There are several things that you need to be mindful of:
1. The Chromecast stick needs to be able to send UPnP packets to whatever device you are using to configure ChromeCast. If you have a software firewall policy on your computer, you'll need to make sure that it accepts incoming connections for the ChromeCast app -OR- for UDP ports 32768 - 61000. On my MacBook pro, I had "block all incoming connections" enabled by default. You'll need to disable this, or make an exception to the relevant UDP ports. If you don't have admin rights on your computer, a work around is to use your phone or tablet where there isn't a firewall setting by default.
2. If you have dual wifi mode enabled (i.e., 2.4GHz wifi and 5GHz wifi), you'll need to disable the 5GHz wifi. Make sure UPnP packet routing is enabled. NOTE: You do NOT need to allow UPnP packets through your router's firewall. Allowing incoming UPnP packets from the outside world into your LAN is a bit dangerous. You only need to make sure that UPnP packets can be routed internally on your LAN.
3. Make sure AP Isolation is disabled. On my router, this is two settings. "Set AP Isolated" and "Isolation between Main and Guest AP". Both have to be disabled.
4. Finally, if you're still struggling with getting ChromeCast to connect:
- Turn on IGMP Snooping.
- Disable WMM and also disable Packet Aggregation temporarily.
- If you can, set the multicast rate to HTMIX (1S) 30 MBps
I'm using the rt-n65u firmware hosted here (https://code.google.com/p/rt-n56u/ ) and had to make the following changes:
Under Advanced Settings --> Wireless 5GHz:
- Enable Radio = OFF
Under Advanced Settings --> Wireless 2.4GHz:
- Goto Professional tab:
- AP Isolated = OFF
- Isolation between Main and Guest AP = OFF
- Enable IGMP Snooping? = ON
- Multicast Rate (Mbps) = HTMIX (1S) 30 Mbps
(optionally, you can change this back later):
- Enable Packet Aggregation = Disable
- Enable WMM? = Disable
Under Advanced Settings --> LAN --> IPTV:
- Enable multicast routing = ON
- TTL correction for multicast packets = NO
- IPTV UDP Multicast to HTTP Proxy Port = 0 [disabled]
- eXtensible UPnP agent (xupnpd), Web port = 0 [disabled]
Multicast traffic - WiFi 2.4 Ghz
- Enable IGMP Snooping = ON
- Multicast Rate (Mbps) = HTMIX (1S) 30Mbps
Under Advanced Settings --> WAN
- Enable IGD UPnP = NO [You'll never want to allow WAN UPnP packets in]
Following the advice of one of the comments, try turning off IGMP snooping after setting up Chromecast. It seems to have resolved issues with dropped casting connections. Also, try enabling WMM as well. I'm testing this pre-Superbowl (http://msn.foxsports.com/foxsportsgo/)