2940 Gig to Fast performance trouble

[Michael Umbricht]

I am having performance issues with a 2940 switch. The network has:

Cable Modem => 2821 => 2970 => 2940 -> desktop

All of the links are gigabit except the last hop to the desktop. The internet connection is 60/20 mbps.

If I connect the 2970 to a 2940 fast port, or manually set the gig port to speed 100 the desktop computers can receive 60 and send 20. If I used the gig port set to speed 1000 or it auto-negotiates 1000 the desktop can only receive 15 and send 20.

I've tried turning flow control on or off. I've tried increasing the interface hold-queue size. I can find a setting that allows the full bandwidth.

Also, where can I find the configuration guide for the 2940? A search of cisco.com or using Google turned up a number of links to the document on the Cisco website, but when I click on them it redirects me to the Products/Switches page.

-mikeu

[Philip D'Ath]

Are you sure it is a 2940?  They are really really old.  Well past the last day of support.  I would not expect to find anything for a device this old.

Note you must set both ports to auto/auto.  You can not set one to 1000 and the other to auto.  Did you definitely configure both of them to auto/auto?

[Michael Umbricht]

Yes, it is an old WS-C2940-8TT-S. I first tried the connection with both ports set to auto speed and duplex. I then discovered that forcing the gig link to fast caused the bandwidth to increase.

I'm seeing the same behavior on 2950 and 2960.

[Philip D'Ath]

Gigabit needs 4 pairs, while FastEthernet only needs 2 pairs.  Are you using a "good" patch lead?

[Michael Umbricht]

The cable connects at gigabit speed to my laptop just fine. The switches auto connect over the same cable at gigabit speed.

The trouble only occurs when the switch is forwarding from the gig uplink to the fast ports.

If the switches have a fast link between them the throughput is 4x faster than the gig link.

But, yes, I tried a brand new one foot long CAT 6 patch cord as a test. Same results.

I'm thinking this has something to do with buffering between the gig and fast ports.