Jmflanag   230 pts.  |   Oct 22 2008  12:39AM GMT

If you are running traditional 802.1D spanning tree, then anytime a link goes up or down, a Topology Change BPDU is generated by the local switch and sent to the Root switch. The Root, after replying back to the TC BPDU, sends a Configuration BPDU with the TC bit set to all downstream neighbors. The effect of this config BPDU with the TC bit set, is that it forces all switches to clear out their CAM tables for non-active sessions. SO if you have a port flapping, or mulitiple devices constantly rebooting, your CAM table is then constanly being flushed. The switch will then go through the process of learning MAC addresses again by flooding out the request for the destination MAC until it learns which port they are on, and do this everytime the edge port goes up or down. THis is not a big deal if it happen a few times, but if it is constant, then it will cause excess broadcasts