3750E jumbo mtu on port-channel?

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Oct 6th, 2008
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Hi,

I have a single 3750E-24PD switch running 12.2(40)SE IOS. I have configured 'system mtu jumbo 9198', and i have configured a port-channel for ports gig1/0/27 and gig1/0/28 (i'm using a twingig module to house two 1 gigabit GLC-SX-MM GBICs). 'channel-group 1 mode on' is configured on both gig1/0/27 and gig1/0/28.


The MTU size on physical ports gig1/0/27 and gig1/0/28 is showing as 9198 bytes, but the MTU size on the port channel is showing as 1500 bytes.


There is no 'mtu' command available under the port-channel interface configuration to increase the MTU size.


Is this reporting of 1500 bytes MTU on the port-channel cosmetic? Will the port-channel inherit the jumbo MTU size from the physical inerfaces when the port-channel becomes active?


VEZ-3750E-1#sh int gig1/0/27

GigabitEthernet1/0/27 is down, line protocol is down (notconnect)

Hardware is Gigabit Ethernet, address is 0007.0e3b.1d1b (bia 0007.0e3b.1d1b)

MTU 9198 bytes, BW 10000 Kbit, DLY 1000 usec,


VEZ-3750E-1#sh int gig1/0/28

GigabitEthernet1/0/28 is down, line protocol is down (notconnect)

Hardware is Gigabit Ethernet, address is 0007.0e3b.1d1c (bia 0007.0e3b.1d1c)

MTU 9198 bytes, BW 10000 Kbit, DLY 1000 usec,


VEZ-3750E-1#sh int po1

Port-channel1 is down, line protocol is down (notconnect)

Hardware is EtherChannel, address is 0000.0000.0000 (bia 0000.0000.0000)

Description: SAN Connection

MTU 1500 bytes, BW 10000 Kbit, DLY 1000 usec,

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j-mccarthy Mon, 10/06/2008 - 18:50
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I've resolved this question.


The port channel interface inherits the 9198 MTU from the physical interface when the physical interface becomes active.

j-mccarthy Tue, 10/14/2008 - 18:36
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hi, the MTU size is applied per interface, so the jumbo MTU size must be supported and configured on every interface, on every switch that the jumbo packets have to traverse. otherwise you will get jumbo frames being dropped as giants or else fragmented and reassembled which defeats the purpose of having jumbo frames in the first place.


I had the jumbo mtu aplied to several interfaces (mostly 10gige) on different switches along the transmission path but the last hop into the SAN is a 2x 1 gig port channel and I wasn't sure if the port channel logical interface inherited the MTU from the physical member interfaces, or if you had to configure it.


hope this helps.

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