Tuesday, August 14, 2012

Hello? ¿Hola? No, Bonjour!

Last fall I helped a local school district install a Cisco wireless network consisting of 1142n APs and a 5508 controller.  All was working well until recently when one of their techs received the district's first AppleTV.

Of course my first question was what VLAN(s) the AppleTV and the iOS device were connected.  That was the first problem which was easily fixed by placing them on the same VLAN.  The next problem is how Apple's devices find other devices on the same layer 2 network.  Apple uses Multicast DNS (mDNS), which they call Bonjour, to locate devices.  Every device on a subnet advertises its capabilities to the multicast address  By default these advertisements are set to have a Time To Live (TTL) of 1 which prevents them from crossing routed boundaries even if multicast routing is configured on the network.

It was easy enough to put the AppleTV onto the same wireless VLAN as the user devices so we didn't have to come up with a solution for that.  To get the two devices talking is actually quite simple.  By default Cisco WLCs have multicast disabled.  This of course prevents Bonjour's discovery process from functioning.  Enabling multicast is done under the Controller tab of the WLC web page.  There are three options, disabled, unicast and multicast.

You might wonder how you can have unicast multicast, but it's not a contradiction in terms.  Because of the way traffic is tunneled from the APs to the controller, the multicast traffic is actually sent to the APs using CAPWAP unicast packets.  If you switch to multicast multicast mode and the switches are properly configured, the CAPWAP packets containing the wireless multicast traffic is only sent to APs that have joined the multicast CAPWAP group.  In a larger environment this can cut down on the amount of traffic sent as well as eliminate traffic going to APs that don't need the packets.

Most of the major networking vendors have released or are planning to release solutions to the problems Bonjour faces in the enterprise network.  In addition there have been petitions online to Apple  for them to make their products work better in enterprise networks.  Hopefully soon these challenges will be a thing of the past and Apple products will "just work" at home and in the office.

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