Stange Timeout from 3750 Switch

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Oct 1st, 2010
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I was hoping somebody maybe able to have some suggestions with a parculiar problem we have with Layer 3 connectivity to a 3750 switch which is deployed in a remote location. The problem we are experiencing is unexpected timeouts when trying to connect either from or two the switch from a remote network. It's unexpected as we have no problems with the windows machines connecting over the same link through the same switch, but the cisco switch seems to be dropping the icmp packets beyween two radio's onwards on one of the microwave links between the two locations. This is all layer two and no routing between any of the networks is involved just one large /16 subnet.

The lay of the the land is a 3750 switch has two vlans configured whereby connected to port Gi1/0/2 that is a 3com switch with a a load of windows servers, workstations attached. Connected on port gi1/0/1 is a microwave radio (or a series of them!) to form the link to an alternative location with another small network both are within the same VLAN on the switch. I have poured over the config and naturally the vlan is up, there is no ACL's at all; i have played with TTL by in an extended ping but to no avail. It's just for some reason icmp packet's are being dropped between two radio's and after that nothing when pinging from and to the switch, there is no ICMP disabled on the radio's's just strange.

If anybody's got any idea's i would be glad to hear them!


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Calin Chiorean Fri, 10/01/2010 - 02:31
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Hello Mark,

There are a couple of things to check in your case:

show proc cpu history -> see if there are spikes when the processor is load 100% and for how long; this can indicate that an event is generating to much load on the switch

show proc mem sorted -> check how much memory you have free / occupied and by who

show interface x/y -> (x/y is a generic name for your interface; replace it with actual interface name) check for any kind of errors and see if the counters are increasing and how fast

show interface x/y status -> check if you have any issue with auto-negotiation of speed and duplex; or if you set them manually check not to have them generate random disconnections

show log -> most important check the logs; you have enable logging you might find there what's causing your problems (if you didn't enable logging, now it would be the right time to do it)

Check for collisions in your wireless devices. Since I don't know how this are configured, I just might assume that they generate a single collision domain. Yes, I know, collisions are old stuff, but sometimes you'll be surprised how many problems they can create.

You can paste here the output of the commands above, and we'll continue to help you!

Good luck


sonofa-gun Fri, 10/01/2010 - 04:25
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Thanks for Response Calin,

I have done a couple of screen dumps of the show results that you mentioned and enclosed in a text file. There is indeed a duplex mismatch shown in the log  although this is on the interface that connects to the 3Com switch and the internal network and not the interface that is connected to the radio links although this is on the same vlan. This i have tried to resolve previously although changing the duplex settings on that particular interface has done nothing more than kick me off and unable to re-connect thereafter. The switch it is connects to is not manageable so i haven't been able to clear it. This is not helped by the fact this is also 6k miles away off the shores of Africa!

A really good point you mentioned about the collisions on the radio link, so i will check that out although what my client has over me which i can't answer is why it affects just the switch and not the windows boxes that connect through it.

Calin Chiorean Fri, 10/01/2010 - 04:30
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Mark, may I ask you to do a basic drawing with your topology and share it with us? On this drawing be sure to indicate from where are you connecting to the switch when you have issues.

I will have a look into the dumps that you attached.


sonofa-gun Fri, 10/01/2010 - 05:44
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Here's a scanned hand drawn diagram of how things are. I hope it's legiable although let me know if anything is not clear.



Calin Chiorean Fri, 10/01/2010 - 06:30
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Hi Mark, and thanks for drawing. I have a now a picture of your topology, but there is something that I don't understand. You said that everything is Layer 2, no routing and you have just a big /16 subnet. Then, why you have 2 VLANs?!


sonofa-gun Fri, 10/01/2010 - 06:50
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Hi Calin, fair question. They have just recently configured an additional vlan with a seperate subnet on the switch so yes they will be routing traffic to that subnet although the rest of the network is one large flat broadcoast domain which is far from ideal particularly with the wireless.

What they want todo is setup static routes on some pc's on the far end network to access the new subnet although as they can't route traffic to the vlan svi on the switch they can't route the traffic to the new subnet (vlan 2 subnet on the diagram). I am working with them in segmenting there network and having some kind of design in place although this is one of there immediate requirments.




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