Packet loss in a Cisco 3750 stack

32,960 pts.
Tags:
Cisco
Cisco 3750
Cisco switches
Packet Loss
Packets
VLAN
I am seeing occasional packet loss in a Cisco 3750 switch stack. There are 5 switches in the stack. The client is on switch 5 and the server is on switch 4. The client is on a separate vlan from the server. I am not seeing any errors on the ports or VLANS or NIC's. Where should I be looking to find where the packets are getting lost? The switches are connected using the switch stack ports. I do not see evidence of flapping. The packet loss is intermittent and causes client session disconnects. Any help is welcome!
ASKED: November 5, 2010  8:22 PM
UPDATED: December 6, 2010  1:06 PM

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1. Are there differences in interface speeds between the switches(ie 100Mbps vs 1Gbps)?
2. Does putting a client on the same switch the server uses make a difference?

If you aren’t seeing any errors on the switch ports or the NIC’s themselves, I would run a packet capture on either of the machines and see what shows up. If it is truly intermittent, you may have to capture for a while before you get anything usable. I would be interested to see what happens when you encounter problems. If the traffic flows are TCP based, do you get any resets or requests to retransmit specific segments? Packet captures will usually tell you where the problem lies. You might have to capture from both sides to really get a good idea of where the problem lies.

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  • Labnuke99
    Client is 100Mbps - Server 1Gbps. Both set to autonegotiate. Captures show packets leave client, server never receives packets (simultaneous capture done on both hosts). There are no RST or FIN packets during the event. The client simply attempts retransmit 4x (.5, 1, 2 & 4 secs between retransmits) and the retransmit packets are never seen @ the server. We have not moved any clients to the same switch as the server as that will not help with the client population that does have to cross the switch stack.
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  • MNorwood
    Does this hold true for any type of traffic sent from client to server? In other words, outside of whatever application(s) you are using, do you see traffic on both sides. I know you stated in the initial question that you were seeing intermittent loss, but I am curious to know if this is related to ALL traffic that can flow between the client(s) and server. I realize moving a client to the same switch won't help the other clients that have to connect via the stacking cable, but in the interest of ruling everything out, it is probably worthwhile in a particular scenario such as this. What about if you move a client to the same VLAN as the server. Still get the same results? Do you terminate L3 on the 3750 stack, or does it terminate elsewhere?
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  • Labnuke99
    The traffic of interest is telnet. Other traffic does not seem to be as sensitive to packet loss as the client telnet sessions. L3 is terminated on the switch stack for each VLAN. External subnets go to an edge router. The server switch does not have available capacity to move the client to share the switch for testing. It would be interesting for sure to try the same VLAN and see if that shows any different behavior. The VLAN interfaces show no errors or queue drops.
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  • Labnuke99
    [...] Packet loss in a Cisco 3750 stack [...]
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