MTU Exceeded on Cat IOS

Unanswered Question
Jan 13th, 2009

Similar to CATOS, is there any entry to view the MTU Exceeded counter on Cat IOS.

I have this problem too.
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rohitrattan Tue, 01/13/2009 - 02:07

Hello Dear,

I believe you can check the command

"show interfaces counters errors"

I guess the last column of the output i.e. "Giants" is the one you should look at.

meanwhile i will dig into it..

Regards

Rohit

cisco_lite Tue, 01/13/2009 - 07:38

I have configured MTU 9000 on server and MTU 9216 on Cat6500 port interface.

And I can see the Giants counter going up on show interface counters errors. Are the jumbo frames being dropped ?

rohitrattan Tue, 01/13/2009 - 08:10

Hello Dear,

If the counters are going up i believe that the frames are being dropped, could be due to FCS error also. If you are having trouble configuring Jumbo Frames, i can provide some info that maybe useful.

Firstly Cat 6500 supports Jumbo Frames up to 9216 bytes but some line cards support Jumbo Frames only up to 8096 bytes due to some ASIC limitations. You can try and decrease the Frame size to 8096 on cat 6500 and a bit lower on the Server.

Secondly you have the following commands for configuring MTU and Jumbo Frames

(config)#System MTU 1500

(this applies to all interfaces)

but if you configure the following command

(config)#System MTU jumbo 9000

(this only applies to Gig ports and 10/100 ports use the system MTU setting)

also note that if you have configured Jumbo Frames that ingress on a Gig ports and egress on a 10/100 port those are dropped.

check this link for details:

http://www.cisco.com/en/US/products/hw/switches/ps700/products_configuration_example09186a008010edab.shtml

Regards

Rohit

cisco_lite Tue, 01/13/2009 - 08:35

Thanks.

In the below output, I can see giants but not CRC or any input output queue errors. Ingress and Egress both are gig ports set to the same mtu.

MTU 9216 bytes, BW 1000000 Kbit, DLY 10 usec,

reliability 255/255, txload 1/255, rxload 1/255

Encapsulation ARPA, loopback not set

Keepalive set (10 sec)

Full-duplex, 1000Mb/s, media type is 10/100/1000BaseT

input flow-control is off, output flow-control is on

Clock mode is auto

ARP type: ARPA, ARP Timeout 04:00:00

Last input never, output 00:00:20, output hang never

Last clearing of "show interface" counters 02:48:16

Input queue: 0/2000/0/0 (size/max/drops/flushes); Total output drops: 0

Queueing strategy: fifo

Output queue: 0/40 (size/max)

5 minute input rate 1000 bits/sec, 1 packets/sec

5 minute output rate 13000 bits/sec, 2 packets/sec

12247 packets input, 19595925 bytes, 0 no buffer

Received 16 broadcasts (0 multicasts)

0 runts, 2995 giants, 0 throttles

0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored

0 watchdog, 0 multicast, 0 pause input

0 input packets with dribble condition detected

15763 packets output, 26639834 bytes, 0 underruns

0 output errors, 0 collisions, 1 interface resets

0 babbles, 0 late collision, 0 deferred

0 lost carrier, 0 no carrier, 0 PAUSE output

0 output buffer failures, 0 output buffers swapped out

Server is set to 9000. Interface on Cat6500 is set to 9216. The line card used is not one of those included on the exception list in the given link.

rohitrattan Tue, 01/13/2009 - 08:54

Can you please provide the configuration you have done..i would like to see how you have configured it.

Regards

cisco_lite Tue, 01/13/2009 - 10:10

The interface configuration is as follows:

interface GigabitEthernet4/11

switchport

switchport access vlan 99

switchport mode access

mtu 9216

spanning-tree portfast

spanning-tree bpduguard enable

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