Jumbo Frames on 3560G

[byates]

We are going to be putting our iSCSI network on a couple of dedicated switches. Want to enable jumbo frames and it looks like you need to enable that for the whole switch, not single ports. So if i have a trunk port going to my Main switch (4510) from these dedicated iSCSI switches,   so i can access it remotely, etc... how would that work? The 4510 is using regular frames while the 3560 would be jumbo. Is that going to be an issue?  

[Georg Pauwen]

Hello,

as I understand it, as long as you are only using GigabitEthernet interfaces, you should be fine. 10/100 interfaces to Gigabit will not work as the system mtu does not match the system mtu jumbo size...

[shelbygomez]

If this connection is just for accessing the switch itself and not passing L2 traffic, you could configure a port as L3 and give it an IP address on your management VLAN. Then connect that IF to an access port. The default routing MTU is 1500 even if the switch is configured for Jumbo frames. For example, with management on VLAN 11:

JumboSw# show 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
JumboSw# configure terminal
JumboSw(config)# interface GigabitEthernet0/1
JumboSw(config-if)# no switchport
JumboSw(config-if)# description Management Access Port to 4510
JumboSw(config-if)# ip address 10.0.0.2 255.0.0.0
JumboSw(config-if)# exit
JumboSw(config)# ip default-gateway 10.0.0.1
JumboSw(config)# exit
JumboSw#


Another option is to configure one SVI on the management VLAN, and configure an access port with that VLAN to connect to the management network switch. I had to do that on a couple of 2960's, and never had problems communicating with them that way. Also, I don't think I used VTP, since the Jumbo switches didn't need the VLANs from the data network anyway.

JumboSw# configure terminal
JumboSw(config)# vtp mode transparent
JumboSw(config)# vlan 11
JumboSw(config-vlan)# name mgmt_network
JumboSw(config-vlan)# exit
JumboSw(config)# interface Vlan11
JumboSw(config-if)# description Management Interface
JumboSw(config-if)# ip address 10.0.0.2 255.0.0.0
JumboSw(config-if)# exit
JumboSw(config)# interface GigabitEthernet0/1
JumboSw(config-if)# description Management Access Port to 4510
JumboSw(config-if)# switchport access vlan 11
JumboSw(config-if)# switchport mode access
JumboSw(config-if)# exit
JumboSw(config)# ip default-gateway 10.0.0.1
JumboSw(config)# exit
JumboSw#

[byates]

Ok cool. I think this would probably work fine. Think i'll go with the routed port. Thanks for the input 

[byates]

Even if i am just keeping the Jumbo frames within the one VLAN?

[Leo Laohoo]

---

@byates wrote:

Even if i am just keeping the Jumbo frames within the one VLAN?

---

Yes.  

Total Output Drops don't look at VLANs.  It looks at how much data the port of a switch can process.   Think of it as going to an buffet.  You stuff yourself with food (like a server pushing/shoving data to the switch port) and then there will come a time when you say, "wait, I need a break" and stop for a while.  

The port will put the still-inbound traffic into a buffer.  Because of the speed of this transmission from the server, the buffer gets filled up very, very quickly.  Once the buffer gets full, it "spills" and this is the Total Output Drops counter. 

So without a well-configured QoS, the server will re-transmit data that were not delivered to the destination.  This will then be a complaint from someone that "there is something wrong with the network".

[Leo Laohoo]

Jumbo frames is the least of your worries.
Make sure you get QoS configured correctly otherwise you'll see Total Output Drops shoot up.

[byates]

Even if i am just keeping the Jumbo frames within the one VLAN?