Two 3750 on a stack

Unanswered Question
Aug 19th, 2007

Dear all,

I'll miss the introduction and proceed to explain my problem.

I have two WS-C3750G-24TS catalyst switches in a stack. Couple months ago I found out an issue with them and two days ago understood with my friends at work what exactly the problem is. Of course we use them as a layer 3 devices and they are responsible for many bgp sessions. When I try to simply ping a host from one vlan to another, where should come the routing process in the stack, there is no problem if the vlan member (host) is on the same switch. If the traffic have to move through the stack cables to the other stack member appears high latency, sometimes big losses of packets. If I try to ping a host on the same vlan but in another stack member, there's no problem and the traffic is great - no issue. One of my friends said it should be a hardware issue of the platform. The stack is build with the desktop routing template. The software is C3750-ADVIPSERVICESK9-M, Version 12.2(25)SEE, RELEASE SOFTWARE (fc2). A single server connected to the second switch (the emptier, low used) did run up the stack cpu usage by more than 20%. Please, provide some light in the tunnel. Is there any possible way to fix it? Thanks in advance!

I have this problem too.
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ralexandrov Sun, 08/19/2007 - 13:23

Which category of the config you would like to view? I am not able to post the entire configuration, due to security and company policy.

Edison Ortiz Sun, 08/19/2007 - 13:34

How about these commands

show platform stack-manager all

show switch detail

show switch neighbors

show switch stack-ports

show switch stack-ring activity detail

Have you considered replacing the stack cables ?

ralexandrov Sun, 08/19/2007 - 16:13

The output of the other commands is in the attachments.

Your question - we've not considered to change the stack cables, just because the switches are forwarding the traffic when there's no inter vlan routing between the stack members. I've explained it on the first post. So I don't think the problem is in the cables.

MUHAMMAD SHAHEEN Mon, 08/20/2007 - 01:27


I think your Switch1 Stackport1 is connected to Switch2 Stackport1 and Switch 1 Stackport2 is connected to Switch 2 stackport2. This does not make it to run in a ring.

I have just double checked my stacks to verify this.

you should connect your cables correctly to make it a ring. Like this....

Switch1 stackport1 <---> Switch2 stackport2


Switch2 stackport1 <---> Switch 1 stackport2


the same rule applies if you have say 9 switches in a stack, means stackport2 from 9th switch will connect to switch1 stackport2 to make it a full ring.

Have a look at the following link.......



ralexandrov Mon, 08/20/2007 - 04:29

Thanks for the reply! I'll take a look at this next time I go to the co-location center. I'd watched them many times, and I thing they are connected by the right scheme, but we'll see. The link you posted and many more others were read by myself before I decide to start the conversation in here (hope i've missed some!)

ralexandrov Mon, 08/20/2007 - 05:03

I've checked the cables. They are correctly connected to the stack ports. If I elect the other switch as a master could the problem be solved? Now it's just forwarding the traffic, and the routing process is done by the master switch according to documentation. I plan to spend some time debugging them. It's a bit silly, but I can not do any experiments that may interrupt the normal working process, because they are the backbone of the network.

MUHAMMAD SHAHEEN Mon, 08/20/2007 - 06:14


Have you tried using just 1 stackcable to link both switches.

In Stage A, Just connect stackport2 from Switch1 to Stackport1 on Switch2. Monitor the situation and let us know.

If you still get issues then disconnect this cable and use the other cable to connect the other 2 stackports between both switches.

If you still get problems, then post the output from "show tech-support", do remove confidential info before porting this here.

Please let us know the outcome



Danilo Dy Mon, 08/20/2007 - 06:32


Can you check the duplex?

show switch stack-ring

Can you post the Switch LED that is lit (Green or Orange) for both switch?

ralexandrov Sun, 09/02/2007 - 10:35


Well there's no such command "show switch stack-ring", but there is "show switch stack-ring activity (detail)" which I don't understand. The LED lights are all green. A friend of mine told me there's no command to show the duplex of the stack. The only way it can be seen is via snmp. I found a script, which checks the duplex and say it is half, but I'm not pretty sure it is true. Anyone know the correct OID to be sure about the status of the stack?


ralexandrov Sun, 09/02/2007 - 11:21

Forgot to say which OIDs are in the script:

cisco_stack_table = ""

cisco_stack_state = ""

cisco_stack_ring = ""


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