Solved: CAM agingtime is different from configured one

sachin · ‎01-22-2004

I m seeing typical problem.Network consist of 5500,6509,4500 series switches and by default cam agingtime is 300sec configured .But i can see for certain interval of time "cam agingtime" is getting changed to 15sec although 300secs is configured.

how its happening ?

smcquerry · ‎01-22-2004

Yes, non-port fast enabled means a port that does not have portfast enabled.

STP runs on all layer 2 switchports because every port is a member of the layer 2 switch topology. When a port goes down anywhere in the network, then it is possible that MAC addresses learned by switches in the network may no longer be out a particular port because of a link going down. Because of this, STP sends out a TCN and Cisco switches will "fast age" CAM entries so that it eliminates the risk of having an inaccurate entry.

This means that every time a port changes states (goes up or down) on any switch, a TCN is sent to all switches in the Layer 2 network and they will fast age the CAM table. A port changes state anytime a PC is powered up or rebooted. Because this happens quite frequently on layer 2 networks, there are a lot of TCNs.

In order to prevent TCNs from occurring when and end device is powered on or rebooted you can configure portfast on the user ports.

To configure portfast on an IOS switch, go to the interfaces you want to configure and enter the command "spanning-tree portfast". For a CatOS device the command is "set spantree portfast mod/port"

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Prashanth Krishnappa · ‎01-22-2004

I would bet my money on Spanning-tree TCNs. Check to see if there are any links flapping. Note that if a PC/end machine reboots on a non-port fast enabled port, it generates TCN and causes this as well. The following tech tips should help you out.

http://www.cisco.com/warp/public/473/17.html

http://www.cisco.com/warp/public/473/143.html#cause2

sachin · ‎01-22-2004

Can you please tell me what is the meaning of "non-port fast enable port" ? Do you mean that machines connected to ports without portfast enabled ? But how the link flapping is affecting CAM agingtime (it is totally local to that switch) ? One more thing how to overcome from this ?

smcquerry · ‎01-22-2004

Yes, non-port fast enabled means a port that does not have portfast enabled.

STP runs on all layer 2 switchports because every port is a member of the layer 2 switch topology. When a port goes down anywhere in the network, then it is possible that MAC addresses learned by switches in the network may no longer be out a particular port because of a link going down. Because of this, STP sends out a TCN and Cisco switches will "fast age" CAM entries so that it eliminates the risk of having an inaccurate entry.

This means that every time a port changes states (goes up or down) on any switch, a TCN is sent to all switches in the Layer 2 network and they will fast age the CAM table. A port changes state anytime a PC is powered up or rebooted. Because this happens quite frequently on layer 2 networks, there are a lot of TCNs.

In order to prevent TCNs from occurring when and end device is powered on or rebooted you can configure portfast on the user ports.

To configure portfast on an IOS switch, go to the interfaces you want to configure and enter the command "spanning-tree portfast". For a CatOS device the command is "set spantree portfast mod/port"