11-09-2007 07:12 PM - edited 03-03-2019 05:46 AM
Hi all,
I'm trying to understand when a layer-2 bridge (switch) would flood an
unknown unicast frame. My understanding is that whenever a device
needs to send a unicast frame, it would use ARP before sending, in
which case the switch would already have the MAC address of the
destination due to it's ARP reply. This seems that there would never
be a scenario where the switch would flood a unicast frame out all
ports. My book lists this as a valid scenario. Am I missing
something, or is this only possible in situations where ARP isn't
used? Thanks.
11-09-2007 07:50 PM
I've seen this behavior on customer's network and often is due to incorrect configuration.
If you set the arp timer higher than your mac-address-table (a.k.a CAM) timer, you will see this kind of behavior.
The switch would have the IP address in ARP but not a corresponding MAC since it has aged out.
11-10-2007 09:04 AM
I agree with most of what Edison says. But not the part about often is due to incorrect configuration. The ARP timer in IOS is 4 hours and the CAM age timer defaults to 15 minutes. This mismatch exists before the customer starts to configure anything.
There are also situations where asysmetric paths can cause unicast flooding.
HTH
Rick
11-10-2007 10:34 AM
some times when mac address table is full, the switch will bwcome as a hub. it broadcast any frame. this is well known attack by flooding the network with unreal MAC address until the mac address table will become full.
11-11-2007 11:21 AM
Thanks for the feedback, guys. What you're saying makes sense, and it's technically true. It's still misleading that my CCIE R&S book v3 says that this is the default behaviour of a switch, when in fact, you shouldn't see this in production.
11-11-2007 01:44 PM
Hi,
as Rick said ARP cache timeout is 4 hours while L2 switch MAC address timeout is only 5 minutes by default.
So it can happen there is the destination MAC missing in the switch forwarding table.
See
http://www.cisco.com/en/US/products/hw/switches/ps700/products_tech_note09186a00801d0808.shtml
and
BR,
Milan
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