Cisco – Disabling NBT via DHCP Option 43

The default setting in windows is to “Use NetBIOS setting from the DHCP server.
If static IP address is used or the DHCP server does not provide NetBIOS setting,
enable NetBIOS over TCP/IP.”  In other words, if you don’t specifically tell your
windows machines to disable NBT, it will remain on.

Microsoft implemented a vendor specific DHCP option to instruct clients to either enable
or disable NBT.  DHCP option 43 is for vendor specific options.  Basically, the DHCP server
can return option 43 and inside option 43 you can have vendor specific sub-options.

I will give you the basic idea:  The DHCP server is configured to return option 43 with the
following subcode structure:

Vendor specific option code: 0×01
Vendor specific option length: 0×04

Vendor specific option data:

This can vary based on what you want to do. The data 0×00000002 instructs the client to
disable NBT

With this knowledge, we can setup our Cisco router DHCP server to hand out this
option 43 with the correct sub-options to disable NBT.

Here is an example :

!
.0002
Disables NetBIOS over TCP/IP for that network interface.
!
!
This option can be used to enable or disable the use of NetBIOS over TCP/IP on the
network interface for which the DHCP message was received. DHCP clients SHOULD
support this option. If the use of NetBIOS over TCP/IP is disabled on the interface,
no NetBIOS over TCP/IP packets can be sent from or received on that interface.

If any NetBIOS over TCP/IP packets are sent to the client on that interface, they are
silently discarded. This option has no effect on NetBIOS over NetBEUI.