System Alternate MTU

Answered Question
Feb 8th, 2012

Hello All,

I'm doing some tests with a new IOS version (12.2(55)SE4) for the ME3400E. During the tests I've noticed that "show system mtu" command shows more information than the previous image:

### IOS 12.2(52)SE ###

EDD_RFS#sh system mtu

System MTU size is 1500 bytes

System Jumbo MTU size is 9000 bytes

Routing MTU size is 1500 bytes

### IOS 12.2(55)SE4 ###

EDD_RFS#sh system mtu

System MTU size is 1500 bytes

System Jumbo MTU size is 9000 bytes

System Alternate MTU size is 1500 bytes

Routing MTU size is 1500 bytes

I've done some research but I couldn't found much info about the command output, and specifically regarding "System Alternate MTU". Could someone help me understand what is “System Alternate MTU”, and the difference between “System MTU” and “System Alternate MTU".

Thanks!!!!

Cheers,

Pedro

I have this problem too.
0 votes
Correct Answer by Ivan Krimmel about 3 years 1 month ago

Hi Pedro,

Alternate MTU, by itself, does nothing until you specifically assign it to a specific 10/100 or GE port

It defines a third interface MTU that fits between the system MTU and jumbo MTU.The system MTU is the interface MTU for the 10/100 ports. The jumbo MTU is the interface MTU for the GE ports. The alternate MTU can be an interface MTU for any 10/100 or GE port. You apply the Alternate MTU with a global command that maps the alternate MTU to the desired interfaces.  A change to the alternate MTU or any of its applied interfaces, like the system and jumbo MTU commands, requires a reload.

HTH,

Ivan.

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Overall Rating: 5 (4 ratings)
pedro.morais Wed, 02/08/2012 - 09:27

Thanks Ivan!!!

Two questions:

- In what version was"Alternate MTU" introduced?

- How can I apply the "Alternate MTU" to a desired interface?

Once again thanks for you help.

Cheers,

Pedro

Ivan Krimmel Wed, 02/08/2012 - 09:34

Hi Pedro,

my pleasure.

it looks like we've got this feature starting from 12.2(55)SE -

http://www.cisco.com/en/US/docs/switches/metro/me3400/software/release/12.2_58_se/configuration/guide/swint.html

the syntax to have it configured is straightforward:

system mtu alternate interface {interface-id | range interface-range}

in global conf mode.

I hope that helped!

Ivan.

Correct Answer
Ivan Krimmel Wed, 02/08/2012 - 09:12

Hi Pedro,

Alternate MTU, by itself, does nothing until you specifically assign it to a specific 10/100 or GE port

It defines a third interface MTU that fits between the system MTU and jumbo MTU.The system MTU is the interface MTU for the 10/100 ports. The jumbo MTU is the interface MTU for the GE ports. The alternate MTU can be an interface MTU for any 10/100 or GE port. You apply the Alternate MTU with a global command that maps the alternate MTU to the desired interfaces.  A change to the alternate MTU or any of its applied interfaces, like the system and jumbo MTU commands, requires a reload.

HTH,

Ivan.

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Posted February 8, 2012 at 9:05 AM
Updated February 8, 2012 at 9:06 AM
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