01-30-2022 02:37 AM
Hi Folks,
We have Cisco FTD 21xx and were in the process of configuring it up and have come to know ISP is using /31 subnet. Our ISP is using a /31 subnet for our internet link and it seems FMC will not support this when configuring the interface.
Is there any work arounds for this ?? We already checked with ISP, they said, they cant change the subnet.
Looking forward to some advice.
01-30-2022 02:43 AM
- FYI : https://community.cisco.com/t5/network-security/firepower-ftd-2110-31-ip/td-p/3912094
M.
01-30-2022 02:47 AM
- Additional : https://bst.cloudapps.cisco.com/bugsearch/bug/CSCvg80765
M.
01-30-2022 02:59 AM
I've never tried it but I think you can configure a /31 for a point-to-point connection, at least on version 7.0.0. However, a better solution may be to ask the ISP to configure a secondary IP address on the interface connected to your FTD with a /30.
01-30-2022 03:12 AM
Could you please confirm, on version 7.x also, its not supported.
01-30-2022 04:33 AM
I just test with version 6.7 does not support /31
01-30-2022 04:55 AM
can anyone test it on 7.x and share your result. Much appreciated.
01-30-2022 05:44 AM - edited 01-30-2022 05:49 AM
Here you go, found this link which confirms in the last table on the page that it is supported (with some limitations) from version 7.0 on:
01-30-2022 05:51 AM
01-30-2022 06:44 AM
- Modernized (smile) : https://www.cisco.com/c/en/us/td/docs/security/secure-firewall/management-center/device-config/710/management-center-device-config-71/interfaces-settings-ifcs-firewall.html#id_39144
M.
02-01-2022 09:44 AM
02-01-2022 10:08 AM
I think that warning is just to make you aware that if you configure a /31 on a broadcast connection it will break it. The /31 should only be configured for a point-to-point link where the connection doesn't care about the network and broadcast IP addresses.
03-26-2023 05:16 PM
Does this only work via FMC? I just tried this locally via FDM on an FTD 2110 running 7.3 and it will not accept it.
10-09-2023 05:24 AM
Unfortunately, RFC3021 is still not supported with FDM in FTD 7.3.1.1
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