In other words, howto do mDNS on Edgerouter.

This is a dead horse that should have been beaten enough by now, but these are my notes to how I got it working.

Setup:
VLAN 5 - trusted wifi
VLAN 20 - iot things

Chromecast uses mDNS to announce itself and since VLANS are routed this breaks but is easily fixed.

I have an Edgerouter er-4 with both vlans on eth1 and this is what I did;

Log in via SSH and turn on the mDNS repeater service for the affected interfaces

configure
set service mdns repeater interface eth1.5  
set service mdns repeater interface eth1.20  
commit; save

Add a firewall rule to open for mDNS from the IOT VLAN to LOCAL, either via gui or cli as below

rule 3 {
     action accept
     description "Allow mDNS"
     destination {
         port 5353
     }
     log disable
     protocol udp
 }

That's it!

Ubiquiti even has an article about Edgerouter mDNS themselves, but omit the firewall rule..

If you want to know a little more about mDNS, consider giving the video below a watch.