Dual Band routers vs mDNS

Wanted to write this up in case someone else is scratching their head with the same issue.

Some routers are able to use both 2.4 GHz and 5 GHz frequencies, allowing devices to appear as though they are on the same network regardless of which they use. These actually provide two different wireless networks, with different SSIDs, and bridge between the two.

This works fine for most things, but today I realised that it’s the cause of a printer problem I’d been having. The printer was using the 2.4 GHz network while my laptop was using the 5 GHz one. When I tried to print, the mDNS name of the printer was needed so a multicast query was sent… but the printer didn’t respond.

The reason is the printer never received the query. The mDNS multicast address is 224.0.0.251, which is for multicast to the local subnet only. My router interprets this to mean that packets sent on the 2.4 GHz network will not reach the 5 GHz network.

5 Responses to “Dual Band routers vs mDNS”

  1. Pete says:

    Can you mention which router you have? Just curious.

    I’m using a Netgear WNDR3700. It is dual band with a separate SSD for each. I’ve connected to both and get the same view with the Avahi Discovery tool. Both bandwidths can also see the local wired network as well.

    • Tim says:

      Maybe it’s yet another firmware bug then.

      • Mike says:

        I just got a WNDRMACv2 with updated factory firmware and have been experiencing strange wireless printer (stops responding) and DLNA (initial discovery and stops playing) intermittent connectivity issues as well. I really don’t think that Netgear’s firmware is passing multicast messages between physical network interfaces properly (including the wired ethernet link). I switched my media streamer to use a SAMBA share and things are fine now. I have yet to resolve my printer issues other than power cycling it before each time I need to print.

  2. Colin 't Hart says:

    Which firmware are you using? I run OpenWRT on a Netgear WNDR3700. All 3 physical interfaces (2 radios and the Ethernet switch) are bridged and assigned a single IP address.

  3. Pierce says:

    As others have suggested, it isn’t a requirement that this not work. Both 2.4GHz and 5GHz networks are on the same ip subnet, and the same “broadcast domain”. It’s just the wifi APs that decide to not propagate broadcast and multicast packets on the other BSSID.

    At the highest conceptual level it should all work like an ethernet switch, regardless of which wireless AP (2.4GHz or 5GHz) devices are connected to. For performance, wifi APs will sometimes not send broadcast or multicast packets to the air, because they don’t use acknowledgements, so they use a low datarate for reliability, and take a lot of air time. Some wifi APs can sometimes do “multicast to unicast conversion” so they can use faster datarates with acknowledgements.

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