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Span tree network Traffic

I recently upgraded my network switches to 3 2950 switches. These 3 switches are configured with cross over cables (port 23 on switch #1 connected to Port 23 on switch #2, Port 24 on switch #2 connected to Port 24 on switch #3)

When I ran a 2 minute ethereal capture on my network more than 50% of the traffic was Cisco spanning tree

This doesnt seem correct to me,

I am very new to Cisco as these new switches are my first Cisco hardware. As I dont speak Ciscoease yet I have been using the Java applet to configure the switches, This morning when I went to the applet it shows my first switch but says that the other 2 are down, However computers on the other 2 switches are working fine.

Any help will be appreciated.

Also one other issue, I am noticing that DHCP seems very slow to issue an address.




Re: Span tree network Traffic

Well, the first thing you have to remember is that if you run etherreal on a switchport of a properly configured network, you will probably only see the traffic that gets broadcast or sent out to each port. If your network isn't very populated (ie: not many arp requests), spanning-tree might be the highest traffic you see. You could try spanning another port to see actual-traffic if that's what you want.

As far as why are you seeing so much spanning-tree traffic, well - thats probably related to why DHCP takes so long to issue an address. On all ports EXCEPT your links to other switches, put "spanning-tree portfast" in the interface configuration. This will allow that port to skip the spanning-tree process when a machine is booting. This 45+ second process will slow down DHCP or PXE machines to the point that they might not get an address in time. All end-user/server ports should be configured this way!!

Hope this helps!!


Re: Span tree network Traffic

Please clarify - are you suggesting that the lan goes through a spanning tree reconfiguration every time a switch port comes up as a computer is booted?

Re: Span tree network Traffic

No, I was not suggesting that...

There are other types of STP traffic that can be sent out, for instance, TCN's are very common, esp. on networks that do not have end-user ports configured correctly with portfast on.

Please see here for more details.