Catalyst Switch "Fast Port" 1

[STF26]

I have an issue in several locations where somoe of the computers that are attached to cisco catalyst switches where they do not recieve and IP from DHCP when they are turned on. It turns out when you reboot or bring up a computer, it resets the ports and I assume it starts the spanning tree process on it. What happens is the computers are booting up so fast that by the time the logon prompt shows up, the port has not gone green.

Any ideas,
Thanks
STF

 

[IPKONFIG]

Turn on portfast. This will solve your problem. You can use the command:

set port host mod/port (on Cat OS switches)

or

switch(config-if)#switchport <----turns port into L2 only
switch(config-if)#spanning-tree portfast <---- turns on STP

This will correct your issue. If you want to know why this was happening, then here goes:

Spanning tree PortFast causes an interface configured as a Layer 2 access port to enter the forwarding state immediately, bypassing the listening and learning states. You can use PortFast on Layer 2 access ports connected to a single workstation or server to allow those devices to connect to the network immediately, rather than waiting for spanning tree to converge.

I hope this helps.


&quot;I can picture a world without war. A world without hate. A world without fear. And I can picture us attacking that world, because they'd never expect it.&quot;
- Jack Handey, Deep Thoughts

 

[STF26]

So, the only ports that you should have the spanning tree running on are ports that are connected to other switches and hubs etc??? Ports that have workstations connected to them should be set to portfast???

 

[IPKONFIG]

Portfast for hosts/servers only.

&quot;I can picture a world without war. A world without hate. A world without fear. And I can picture us attacking that world, because they'd never expect it.&quot;
- Jack Handey, Deep Thoughts

 

[STF26]

What are examples of ports that you would leave the port fast off?

 

[IPKONFIG]

connections to other network hardware such as switches and routers. It was designed specifically for switches and to prevent loops in your L2 network. That's why you only put portfast on your host connections only.

&quot;I can picture a world without war. A world without hate. A world without fear. And I can picture us attacking that world, because they'd never expect it.&quot;
- Jack Handey, Deep Thoughts

 

[STF26]

Thanks for your help everyone.

Also, is there a way to turn on port fast on many ports at once? These are 48 port switches so it could take a while. By default the switch comes with the STA turned on for everyport. Why do they do that?

 

[IPKONFIG]

No problem. Yes you can turn on STP on multiple ports at the same time.

Cat OS: set port host 2/1-48 (this will get all 48 ports on module 2)

IOS: interface-range fa 2/1 - 48 (This will put you in the interface range prompt, then you can type your portfast command)

router(interface-range)#spanning-tree portfast

&quot;I can picture a world without war. A world without hate. A world without fear. And I can picture us attacking that world, because they'd never expect it.&quot;
- Jack Handey, Deep Thoughts

 

[STF26]

Is there a command that I can run that will tell which ports have computers on them and which ones have switches or hubs plugged into them?

 

[MJewell]

You could look at the CAM tables and see which ports have more then 1 MAC address associated with it...

-Mike

 

[STF26]

How do I look at the &quot;CAM&quot; tables?

 

[MJewell]

on Cisco IOS, you do:
show cam-address-table

on CatOS, you do:
sh cam

on both you can add a ? to refine the commands further...

-Mike