Dell equalLogic - cisco 3750x and jumbo mtu 9000 - seeing oversize packets

beanadmin · ‎10-31-2011

Hello,

We resently changed ISCSI NIC's on all servers to jumbo fames 9014 and flow control (generate and Rx). Then programed C3750 identical as listed in document link below setting system MTU and flow control. I have provided output from C3750 showing configuration and yes saved and reload after setting MTU to 9000.

The issue is I'm now seeing overside packets on all ports in VLAN15, which show MTU set to 9000. Should I have set the MTU higher then 9000?

http://www.cns-service.com/equallogic/pdfs/tr-cisco-3750-2970-switches.pdf

From switch:

>ISCSI-STACK#sh 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

-------------------------------------------------------

>VLAN

vlan 15

name ISCSI

mtu 9000

------------------------------------------------------

>port setting:

interface GigabitEthernet1/0/3

switchport access vlan 15

switchport mode access

flowcontrol receive desired

------------------------------------------------------

ISCSI-STACK#sh interfaces gigabitEthernet1/0/3 mtu

Port Name MTU
Gi1/0/3 9000

Thank you,

JC

Vu Phan · ‎10-31-2011

Catalyst 3750 series switches support an MTU of 1546 bytes for all
10/100 interfaces. Jumbo frames up to 9018 bytes are supported on all
Gigabit Ethernet interfaces. The MTU cannot be changed on an individual
interface, it must be set globally. The switch must be reset afterwards
for the MTU change to take effect.
 
Since your 3750 is a gig port switch it supports jumbo frames up to 9000
bytes. Here is the configuration to follow:
 
3750(config)# system mtu jumbo 9000
3750(config)# exit
3750# copy run start
3750# reload
 
To check configuration:
 
3750# show system mtu
 
For more information please make reference to the following links:
 
http://www.cisco.com/en/US/docs/switches/lan/catalyst3750/software/relea
se/12.2_25_sed/configuration/guide/swint.html#wp1154596
 
http://www.cisco.com/en/US/products/hw/switches/ps700/products_configura
tion_example09186a008010edab.shtml#c3

jumbo frame max MTU is 9000, but you may see some frame slightly larger up to 9018

beanadmin · ‎11-01-2011

Thank you for the reply.

The question is not how to, that is done exactly according to Cisco and DELL recommendations.

The outputs above from switch shows all is set correctly. My issue after completing settings on switch and server NIC's and reloading the stack, I'm now getting RX oversize packets on each of the ports that are in VLAN 15 that has MTU set to 9000.

According to Cisco RX oversize packets are discarded and a sign that the MTU is not configured correctly. Based on switch settings listed above settings match server NIC jumbo settings. The only difference is on server NIC's your choice for Jumbo frames is Disabled, 4088, or 9014bytes. From your reply the MTU 9000 on switch should not have an issue with servers jumbo MTU 9014, but I'm getting RX oversize packets on each server port configured with Jumbo frames 9014-- via the VLAN MTU setting 9000, which these ports are assigned and a (show interface gigabitEthernet1/0/3 mtu) clearly shows the port is using the VLAN MTU setting of 9000.

Thanks,

JC

beanadmin · ‎11-01-2011

After more testing it appears that VLAN 15 and those ports associated with it are not actually handling MTU 9000 and still 1500. I changed one of the servers to 4000MTU and the port still shows oversize packets RX. I'm hoping another reload of the stack will correct the issue. I think what was missing was a reload after setting the VLAN MTU to 9000. I reloaded after setting the system jumbo mtu to 9000, but not the after changing the VLAN MTU.

Wish me luck!