UDP packet flow over 2950 switch ports

Unanswered Question
Mar 2nd, 2007

I have a question that concerns UDP packet flow over a 12 port switch. When a UDP packet is sent to a IP destination will the UDP packet flow over every port on a switch? When I sniff each port these packets are flowing over ever port. Is this normal?

I have this problem too.
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Jon Marshall Fri, 03/02/2007 - 07:56

Hi Tim

If the destination IP address is unknown to the switch then the switch will flood the packet out of all ports except the one it was received on. Once the switch has learned the mac-address to IP mapping then if the destination IP address is a unicast IP address then no you should not see it on all ports. There is nothing special about UDP compared to TCP in this regard.

Is the IP destination address definitely a unicast address ie. if it begins with 224.x.x.x and you do not have multicast enabled then the switch has to flood these packets to all ports as it would with a broadcast.

Hope this explains things, let me know the IP address if there is still some confusion

Jon

tim.holden@hctra.com Fri, 03/02/2007 - 08:53

I have a 6509 switch between two networks. On one network the source is 10.2.23.207 and the destination is 10.2.25.159. the network ip range on the switch i see all ports with this udp packet flow is 10.2.23.0/23 and the network destination side is 10.2.25.0/23. on the 10.2.23.0/23 side i have 5 or 6 switches connected via fiber. all switches i see this traffic on all ports.

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Posted March 2, 2007 at 7:05 AM
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