Bridging networks on a VM

So, you’ve got your shiny new Mac and you’re in that ‘in-between’ time where you’re running a VM to support all of your Windows needs. You decide that your VM needs to be connected to the same Layer 3 network as your physical box so you decide to change your VM network settings from ‘NAT’ to ‘Bridged’. This seemingly simple configuration change has some pretty significant ramifications in the Cisco wireless world however so you may be shocked to find out when you take your beloved Mac back to work that your VM stops getting an IP address! As it turns out, there is a feature enabled by default on a Cisco lightweight wireless infrastructure that is spelled out thusly:

In the controller software Release 5.2 or later releases, the controller enforces strict IP address-to-MAC address binding in client packets. The controller checks the IP address and MAC address in a packet, compares them to the addresses that are registered with the controller, and forwards the packet only if they both match. 

Since your Mac(intosh) uses a single adapter (your WLAN adapter) for the connection to the network, the controller only sees a single MAC address. This means that it will only let a single IP address talk on the network since it’s expecting a 1:1 mapping of MAC address to IP address. The quickest way around this is the following global command on your WLC:

config network ip-mac-binding disable

Which will remove this 1:1 mapping expectation. Don’t forget to save your config and you should be good to go with IP addresses issued via DHCP to both your real machine and the Virtual Machines living behind the bridged VM network!

It should also be noted that many ‘security appliances’ serving as your DHCP server will refuse to issue multiple IP addresses to a single MAC address, effectively recreating identical symptoms (a VM that get’s no IP address). As far as I know, there is no workaround aside from not using a security appliance for your DHCP server. This is believed to afflict both Palo Altos as well as ASAs and is likely to impact anything else under the guise of a ‘security appliance’. Your best bet is to try and put DHCP services on a real server (Windows DHCP or Linux ISC-DHCPD) or try running it in IOS on your next hop Catalyst switch. You *do* have a next-hop Catalyst switch, right? 🙂