Jumbo packets configured, but not working

Stephan Romano · ‎02-24-2012

I have a switch that I have configured for jumbo packets, but they don't seem to be functioning.

I have set system mtu jumbo 9000. The hosts are connected via 2x EtherChannel links. The hosts are jumbo frame enabled, and can ping their own local address using jumbo packets & do-not-fragment flags on the pings. They cannot, however, ping each other or the switch that way - it always says that the packet requires fragmentation.

I'm pretty stumped here. I know the attached machines (they're all VMs) and virtual switches support jumbos because I can ping within the virtual interfaces of the VMs. It's just traffic that goes over the switch that fragments.

The switch is a WS-2960G-48TC. Here are the various outputs, with a section of config at the end.

hrnacancwtdevs3#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

hrnacancwtdevs3#

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

hrnacancwtdevs3#show vlan mtu

VLAN SVI_MTU MinMTU(port) MaxMTU(port) MTU_Mismatch

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

1 1500 9000 9000 No

1002 - 9000 9000 No

1003 - 9000 9000 No

1004 - 9000 9000 No

1005 - 9000 9000 No

1100 1500 9000 9000 No

hrnacancwtdevs3#

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

(attempt to ping the local address on the switch itself with a large packet)

hrnacancwtdevs3#ping 10.7.100.1 source 10.7.100.1 size 2000 df-bit

Type escape sequence to abort.

Sending 5, 2000-byte ICMP Echos to 10.7.100.1, timeout is 2 seconds:

Packet sent with a source address of 10.7.100.1

Packet sent with the DF bit set

.....

Success rate is 0 percent (0/5)

hrnacancwtdevs3#

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

hrnacancwtdevs3#show interfaces mtu

Port Name MTU

Gi0/1 9000

Gi0/2 9000

Gi0/5 9000

Gi0/6 9000

Po1 SAN 1 9000

Po2 VMSRV3 9000

hrnacancwtdevs3#

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

(Interface config)

interface Port-channel1

description SAN 1

switchport access vlan 1100

switchport mode access

!

interface Port-channel2

description VMSRV3

switchport access vlan 1100

switchport mode access

!

interface GigabitEthernet0/1

switchport access vlan 1100

switchport mode access

channel-group 1 mode on

!

interface GigabitEthernet0/2

switchport access vlan 1100

switchport mode access

channel-group 1 mode on

!

interface GigabitEthernet0/5

switchport access vlan 1100

switchport mode access

channel-group 2 mode on

!

interface GigabitEthernet0/6

switchport access vlan 1100

switchport mode access

channel-group 2 mode on

!

interface Vlan1100

ip address 10.7.100.1 255.255.255.0

no ip route-cache

!

acampbell · ‎02-24-2012

Hi,

After you configured the jumbo frame support did you reload the switch

For 2960/2970 see this link :-

http://www.cisco.com/en/US/products/hw/switches/ps700/products_configuration_example09186a008010edab.shtml#c4a

2970(config)# system mtu jumbo 9000
2970(config)# exit
2970# reload

Regards

Alex

Regards, Alex. Please rate useful posts.

Stephan Romano · ‎02-24-2012

Hi Alex -

Yes, I've reloaded the switch a couple times. I even tried upgrading to the most recent firmware.

That guide was the one I initially followed when setting up jumbo frames.

Reza Sharifi · ‎02-24-2012

Can you change the size to 1998 and test again?

According to the command reference guide:

The size of frames that can be received by the switch CPU is limited to 1998 bytes, regardless of the value entered with the

system mtu

command. Although forwarded or routed frames are usually not received by the CPU, some packets (for example, control traffic, SNMP, Telnet, and routing protocols) are sent to the CPU.

http://www.cisco.com/en/US/docs/switches/lan/catalyst2960/software/release/12.2_58_se/command/reference/cli3.html#wp1948947

HTH