interface

[terje]

Hi, the FTD software (6.2.3.10) on ASA 5506-X does not seem to support point-to-point (/31) ipv4 subnets. When trying to set my static /31 public ip on the outside interface in FDM the error message "You cannot assign a broadcast address as the IP address of an interface" is raised. A fix would be most appriciated ! 

[balaji.bandi]

This message is clearly says you can not assign ""You cannot assign a broadcast address as the IP address of an interface"

can you please show us more information, what is the exactly IP address you trying to Assign ?

[terje]

Hi, any attempt to assign an ip address with netmask 255.255.255.254 to an interface in FDM will either raise the error message above or "You cannot assign a network address "x.x.x.x/255.255.255.254" as the IP address of an interface" based on whether you are using the first or last address in the point-to-point subnet.  

/31 subnets is not a problem on a IOS router or ASA:

interface GigabitEthernet1
ip address x.x.x.81 255.255.255.254

TB

[Abhijeet Kumar]

I just tried that on my FPR 2110 and it does work. What's the extact IPv4 address you are entering?

[terje]

I'm using a new ASA5506-X reimaged with FTD and FDM local management (no FMC avail). The smallest subnet possible on the interfaces is /30. If the problem is FDM, could the interface ip be configured from CLI Expert mode ?

TB

[terje]

... and every ip (i.e 10.20.30.40) with 31 bits mask (255.255.255.254) is rejected.

[terje]

Abhijeet- just got confirmed from TAC that the /31 addresses is a restriction on ALL devices running FTD due to limitations in FMC. This feature is planned implemented in a future FTD/FMC vesion. Could you share the running FTD/FMC version on your FPR2110 ?

Thanks,

Terje

[Abhijeet Kumar]

Please see attached.

[terje]

Update: 

Still no support for /31 mask on interfaces in FTD 6.7.0.1. Tested on a FPR2110

Today upstream providers is consolidating IPv4-space using /31 links which prevents us from using the Firepower NGFW series as WAN/Internet edge devices.

[Marvin Rhoads]

Correct - not supported. There's an open ENH bugID:

https://bst.cloudapps.cisco.com/bugsearch/bug/CSCvg80765

[terje]

Update - Firepower 1010

Still no support for /31 netmask on routed interfaces in FDM on the latest FTD 7.3.0 !!