Errors on C6500 CATOS

Unanswered Question
Jun 26th, 2009


We have a C6500 running in hybrid mode and have seen alot of errors on 3 interfaces which are bundled together and connect to another C6500.

The errors are jumbo frames. in the counters of the port we see alot of rxOversizedPkts and dot3StatsFrameTooLongs errors.

The weird thing is that on the other side, no jumbo frame support is configured, MTU is 1500.

Anyone faced this problem? Versions are:

8.3(3) and 12.2(18)SXF10.

Thanks in advance!



I have this problem too.
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Giuseppe Larosa Fri, 06/26/2009 - 05:54

Hello Dario,

is MPLS in use that could add 4 or more bytes overhead ?

Hope to help


dario.didio Fri, 06/26/2009 - 06:02


thanks for the quick reply.

No, MPLS is not used.

Physically its like this:

C6500 CATOS --- Po of 3 Gig links ---- C6500 IOS

I get the errors on the side of the CATos.

The strange thing is, Jumbo frames are not configured anywhere on the network.

If I look at the counters on the IOS side, no Jumbo frames are received.

If I look at the counters on the CATos side, jumbo frames are received on the 3 links in the port-channel.

It seems like the the jumbo frames are originating from the IOS switch itself, neverless MTU is 1500 on all 3 Gig ports.



dario.didio Mon, 06/29/2009 - 04:08


Did some sniffing and I saw alot of frames that are 1514 bytes in Wireshark (it is a trunk, but I can't sniff .1q tags), so I assume the frames are 1518 bytes which should be Ethernet II compliant.

However, on the CatOS I see the errors incrementing (rxOversizedPkts).

Is the MTU by default 1500 or 1518 bytes?

Is it normal that these frames are marked as too large?

Thanks in advance.



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