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IP Packet Multicast it is not work in my Switch


Hi Guys,

I have a Switch Core 6509-E in my network , my Supplier developed a system that work with Multicast IP however I cant see this packet arriveing in my Sniffer in the same VLAN without any config span. I'm Know that the Switch do flood ip multicast by default in all port in the same VLAN. I would like know if is there some information inside the packet/system that may cause this problem ?? or is there a command in Switch that i must use ?

Regards !!

7 Replies 7


1-is 6509 routing multicast ot switching it ?

2-is there a host listening to ?

3-if there are other devices in the setup, troubleshoot basics: RPF, PIM enabled, RP etc.. depending on how u are set up.

a drawing of set u might help




1-is 6509 routing multicast ot switching it ?

R: No

2-is there a host listening to ?

R: Yes , there is a Server uses a aplication that send a multicast continuously , my goal is install more two Server in the same Network , because I want to do a Cluster , but the multicast packet is not arriving in a other Servers , because the Switch is not doing a flood in other port in the same Vlan.

3-if there are other devices in the setup, troubleshoot basics: RPF, PIM enabled, RP etc.. depending on how u are set up.

R: I dont have any configuration in my Switch.





It's not true that the switch floods multicast by default. It's only true if the there is a source and no receipient in the same vlan, then the traffic is flooded. As soon as a receipient register it's interest in the stream with IGMP, then the switch starts building a forwarding table and stops flooding the traffic. That holds true when you have igmp snooping enabled, which is the default for most switches.

If you want to see the traffic on all ports, disable igmp snooping, which is actualy not a good idea to do.

Best regards


Hi Simon,

I foud out what is the problem now , i dont know how happened but my Switch Core recorded the mac-address of Multicast IP (0100.5e06.0708) in table mac and I want my Switch sends a flood multicast in all port in the same Vlan. I tried to remove this mac-address from mac table , but i Could not. How can I do this ?? how is command ? Is there a Unavailability in my enviromente after of the command ?

Follow the Mac-addres table of my Switch:

SPJNDVACEYS6002#show mac-address-table static vlan 21

Legend: * - primary entry

age - seconds since last seen

n/a - not available

vlan mac address type learn age ports


* 21 0019.a982.3400 static No - Router

* 21 0100.5e06.0708 static Yes -

* 21 3333.0000.000d static Yes - Gi5/4,Gi5/5,Gi5/6,Gi5/8


"no ip igmp snooping" will disable igmp snooping (can also be done per VLAN), then multicast should be flooded again. But I don't think thats a good thing to do. It's better when your clients interessted in the multicast are able to signal that via IGMP.

Best regards



Yesterday I foud out what was problem in my Switch. I got a application that generate multicast packet and did some test, and I saw that when I change the IP addres of application from to 224.0.0.x , my Switch flood multicast in all port normally.

I checked this link documentation (see below) and i see that there are a "Local Network Control Block". In my opinion the Switch is check multicast IP address and did not flood this multicast address because i think that it work in compliance with RFC-3171.Would like Know if you have some Cisco Documentation that show about this kind of behavior ???

Best Regards

Fabio Santos


Did find out some cisco documentation that show about this kind of behavior(See msg 03/14/2008)???


Fábio Santos

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