Cisco, Switch, 2950, Configuration

Posted on 2008-11-05
Last Modified: 2011-10-19
I have 3 Cisco 2950 Switches. Recently my peripheral devices i.e., wireless access points and switches in laboratory environments on the LAN lost connectivity. If I unplug these devices from the Cisco switches into plain jane 3COM switch set at factory defaults these devices can connect to the network.  I know nothing about configuring switches outside of some basic terminology. I need to resolve this issue as these 3 switches along with the router are the backbone of my LAN.
I have attached the config from the switches I got from telnet sessions.

Question by:lhzeigler
    LVL 15

    Expert Comment

    Need to know what ports your switches are attached to for trunking. Other then that there is not much configuration on the switches. I notice on switch 3 you have portfast turned on all your ports. If you plug a switch into a portfast interface and then plug that same switch back into another switch you can crash your stack.
    Loop free is the only thing you need to worry about on a layer 2 stack like this. Since your not running any vlans.
    LVL 16

    Expert Comment

    by:Aaron Street
    if you have portfast on a interface, and BDUP gard enababled. then If you plug a switch in to that interface it wil be disababled.

    Portfast and BDUPgard are to do with spannign tree and preventing lops in the network. (or more allowing loops, but shutting one arm of the loop down automaticaly, but renabeling it is the other part of the loop fails. So good for redundence.)

    If you put a port in to port fast state, then it assumes it will not be connected to another switch. If it detectes that is has been it will be shut down.

    try running a "show log

    this will show you what happens on the ports, if any have been disababled, or any errors that have shown up.

    (see my post in for an example of a disababled port due to BPDU and portfast)

    post the log here if you are not sue and we can have a glance through it. But be warned if you reboot a cisco switch you will lose all the logs (unless you have set them up to save somewhere of the switch, or in non volitial memory)


    Author Comment

    1. Could be that the stack does it recover?

    2. I understand  from reading above that if the ports are on "port fast" that they will disable the switch port itself or the other device on the other end if it is not a switch.  Is this correct?

    3. If so then should I disable port fast on all ports on the switches except for those ports that are attached to another switch. Or can I just disable port fast on all ports?

    4. How do I re-enable the ports that are error disabled?

    5. Trunking..I have no clue what trunking is.....assuming it is those ports used to connect switch to switch.
    Switch 1>>>>>Switch 3 port 24
    Swtich 2>>>>>>>Switch 3 port 23

    I can't show log b/c I restarted the switches in an attempt to recover.

    I hope I've answered your questions.

    LVL 15

    Accepted Solution

    1) If you have a loop that creates so much traffic it could take up all the CPU of a switch and basically crash the stack.
    2) No what portfast does is stop a port from going through the spanning tree (which is a loop prevention algarithm) steps to keep loops from happening. Instead it brings up the port faster.  If you know you have a PC or end device plugged into a port you can enable portfast to make it so that when you turn on the PC your network comes up faster.
    The BDUguard that Devilwah was talking about needs to be enabled seperatly like he said to prevent possible loops by shutting down a port that has portfast enabled if it sees a switch on the other end of a link (switches send out bdu's which are small packets needed for root bridge election proccess).
    3) just the opposite you need to keep it off the switch ports.
    4) shut / no shut will reenable
    5) Trunking is when you have to send multiple vlan traffic over a single port. A trunk link is what you call the link between two switches.
    You have no VLANs configured so by default all traffic is flowing on VLAN 1. I wouldn't worry to much about trunking as long as you can pass traffic between switches everything is working fine.
    LVL 16

    Expert Comment

    by:Aaron Street
    just to clarifie portfast a bit further,

    normaly when you attacht a device to a port on a switch that has spanning tree enababled(the protical that runs the algrothems over your entire network to prevent loops)

    the port starts in a shut down state and then the switch checks with the rest of the network that enabeling the port will not casue a problem. if eneabling the port will not cause loops then the port it enababled. if it will casue a loop the port is left shut.

    however this can take 30+ seconds to go through the steps, and with many PC's able to boot in to windows in underthis time, it can mean that a PC will boot with out the network!!! to get round this, many switchs allow you to place a port in to Protfast. this means that the switch will enabable the port fully as soon as a device is attached to it. and not run the span tree protical on it. After all if it is only a PC being attached, then this will not casue loops, so there is no need to run spanning tree. However becuase it dose nto run spanning tree it can be dangrous if some one plugs a switch in to a port. To avoid this you can enable BDU gard, this can detect if a switch has been attached and will shut down the port if it has.

    LVL 15

    Expert Comment

    Did you get your nodes plugged back into the Cisco switch? Did you need anything else?

    Author Comment

    Thank you all or your help. After getting portfast off of ports that switches were connected to and restarting all 3 switches things are back to normal.

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