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Flooding of ip unicast traffic

Hi there,

I capture packets from the network from my workstation during 30 minutes without putting the port of the switch in monitor modus. So my guess was that i would only see broadcast messages and some unicast packets where the mac address is not know to the switch.

But i saw some other packets that i could not explain why i can see them. I saw for example a Syn, Ack package which in my optinion i shouldn't see since the mac address should be known to the switch.

I understand that if i see udp messages that it could be possible that i can monitor the whole traffic if the destination host never sends a packet since than the switch will never know where the machine is located and has to flood all the time. But for tcp? A Syn packet ok no problem but i think that i shouldn't see Ack of Syn, Ack....or everything else.

I hope i was clear describing my problem and to resume i have the impression that sometimes packets are flooded even if the switch knows the destination port.

Is there any way how i can verify this?

Thanks a lot.

regards,

ycae

3 REPLIES
Silver

Re: Flooding of ip unicast traffic

Flooding also occurs in such cases. The best way to carefully verify the packets is to use a good analyzer.

Re: Flooding of ip unicast traffic

I have seen a configuration of ISA servers where the server deliberately causes all unicast traffic to the server to be flooded. It does this as part of an elaborate load-balancing technique. The way it tricks the switch into flooding all its unicast traffic is by using its NIC MAC address as the source of its outgoing frames, but giving the client a different MAC address in its ARP response. The client sends its frames to a MAC address that the switch has never seen as source, so it floods them.

There are several other reasons why a switch might flood. Are you sure these are MAC unicasts? The first byte of the destination MAC address ... is it even or odd? If it is odd, then you have a multicast or broadcast destination.

Does that help?

Kevin Dorrell

Luxembourg

Green

Re: Flooding of ip unicast traffic

Welcome back Kevin, good to see you back online!

I suppose it's also possible that something caused the ARP in the switch to flush ... or on some of the older Cisco switches, a heavy traffic load will cause it to go into "hub" mode and flood everything (versus freaking out and dumping most traffic to get caught up).

FWIW

Scott

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