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Using 2 consumer grade access points what is the behavior of when multicast packets are sent between them.

Consider -> AP1 -> AP2

(Note that AP1 & AP2 are connected over the LAN ports to the network. WAN connections are not used and everything can be considered on the same network.)

Consider a muticast packet which is seen by AP1, sent from the network. Assuming these access points know nothing about multicast then they will behave the same way as it it was a broadcast.

Even with nothing connected to AP2 the packet would still be transmitted to AP2? And then retransmitted by AP2? (I think this would be only if AP2 had anything connected it by wireless, other than AP1)

With multiple PC behind AP2 (wired connection) would this still be the same behavior or would AP1 send multiple packets to each of the PC behind AP2?

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Your diagram isn't clear, how are the two access points connected? If they are consumer gear, and you have connected the WAN port of AP2, to a lan port on the integrated switch in AP1, then broadcasts/multicasts will not cross from one to the other at all. –  Zoredache Oct 20 '11 at 21:50
    
Clarified to show that WAN ports are not being used in this scenario. Everything is on the LAN side at both ends. –  Callum Oct 20 '11 at 22:02
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1 Answer

Layer2 switches that are not aware of IGMP/PIM joins normally flood multicast traffic; wifi access points behave as a kind of Layer2 switch.

From RFC 5110 - Overview of the Internet Multicast Routing Architecture

2.0 Multicast Routing
... When multicast transmission arrives at the receiver's LAN, it is flooded to every Ethernet switch port unless flooding reduction such as IGMP snooping is employed (Section 2.7).

Then in Section 2.7...

2.7. Restricting Multicast Flooding in the Link Layer
Multicast transmission in the link layer, for example Ethernet, typically includes some form of flooding the packets through a LAN. This causes unnecessary bandwidth usage and discarding unwanted frames on those nodes which did not want to receive the multicast transmission.

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This is as I thought. So as long as the WiFi access point "AP1" only has 1 devices connected to it wirelessly "AP2". And this "AP2" has no wireless connections (other than "AP1) then there will not be excess traffic over this link. AP1 will only transmit to AP2. AP2 will not retransmit over wifi because it dosn't have any other connections... –  Callum Oct 20 '11 at 22:01
    
If AP1 and AP2 are in the same ethernet broadcast domain, then AP2 should see multicast flooded to it, unless you enable IGMP snooping in your infrastructure –  Mike Pennington Oct 20 '11 at 23:27
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