Can only certain ports on a Cisco Catalyst be set to jumbo frames?

Hello.  I am looking to try and improve, as much as possible, the throughput for some of our office users, who are connecting to Cat 5 infrastructure cabling through the walls (until it can be upgraded to Cat 6).  The speeds of the backups for some of the users are extremely slow, and are not able to be completed due to the slow speed.  So my goal is to configure the Cisco Catalyst switch they are on to the best possible port speed, and then make sure the computers are configured as well to recognize the fastest speed for data transfer - up and down.  So - I know there is an MTU rate that can be set on the switch.  From what I'm reading so far, it looks like there is a "Global" setting and then a "System" setting.  My first question is 1) how can I tell the current MTU status of the switch? 2) Can the switch be set to a global, port-wide 9000 jumbo setting, without affecting those computers that are NOT set to "jumbo" MTU (still at 1500). and 3) Can the jumbo packet setting be used on only select computers going thru the switch?   Additionally - I'm all ears for any other tips on how to increase the throughput for a client workstation through a switch, besides setting it to jumbo packets.  

Thanks
Damian_GardnerAsked:
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Don JohnstonInstructorCommented:
It would help to know which Cisco switch since there are some exceptions.

But for the most part, the MTU is a global (system) setting.  If you enable it, devices that aren't set to send jumbo frames will not be impacted.

"show system mtu" works on some switches to see the current setting.
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JustInCaseCommented:
1) In CIsco article you can see MTU per switch and options to how and where can be configured per model (global, interface). Catalyst Support for Maximum Frame Sizes
2) Yes, frames that are smaller than MTU will not be affected.
3) Not sure

Additionally - since network topology and additional information about network is not available, it is impossible to give any improvement suggestion.
There are simply too many things that can cause network to be slow - half duplex instead of full duplex on important links (like trunks), high packet loss (EM interference, or bad cables), oversubscription of links, switches turned to hubs at some point in time (MAC learning don't exists), root bridge misplacement and many more....

Also, bigger MTU in some cases like EM interference or bad cables can have negative impact on network performance. Change MTU can only improve network performance if network is "healthy", otherwise can have negative impact.
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Damian_GardnerAuthor Commented:
Don & Pedrag - thank you for your responses.  Sorry for neglecting to tell you these are a mixes stack of older Catalyst 3750 and newer Catalyst 3850 switches - stacked together.  We will be phasing out the 3750's soon.  but this will be after we get the funds to upgrade the Cat 5 infrastructure throughout the building.  In the meantime, I'm hoping I can squeeze as much bandwidth out of the existing connections as possible.  

Per Don's suggestion - I've issued the show mtu command on the 3850 stack , as well as the 3750 stack, and find that the 3850 is at 9000 MTU, while the 3750 is at 1500.  How do I bring the 3750 up to 9000 MTU?

Pedrag - I understand that there's insufficient info in order to suggest a general network improvement plan, and thank you for your effort.  and thank you for clarifying that clients which are not set to jumbo frames will not be impacted - this is good news.  So - if I can set the 3750 up to 9000 MTU, then I would simply need to set the client NIC adapters in question to 9000 MTU as well - and as long as the network is "healthy", this should provide a boost in the packet transfer rate.  is this correct?
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Don JohnstonInstructorCommented:
How do I bring the 3750 up to 9000 MTU?
system mtu jumbo 9000

Increasing the MTU on the switch will improve throughput if the clients are set to use jumbo frames and there are no errors due to wiring.  Non jumbo clients will see no change in their performance.
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Damian_GardnerAuthor Commented:
Ok - great.  I found this summary of steps as well.  do these look accurate:

How to Configure MTU Sizes


Configuring the System MTU


 
Follow these steps to change the MTU size for switched packets:
SUMMARY STEPS
1.    configure terminal


2.    system mtu bytes


3.    end


4.    copy running-config startup-config


5.    reload


6.    show system mtu
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Don JohnstonInstructorCommented:
No. That looks like what you would do to set the MTU to a value less than 1500.  If you want jumbo frames (anything larger than 1500) you would use "system mtu jumbo 9000" on a Catalyst 3750.
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Damian_GardnerAuthor Commented:
ok - and so I would not have to RELOAD or WR MEM, or anything like that?  just looking for any extra steps.  I'll go ahead and try it...thanks
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Don JohnstonInstructorCommented:
Well yeah.  You should save the configuration.
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JustInCaseCommented:
#system mtu jumbo 9000
Will affect only Gigabit ports on Catalyst 3750. Max MTU size on 10/100 ports is 1998 bytes.
Have in mind (from link above)
Note: If Gigabit Ethernet interfaces are configured to accept frames greater than the 10/100 interfaces, jumbo frames that ingress on a Gigabit Ethernet interface and egress on a 10/100 interface are dropped.
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Damian_GardnerAuthor Commented:
ok - sorry for the delay.  I just issued the command on my 3750 stack, and it says "will not take effect until next reload".  I assume I canot reload the switch while users are actively on the network, correct?  I assume they will be dropped.
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Don JohnstonInstructorCommented:
Yeah... Some platforms are like that. So you'll need to reload the switch.
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Damian_GardnerAuthor Commented:
thank you for your help guys.  the mtu size command is helping and I'm getting better throughput now.  thanks you
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