what do pc's do with multicasts

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May 30th, 2008
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Hi all, as switches treat multicasts like broadcasts, does my pc have to process each multicast like it does with a broadcast to see if its ment for that pc? also how do you stop the switch flooding multicasts, would I just enable igmp snooping ?

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Joseph W. Doherty Fri, 05/30/2008 - 15:59
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I believe a PC NIC should only forward multicast packets to the host's IP stack if the host is interested in them, although since there's overlap in Ethernet multicast addressing for unique IP multicast addressing, the NIC might forward multicast frames that the host isn't interested in at the IP level.

Yes, IGMP snooping is what's normally used to stop flooding multicast to switch ports that are uninterested in receiving multicast packets. (Not all switches have this capability.)

husycisco Sun, 06/01/2008 - 18:33
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Hi Carl,

PCs send multicast group membership report to the router/switch for multicast group that it wants join/to be a part of. It does nothing with a multicast traffic that is flooded to a multicast group, which it is not a member of, unlike broadcast.

Although PCs do not utilize the unrelated multicast packets, flooding multicast traffic to unnecessary ports affects throughput, bandwith and resource utilization. You can either use IGMP or CGMP depending on your equipment. CGMP is recommended if you are low on switch resources.


bvsnarayana03 Sun, 06/01/2008 - 21:43
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For the switch to avoid flooding multicasts out all its ports, it should be configured for CGMP or IGMP snooping. By enabling either of these, switch prepare CAM entries for hosts willing to listen mcasts for specific groups. remember this information, which hosts are interrested to listen mcasts & belong to which mcast group is communicated to the switch by the router in case of CGMP(hosts doesnt indicate switch of hteir interest in mcast traffic). In case of IGMP snooping, switches sniff the pkts send by the hosts & thus bulit the CAM for mcast. Separate CAM entries for each group are maintained by switch.

carl_townshend Tue, 06/03/2008 - 01:56
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so am i right in saying pc's have to process every broadcast, but not multicasts ?

javickas1 Mon, 06/02/2008 - 06:06
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Keep in mind also that there are two different multicast behaviors. Some multicasts can be forwarded beyond your local segment. SOme multicasts stay on the local segment and are not forwarded beyond it.

The range of addresses between and, inclusive, is reserved for the use of routing protocols and other low-level topology discovery or maintenance protocols, such as gateway discovery and group membership reporting. Multicast routers should not forward any multicast datagram with destination addresses in this range, regardless of its TTL. RFC 3171

You might also want to use PIM if your multicasts are spanning multiple routers. As well as IGMP snooping.

But I don't think IGMP snooping would affect the traffic in the to range.


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