Cisco Support Community
Showing results for 
Search instead for 
Did you mean: 

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. If you'd prefer to explore, try our test area to get started. And see here for current known issues.

New Member

bad 3750 performance


i have 3750-12G with all SX SFP's. c3750-i9-mz.122-20.SE4

All ports are 802.1Q trunks, with 3 ports trunking all (10) vlans, 9 ports trunking 2-4 vlans.

All routing is done via SVI vlan interfaces.

VLAN1 is workstations with ~500 nodes.

VLAN2 is voip with ~100 nodes.

VLAN4 is servers with ~50 nodes.

VLAN10 is switch management with ~25 nodes.

All ports are connected to either HP4108 or pair of chained HP2524's, except one which is Cisco 3560-48PS and HP2524 chained.

STP is enabled on most switches and is pvst on 3750 and designated root.

sdm prefer routing has been enabled.

Routing between vlans appears to slow down or stall, especially between vlan1 and vlan4, the busiest ones. But no interfaces errors or packet drops.

sh proc cpu hist gives

<10% ave per second

<10% ave, <50% max pre minute

<10% ave, <40% max per hour

I have tried with mls qos enabled and disabled, using only mls qos trust cos on each interface and with auto qos voip trust.

I have tried applying the queue/buffer changes as posted in other qos related threads with no difference.

This switch role should be core aggregator and router for the network, but seems to be struggling.

Is this the wrong equipment/setup for this relatively small network ?

Any suggestions for troubleshooting or any info from switch required to help further ? The hardware/network is getting some flak at the moment.



  • Other Network Infrastructure Subjects

Re: bad 3750 performance

Hi Craig,

In this case I will first check the kind of traffic flowing in my Vlans. There could be some malicious traffic or some unusual boradcast traffic that might be causing the slow proformance on the network. Check for Vlan interface and the port interfaces to see if its getting a lot of broadcast on those interfaces.

Try to see some errors on the interfaces on each side of the link.

Use a sniffer like ethereal to see the traffic pattern on the network and catch the culprit if any.


-amit singh

VIP Purple

Re: bad 3750 performance

Hello Craig,

in addition to Amit´s post, how are your HP (ProCurve) switches trunked to the 3750 ? And when you say that STP is enabled on most switches, do you have it turned off on some ? Also, can you check if the 3750 is definitely the root for your VLANs (if not done so yet, you can use the command ´spanning-tree vlan x root primary´ on the 3750).

Also, can you post the configuration of the 3750 ?

By the way, can you check if the user ports on your HP switches are configured with ´spanning-tree mode fast´? And you might want to set the ports connecting the 3750 to the HPs to fixed speed and duplex settings...



New Member

Re: bad 3750 performance

Hi Amit, GP,

thanks for the feedback.

Amit, i have been using etheral to identify as much as possible already, and have eliminated a lot of needless broadcast, ipx, and other stuff that could have been causing a problem.

I have found no trace of any malicious stuff either.

None of the interfaces show any errors at all, on the 3750 or the HP's.

GP, all HP's are SX fibre trunked with untagged vlan1, and tagged other vlans as required, and the 3750 is set with this type of config on each port.

interface GigabitEthernet1/0/2

description Server_Switch_1

switchport trunk encapsulation dot1q

switchport trunk allowed vlan 1-4,9-11

switchport mode trunk

mls qos trust cos

The HP4108's are not STP enabled yet, as the firmware version that was on them caused the switch to stop responding on telnet. They have been upgraded, but not enabled yet. Although i will be using RSTP not STP.

The 3750 is defintely root as it is set with the priority, and all other switches see it as root, however i havent tried the command you mention, so will use that if it makes a difference. Config shown.

spanning-tree mode pvst

no spanning-tree optimize bpdu transmission

spanning-tree extend system-id

spanning-tree vlan 1-20 priority 24576

The spanning-tree mode is relevant to 802.1D STP, but i have my HP's set for 802.1w RSTP, so i dont get that command. I have these options instead:-

spanning-tree [interface(s)] edge-port (Set that port is connected to a LAN segment that does not have any bridge connected to it.) For non trunk ports, similar to fast mode.

spanning-tree [interface(s)] point-to-point-mac (Set if the port is connected to a point-to-point segment or not.) For trunk ports ?

Not sure if i can get the right marriage between HP RSTP and Cisco PVST (or pvst+)? but i dont see why not.

I see no interface flaps or resets, and both sides have auto-negotiated fine, but i can certainly have a try at manually setting the speed and duplex if you think that may be a problem.

Other than some static routes, and vlan interfaces with IP and HSRP standby commands, most of the physical interfaces are configured similarlry.

Unless you really need the full config, i am sure you can piece together from the parts i have posted ?

There are no ACL's, no maps, policies etc. Just basic switching, trunking, vlanning and static routes, not even RIP.




Re: bad 3750 performance

Cisco 3750's also can use rapid spanning tree and you should probably make them all the same . Cisco's version still uses a separate instance of of spanning tree for each vlan even with running RSTP .

This widget could not be displayed.