MTU in switches

[Cisco Freak]

Hi Experts,

I was looking into the configuration of a 6500 switch.

6509-01#sh int GigabitEthernet1/3/31 | i MTU
MTU 1500 bytes, BW 1000000 Kbit, DLY 10 usec,
6509-01#

It shows the MTU of its interface as 1500 bytes. When I tried to change that value, I get only the 9216

6509-01(config)#int GigabitEthernet1/3/3
6509-01(config-if)#mtu ?
<9216-9216> MTU size in bytes

This is confusing me. Why is it so?

CF

[Joseph W. Doherty]

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Posting

Only Cisco could answer why, but it might be the hardware can only easily handle the default of 1500 or jumbos of 9216.

[Reza Sharifi]

Hi,

That is because the interface is configured as switchport. If you want to see <1500-9216> MTU size in bytes than you need to change the port to a layer-3 routed port (no switchport).

HTH

[Cisco Freak]

Thanks Reza!

I got one more query. As I understand this MTU command is specify the size to L2 frame without any header. So even if we set the MTU as 1500, it can go upto 1518 with L2 headers. Am I right?

Also, what is the relevance of setting MTU at switch interface level, if the host machines has got a higher MTU and it has set DF bit in the IP header.

CF

[Reza Sharifi]

Correct.  It will go up to 1518 without the vlan tag.  With vlan tag it will go up to 1522.

see link for breakdown of the segments.

http://stackoverflow.com/questions/12768566/confused-about-the-header-size-for-a-ethernet-frame

HTH

[Cisco Freak]

Thanks again Reza!!

I missed the 4 bytes CRC portion, as Wireshark never showed that one.

CF

[Joseph W. Doherty]

Disclaimer

The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

Liability Disclaimer

In no event shall Author be liable for any damages wha2tsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.

Posting

As a guess, perhaps Cisco restricts the choice of MTU on a switchport, to avoid having switchports, in the same VLAN, with different MTUs.  Consider on smaller Cisco switches, setting a jumbo MTU is often a system wide global setting.