Mac Address Table Entry - Flood To Vlan

matthew.reader · ‎01-09-2009

Hi, quick question here. On a 6509-E with dual SUP720 I can see the following entry in the mac-address table:

7 0800.0f27.4991 dynamic Yes 170 flood to vlan7 (unicast)

There are a number of entries similar to this, all in the same vlan. My question is when would you expect to see, or in what situation, the "flood to vlan (unicast)" event occuring?

There is a bit more to this (i.e. other probs!) but I am trying to understand what events could cause the above.

Thanks

Matt

Roberto Salazar · ‎01-09-2009

The Unicast flooding means when a packets comes in the switch , the switch looks at the

Destination mac address of the packet and does not know what port to send out the traffic

to , then what switch does is floods that packet in the entire vlan just like a broadcast

in hopes that whoever has that address will pick it up and rest of the Hosts will drop the

traffic after looking at the MAC.

That is the information you are looking at in the output of "show mac-address-table"

command that for certain Destination MAC addrresses , the switch does not know what port

these addresses are on, and hence the "Flood to VLAN" argument.

Now why would a unicast flooding occur . A lot of times , unicast flooding can be

attributed to asymmetric routing. And usually bringing the arp and CAM timers as close to

each other as possible would eliminate unicast flooding to a great extent. In order to

better understand unicast flooding , I would suggest that you take a look at case study 8

in the following link.

http://www.cisco.com/warp/customer/473/62.shtml

You can configure unicast flooding protection starting from release 12.1(19)E1. The

following link explains the usage of the command.

http://www.cisco.com/univercd/cc/td/doc/product/lan/cat6000/12_1e/comref/i1.htm#31289

Other possible bugs that you might want to look at:

CSCsa84231

CSCeh73110

Hope that helps.