Setting MTU on Cisco 3750G Stack

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Dec 5, 2003
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I am currently setting up an independent network for my SAN traffic. One of the recommendations from the manufacturer was to enable jumbo frames. On the 3750G it appears as though this must be done globally. I entered into priv exec mode and entered "system mtu jumbo 9000." After that I reloaded the switch and checked the MTU on my interfaces with "show interface gi 1/0/1." Further, the instructions indicated that I need to enable an individual VLAN to use Jumbo Frames (doesn't make sense according to the show interface command). Back into priv exex mode I entered VLAN 500 and then mtu 9000. I saved and reloaded the configuration.

After the reload I checked the configuration. Show interface VLAN 500 incidated the MTU was 1500, show interface GI1/0/1 (assigned to VLAN 500) indicated the MTU was 9000, and finally show vlan indicated the MTU was set at 9000.

"show vlan"
VLAN Type SAID MTU Parent RingNo BridgeNo Stp BrdgMode Trans1 Trans2
---- ----- ---------- ----- ------ ------ -------- ---- -------- ------ ------
1 enet 100001 1500 - - - - - 0 0
500 enet 100500 9000 - - - - - 0 0

"show interface vlan 500"
Vlan500 is up, line protocol is up
Hardware is EtherSVI, address is 0024.98af.5d41 (bia 0024.98af.5d41)
MTU 1500 bytes, BW 1000000 Kbit, DLY 10 usec,

"show run interface Gi1/0/1"
interface GigabitEthernet1/0/1
description BAMBINO 1 - 1
switchport access vlan 500
switchport mode access
flowcontrol receive desired
end

"show interface Gi1/0/1"
GigabitEthernet1/0/1 is up, line protocol is up (connected)
Hardware is Gigabit Ethernet, address is 0024.98af.5d01 (bia 0024.98af.5d01)
Description: BAMBINO 1 - 1
MTU 9000 bytes, BW 1000000 Kbit, DLY 10 usec,

"show system mtu"
System MTU size is 1500 bytes
System Jumbo MTU size is 9000 bytes
Routing MTU size is 1500 bytes



What is going on here? Did I miss something? Why do I need to change the MTU on a VLAN if the global MTU setting applies to every interface? What is the explanation for the above results?
 

mattjw916

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This sounds like a good candidate for a TAC case really.

I suspect it will be something like the switching engine can do jumbo but the routing engine can't. A switched virtual interface (SVI) is created in software, it's not the same thing as a physical interface. There are plenty of IOS commands that behave differently on an SVI versus a physical interface whether it be a switchport or a routed port.

Either that or there are additional config commands needed. It's been a long time since I played with jumbo though.
 

QHalo

2[H]4U
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Reading over the Cisco documentation on setting them up it appears that the system mtu jumbo 9000 command is applied to all Gig interfaces on the switch. The only documentation that I see where you can change the MTU on a VLAN is on a 6500 series switch. Someone else might step in and trump my answer though. I'd also try sending some traffic through that interface and see if jumbos are being sent.

http://www.cisco.com/en/US/products...onfiguration_example09186a008010edab.shtml#c3
http://www.cisco.com/en/US/docs/swi...ive/configuration/guide/sw_int.html#wp1048529
 
Last edited:

xphil3

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Open a Tac case? How about read the documentation ;)

http://www.cisco.com/en/US/docs/swi...2_44_se/command/reference/cli3.html#wp1948947

To me, its pretty clear. Its not supported in this platform, plain and simple.

You cannot configure a routing MTU size that exceeds the system MTU size. If you change the system MTU size to a value smaller than the currently configured routing MTU size, the configuration change is accepted, but not applied until the next switch reset. When the configuration change takes effect, the routing MTU size defaults to the new system MTU size.
Since this platform can support a maximum MTU of 1998, your SVI's are going to be limited to below that. Why you may ask? You say... well the CPU isn't switching the packets! ASICS w/ CEF is why I spend so much moneys!! Think of the packet flow, the first packet MUST be switched by the CPU(not matter what platform you're using, unless its hardware assisted), if the switch receives a packet with an MTU greater than 1998 the system will drop the frame and never create the CEF entry. Again, simple and easy answer is that you cannot route packets greater than 1504 bytes.

HTH
 

xphil3

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Yeup, my time is more valuable than TAC's... unless I'm bored.
and this is the reason why TAC engineers give customers like you bad service. Learn the technology if you're going to work with it, if you're not willing to do that perhaps you should start thinking about another career :eek:
 

mattjw916

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and this is the reason why TAC engineers give customers like you bad service. Learn the technology if you're going to work with it, if you're not willing to do that perhaps you should start thinking about another career :eek:

Bad service? So you're telling me you know every command for every platform and IOS version out there off the top of your head? Don't think so chief.

I've been doing this for 15 years. Part of being a senior engineer is to know when to delegate. Why pay tens of thousands of dollars a year for support contracts if you aren't going to use them? My six figure salary thinks my career decisions are just fine kthx.
 

xphil3

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Bad service? So you're telling me you know every command for every platform and IOS version out there off the top of your head? Don't think so chief.

I've been doing this for 15 years. Part of being a senior engineer is to know when to delegate. Why pay tens of thousands of dollars a year for support contracts if you aren't going to use them? My six figure salary thinks my career decisions are just fine kthx.
15 years, 6 figure salary.... shens boss, shens... With some of the crap that you posted thats very hard to believe as you are most likely a fresh of of college young buck! . But thats not the point of this post, but please feel free to tell me where I said I know every command for every platform? Oh... I didn't:rolleyes:

Another thing, again... if you spent more than 30 seconds reading the 3750 documentation about changing MTU's you would have hit the exact section that I posted. Again, learn how to read. Whatever though, just know that you are the type of people that ALL the TAC engineers laugh at after closing a case(all of my buddies did a TAC rotation).
 

horndog

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Matt,

Why don't you go somewhere else, since you cannot add anything helpful. Besides, you probably don't even know how to get into priv mode on a Cisco.
 

QHalo

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There's a significant difference in providing someone with a helpful response that's completely devoid of sarcasm and patronization and the response that was given in this thread. You might as well just said RTFM and that's it. Why bother posting if you're going to inject a 'holier than thou' attitude?
 

xphil3

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There's a significant difference in providing someone with a helpful response that's completely devoid of sarcasm and patronization and the response that was given in this thread. You might as well just said RTFM and that's it. Why bother posting if you're going to inject a 'holier than thou' attitude?
I hope that you're not refering to my detailed explanation of why changing routing frames over 1504 bytes isn't supported, as well as providing the correct documentation :confused:

Whatever, Civic00typer you got your answer... please see my post before the shit storm
 
Joined
Dec 5, 2003
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Thanks for the information. Makes sense now... shouldn't matter if the ports can support jumbo frames and I am only working with jumbo frames within my storage VLAN.
 
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