Port Diagnostics in 48 port Switch Modules(Catalyst 4006 with IOS 12.1)

[s.kadagathur]

I have seen some bad ports in 48 port switch modules in different switch chassis. We have VLans configured on these different switches and all the ports I am referring to are access ports. These bad ports were working O.K. and for some reason the end devices connecting to these ports couldn't get DHCP lease or even after assigning static IP address(eliminating the DHCP malfunction) for the end devices connecting to these ports wouldn't communicate.

I have checked the error disabled command and checks O.K. When a cabled is patched to the suspect port the link light turns green. I am puzzled because these modules are new. I do know about ESD prevention and assuming that we have taken that precaution what could have gone wrong. I appreciate to know any other way to check the hardware integrity of the the RJ45 ports.

Thanks.

[Prashanth Krishnappa]

You can do the sh diagnostics command to check for POST failures, but I do not suspect bad hardware in your case.

W2S-2.7-c4507R-A#sh diagnostics online module ?

<3-7> module slot number

W2S-2.7-c4507R-A#sh diagnostics po

W2S-2.7-c4507R-A#sh diagnostics power-on ?

active show POST results for active supervisor module

standby show POST results for standby supervisor module

| Output modifiers

W2S-2.7-c4507R-A#sh diagnostics power-on

Have you enabled portfast on the ports connected to end devices? If you have your DHCP server in a different VLAN, have you configured helper addresses? The following page should help as well

http://www.cisco.com/warp/public/473/100.html

http://www.cisco.com/warp/public/473/12.html

[s.kadagathur]

I appreciate the response. It is very informative. I will try this solution. But what does portfast got to do with the access ports. Please correct me if I am wrong. I understand portfast, uplinkfast and backbone fast are spanning tree protocol implementations and for the access ports if we know that we are not creating a paralell path we can as well disable it. I have read some where that if we know our topology well and we are in control, one could disable spanning tree altogether and also it would save some unnecessary BPDU's.

Thanks.

[Prashanth Krishnappa]

Disabling STP is not a very good idea. Is STP enabled in your network ? Try using the Macro switchport host which will turn trunking and channeling off in addition to enabling portfast

W2S-2.7-c4507R-A(config)#interface range fastethernet 3/1 - 48

W2S-2.7-c450(config-if-range)#switchport host

switchport mode will be set to access

spanning-tree portfast will be enabled

From the Port-fast document

A port running STP can have 1 of 5 states: blocking, listening, learning, forwarding and disabled. STP dictates that the port starts out blocking, then immediately moves through the listening and learning phases. By default it will spend approximately 15 seconds listening and 15 seconds learning. During the listening state, the switch is trying to determine where the port fits in the spanning-tree topology. The switch especially wants to know whether this port is part of a physical loop. If it is part of a loop, then this port may be chosen to go into blocking mode. Blocking means it won't send or receive user data for the sake of eliminating loops. If the port is not part of a loop, it will proceed to the learning state which involves learning which MAC addresses live off of this port. This entire STP initialization process takes about 30 seconds.

If you are connecting a workstation or a server with a single NIC card to a switch port, this connection cannot create a physical loop. These connections are considered leaf nodes. There is no reason to make the workstation wait 30 seconds while the switch checks for loops when the workstation cannot cause a loop. So, Cisco added a feature named "portfast" or "fast-start," which means the STP for this port will assume that the port is not part of a loop and will immediately move to the forwarding state, without going through the blocking, listening, or learning states. This command does not turn STP off. It just makes STP skip a few (unnecessary in this circumstance) steps in the beginning on the selected port.

Note: The portfast feature should never be used on switch ports that connect to other switches, hubs, or routers. These connections may cause physical loops and it is very important that spanning-tree go through the full initialization procedure in these situations. A spanning-tree loop can bring your network down. If portfast is turned on for a port that is part of a physical loop, it can cause a window of time where packets could possibly be continuously forwarded (and even multiply) in such a way that the network cannot recover.

[s.kadagathur]

I appreciate your answer. We have Nortel's Bay 450 and Passport Switches also in our environment. We have disabled spanning tree altogether in Nortel's switches for the access ports although Nortel comes with a feature similar to portfast from Cisco called fast start. The reason we have disabled stp is for Novell's clients which does behave badly when stp is enabled on the access ports. I understand that it is not a good idea to disable spanning tree and so far we do not have any probelm from disabling spanning tree on our Nortel's switches. We have enabled spanning tree for uplink ports although it may not be necessary when there are no redundant paths. My question is and the answer from your statement "If you are connecting a workstation or a server with a single NIC card to a switch port, this connection cannot create a physical loop". I am trying to decipher the logic of portfast for access ports. Thanks.

[g.shashank]

Hi,

Try swapping the patch cable from a working port to the port u suspect.

This help in troubleshoot the root cause.

1. Did u get the connectivity after swapping.

2. Also let me know your os version.

regards,

[s.kadagathur]

Yes indeed I changed the patch cable. I didn't get connectivity after changing the patch cable. Infact I used my laptop computer directly patched to the suspect switch por and no luck!

Thanks.

[ebuena]

Do you encounter some errordisable on the port you're encountering a problem. Did you try to modify the speed and duplex setting of the access switch. what nic card you're using on the client side when connecting to this particular switch.

[s.kadagathur]

Thanks for responding. No there is no error disabled reported on the port. I have not run the show diagnostics yet on the said port or ports yet and I want to be sure running some command doesnot adversely affect the switch environment globally(since I am new to Cisco switches). Thanks to PRKrishn earlier for his help in giving me hints and tips. I do not have the NIC card information with me. I understand speed settings would make the port and the client experience errors(like FCS due to duplex mismatch) and this could result in slow network responses and how does this affect the port overall? and in our case the client is not initializing like getting a DHCP? But trying different scenarios won't hurt. Thanks again.