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Multicasting, IGMP Snooping and MTU

Tensulp
Grafter
Posts: 38
Registered: ‎20-09-2008

Multicasting, IGMP Snooping and MTU

I have just changed to a new Asus fibre router and wanted some advice on a couple of settings.
The Plusnet router had Multicast enabled but on the new Asus router this appears to be turned off. I have the option to "enable IGMP Snooping" and set a "multicast rate".
Can someone help exlain multicast as I have read many different explanations and I am getting more confused.
Also the MTU of the router is 1492 on the Asus and 1500 on the Plusnet, I understand that this relates to the maximum packet size and wondered if its better to increase the MTU or to leave it at its default Asus setting? Why would Asus have lowered the limit, is it purely for stability?
Thanks in advance
2 REPLIES
jimbof
Grafter
Posts: 348
Thanks: 2
Registered: ‎02-05-2013

Re: Multicasting, IGMP Snooping and MTU

On the MTU - the standard rp-pppoe source (in, for example, openWRT) can't do MTU larger than 1492 for PPPoE due to the 8 byte PPPoE overhead required to fit in a standard ethernet frame. 
There is a patch for rp-pppoe to add support (which uses mini jumbo frames to send 1508 byte sized frames to the modem, including the PPPoE overhead).  I believe the Asus firmware is WRT-based and so this might be the reason (I don't know though).
See here:
http://wiki.aa.org.uk/FTTC_Modem
Multicast is used for streaming TV channels - basically the cabinets get sent streams of channels, and via a protocol called IGMP IPTV boxes (such as youview boxes) on your network can get access to these streams.  They basically say "I'd like to watch channel blah" and channel blah gets put onto your incoming network connection, and then when you stop watching it gets removed.  It is used to reduce the amount of bandwidth taken up on ISP networks by streaming TV services.
If you don't have a working mutlicast setting you won't be able to receive channels sent over multicast on such boxes.  Note this is not relevant to things like youtube, netflix, iplayer, etc - only to stuff like real channels but sent over the ISP network instead of TV aerial / dish. 
Tensulp
Grafter
Posts: 38
Registered: ‎20-09-2008

Re: Multicasting, IGMP Snooping and MTU

Thanks for the reply and brilliant explanation.
The MTU makes sense now, I wasn't aware a patch for rp-pppoe but it looks like you are correct about the asus router. Makes little difference in the short term anyway.
Multicast is what I thought it was, you just managed to explain it better than othr sites that confused me.