cisco cat2960: large frames

Unanswered Question
Oct 7th, 2009

Hi All.

I was checking my ports on errors on a subject switch and all was OK. but one ios command -

"show controllers ethernet-controller <my interface>" has shown me that some ports have

"Too large frames" in transmit section and "Valid Frames, too Large" in receive section.

does it mean that my switch drops this frames?

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dhananjoy chowdhury Wed, 10/07/2009 - 03:22

The default MTU size for frames received and transmitted on all interfaces (10/100 or GigE) on the 2960 switch is 1500 bytes.

However, the 10/100 interfaces can be configured for jubmo frames upto 1998 bytes and for GigE interfaces to 9000bytes.

If GigE interfaces are configured to accept frames greater than the 10/100 interfaces, then jumbo frames received on a Gigabit Ethernet interface and sent on a 10/100 interface are dropped.

alex-talker Wed, 10/07/2009 - 05:21

It needs me because i had set up rspan for analyzing traffic in interesing port and I wish to know what device sends large frames.

At the start of my experiment I had default mtu on all ports - 1500.I have analyzed network traffic in interesting port and seen nothing. then i have changed mtu on my port (GigE) to 9000 - max value and repeated experiment and again seen nothing.

Does it mean that frames bigger than 9000 and switch drops it?

dhananjoy chowdhury Wed, 10/07/2009 - 20:43

Frames bigger than the configured size will be dropped.

Coming to the RSPAN part, how are your switches stacked? Is it a GigE port on both the switches with 9000 mtu?

alex-talker Thu, 10/08/2009 - 02:18

I have no stack. My cisco-cat switches are connected through twisted pair (utp5).

с2960[gi0/1]======[gi0/1]с2950[gi0/2]======[gi1/0/15]с3750

====== - it s gigabit trunk connection.

i have checked ports on other switches and it has surprised me: c2950 have no large frames , but c3750 have large frames.

I have set mtu in 9000 only on c2960. i thought that some one who is connected to this switch(PC or other network device) sends large frames. I did not think that large frames can be on other switches.

it seems to me that problem has occured because of ipx traffic in my network and fallback bridging on my c3750.

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