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New Member

strange broadcast

We have a Catalyst 2948GL3 working as a router, and connected to a Catalyst 3524XL working as a switch. Some kind of traffic is sent from the 2948GL3 to the switch, is unicast traffic (unicast MAC's and IP addresses), but the switch sends the packets to all ports of the switch. It seems as if the packet was broadcast, but I captured them and they are unicast.

TIA

4 REPLIES

Re: strange broadcast

This is the default behaviour of a switch. When a mac adress is unknown (probably located on another switch) it is forwarded to all ports.

You will always obeserve these packets in a network with multiple switches, while the arp-tables are local. Devices on another switch are not learnt.

New Member

Re: strange broadcast

But a packet is forwarded to all ports THE FIRST TIME, after that the MAC is learnt and packets are forwarded only to the right port. But in this case there are a lot of unicast packets with the same source and destination addresses that are forwarded to all ports.

Re: strange broadcast

Then check if the PC sending/receiving these unicasts is on that switch. You will find that it is not. Within a VLAN all traffic that does not match a local ARP is flooded to all other switches in that VLAN. On the one containing the node it will be sent to the corresponding port. On all others, it is flooded. Does not match with what they teach you hey? But this is how it works.

Cheers

Re: strange broadcast

Take a look at this link and see if it applies to you: http://www.cisco.com/en/US/products/hw/switches/ps708/products_tech_note09186a00800a875c.shtml

This one may help as well (good show commands for the 2948G-L3): http://www.cisco.com/en/US/products/hw/switches/ps606/products_tech_note09186a0080093fff.shtml

Hope it helps.

Steve

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