3750(G) / Jumbo frames

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Dec 4th, 2009
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On a specific VLAN - used for storage - I'd like to enable jumbo frames (9000 bytes).

If I check the manual, it states than it's possible to do this on a VLAN (command VLAN MTU 9000), but on the other hand, Jumbo frames need to be configured via SYSTEM MTU JUMBO 9000, affecting all gigabit-ports.

Is it possible to use jumbo frames for a single VLAN, or what does it do on the VLANs with an MTU of 1500 when the system mtu jumbo-parameter is set  ?  And can this be forwarded to another stack as well ?

I have some difficulties in the traffic between some stacks, which contain also C3750-switches with fast-ethernet ports, and I think it's because of the jumbo frames-setting in the stack in the server-room.  Also the connection to other switches (2924 / 2950) seem to have some problems, and also here I'm thinking the trunking is having problems since the configuration of jumbo-frames...

Current IOS: Cisco IOS Software, C3750 Software (C3750-IPBASE-M), Version 12.2(35)SE5

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Alain Cloet Tue, 12/08/2009 - 06:12
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Really no-one that has a clue? 

It's already certain that the jumbo-frames were causing problems on another vlan, so working without 'system mtu jumbo' is necessary...After disabling this, the network was running normal again...

If I can't do it with VLAN's, I either have to go for physical separation, meaning buying 4 new switches or just live with the fact I can't use Jumbo-frames...or am I overlooking something else ?

corjam Wed, 12/09/2009 - 11:28
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Do you need to have Jumbo frames set for specific servers or are you just trying to get more bandwidth throughout your entire network by enabling this globally? If you are just wanting to get better throughput for servers, I would suggest only setting the Jumbo mtu on the ports that the servers are connected to not the entire Vlan.

Alain Cloet Thu, 12/10/2009 - 01:12
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I need it for all the servers on the VLAN (as that VLAN is exclusive for storage), with the 3750G-switches it's even impossible to set it port-based.

With the current settings (system jumbo at 1500 and storage-VLAN set to 9000)  it shows the 9000 for the specific VLAN with "show VLAN", but the ports which are access-ports on the VLAN, it still shows 1500 with show int Gi1/0/xx ...

So I don't know if it's working with Jumbo frames or not on the VLAN or not.

Is there a way to verify this ?  Didn't try monitoring at the moment, as we are in a temporary room, connected to switches that don't allow remote monitoring ...

corjam Thu, 12/10/2009 - 06:24
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After you applied the jumbo command globally, did you reload the switch? If not you need to reload it to make sure that it applied and I would also recommend that you create a span to monitor the traffic with a laptop and maybe wireshark to see if it is indeed passing jumbo frames. That would at least let you know if your switch is configured correctly.

Alain Cloet Fri, 12/11/2009 - 00:34
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Yes, I did a reload of the switch, that's how we noticed it didn't work like we wanted .  Than I turned off the jumbo frames again, and did a new reload to undo the change...

I got info that it should work correctly bases on info given by the storage device, but I want to verify this...(but probably will only be able to do so in 2 weeks... )


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