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TCP delay on catalyst switch

i experienced a TCP delay on catalyst 4506, avoid the problem when i replaced 4506's with dummy unmanaged switches.

i used two PCs(PC 1 and PC 2) and two 4506 switches (S1 and S2)

PC 1 is connected to S1 (fast ethernet port)

PC 2 is connected to S2 (fast ethernet port)

S1 is connected to S2 (SFP gigabit ethernet port)

-I started continuous UDP,TCP,MULTICAST and PING from PC1 to PC2

-I unplugged link between Switch 1 and Switch 2

all communication stopped.

-I plugged link between Switch 1 and Switch 2

-UDP,MULTICAST and PING started immediately but TCP started with approximately 15 seconds delay. :-(

I repeated same procedure with unmanaged dummy switches instead of 4506, there wasnt 15 seconds delay. TCP showed up in 1 second.

How can I avoid TCP delay on catalyst switches? Probably some tuning with configuration would do the job?

tx for helping

VIP Purple

Re: TCP delay on catalyst switch


do you have the ports on the 4506 switches configured with ´spanning-tree portfast´ ?

And are the ports configured as static access ports, and with fixed duplex and speed settings (configure fixed speed and duplex on the PCs as well) ?

interface FastEthernet0/1

switchport access vlan 1

spanning-tree portfast

speed 100

duplex full



Community Member

Re: TCP delay on catalyst switch

hi gp and thank you very much for responding to this unusual problem.

- switch ports to the PCs are configured as portfast.

- switch ports between two catalyst switches are not configured (default)

- i didnt use the 'switchport access' command since they are default layer 2 interfaces. would 'switchport access vlan 1' command make any difference?

- i looked at the port status and confirmed connection is 100 mbps full duplex.

unusual issue is; ping, udp, multicast shows up in a very short time after I re-plug the uplink. that proves all ports are in forwarding state. only TCP shows up with delay, which doesnt occur on 200 $ unmanaged switch??

thanks in advance for any suggestions

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