multicast problem


I have this problem that has been bugging me for days. I "searched through the whole Internet" but couldn't seem to find any mention of my problem.

I have 2 machines connected via a Layer 2 Cisco Switch.

machine A will listen for while machine B will send multicast on

On the first send, machine A will receive the datagram. On subsequent sends, machine A will not receive them until a "certain time" later.

An interesting to note is this problem will not happen for multicast addresses in the range x.0.0.x. That is, machine A will be able to receive multicast datagrams from,, etc any number of times.

Any one has any idea what could be the problem causing this?

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We have seen a similar problem, though with non-Cisco switches. It does seem to be related to the switch allowing certain multicast MAC addresses through by default and blocking others. You may be aware that the 28-bit multicast IP address space is mapped onto a 23-bit MAC address space by mapping the lower 23-bits across from the IP address to the MAC address. Thus shares the same MAC address as (and,, and so on up to is the group for all IP hosts and might be expected to be treated specially, along with most of the 224.0.0.x numbers.

Our problem is on the back burner at the moment, but anything you come up with may be useful.


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Sam Wilson

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"skybach" ha scritto nel messaggio news:


The switch's behaviour is logical from my point of view: the range 224.0.0.x is not routable so there should be no IGMP directed to a router for this address. (so it is never pruned)

The switch will "snoop" for any IGMP join/leave messages from your hosts to a router in order to filter or let pass the traffic... if there is no IGMP messages you could use a static entry (should be something like ip igmp snooping vlan 1 static interface fa0/1)

Regards, Gabriele

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Gabriele Beltrame

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