HUB to Switch Multicast problem

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Mar 14th, 2007
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We have 2 Alpha DS10 machines running Magic Meditech OS. If they are networked through a hub - they work fine. When plugged in a Cisco Catalyst 2950, machines will NOT boot up past JOIN MULTICAST GROUP part. IGMP snooping is configured on the switch as per vendor requirements. The switch seems to be blocking multicast communication between the machines. Any suggestions?

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glen.grant Wed, 03/14/2007 - 16:16
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Do you have portfast turned on for all your client machines switchports , may be timing out waiting for spanning tree to run . On each port use the "switchport host" command and see if that helps...

scottmac Wed, 03/14/2007 - 20:03
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If both machines are only on a switch (or only switched network of one VLAN / broadcast domain), then the switch should not interfere with the multicasts.

By default, and in "plain ol' switch" mode, a switch will pass all broadcasts and multicasts, just like a hub.

If anything, I believe you'd want to turn *OFF* all broadcast / multicast bell & whistles, as most are there to *reduce* the amount of broadcast / multicast traffic.

As the previous poster mentioned, it is likely related to some other feature, like spanning-tree, than the broadcast / multicast features.

You can also kill the spanning-tree action on the port by going into interface config mode (config t, int fa0/XX) and issuing a "spanning-tree portfast" command (close config mode with ctrl-z, then copy run start to save it in case of power-cycle).

You may also want to do a "show vlan" to make sure the ports that the hosts are connected to are both in the same VLAN (by default, all ports are in VLAN 1, the "default" vlan.

Give it a shot and let us know.

Good Luck



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