Can't ping above 1500 mtu between two 4510 switches

[Kgrevemberg]

Hi all!

I have an interface vlan on two 4510 switches. these switches are trunked together. There is a service running across these switches that isnt working and requires large mtu sizes. Pinging between the interface vlans linking the switches over ospf, I cannot ping above 1500. It drops out at 1501.

Any insight into this would be greatly appreciated. Do I need to enable large frames by some means?

Thanks

[Giuseppe Larosa]

Hello ,

>> Any insight into this would be greatly appreciated. Do I need to enable large frames by some means?

Yes, however there are different ways to achieve this.

You can use a global command like

system mtu  <size>

(Warning your OSPF adjacencies will flap  so you need a maintenance window)

Or you can enable jumbo frames.

You need to ensure that all L2 interfaces in the end to end path are configured and support the increased MTU.

but, first of all you need to understand what MTU is needed by this application to work.

On the other hand you can find the involved hosts likely servers and check their MTU settings.

They have likely a non standard MTU like 9000 bytes and this can cause the issue more then the application itself.

Hope to help

Giuseppe

[Kgrevemberg]

The technician was only able to tell me he thinks its around 2048. I've set 2248 on the trunk ports between the two 4510's and on the int vlan that they ospf link over. Still can't ping above 1500. I'm pretty sure i'm missing something. thanks for your help.

[Giuseppe Larosa]

Hello,

check the effective IP MTU with

show ip interface vlan X

The system mtu 2248 may solve your ping issue between the C4510 in some platforms / IOS versions is needed to be able to use greater packets in the switch TCP/IP stack (the route processor)

Be aware that also L2 ports connecting to servers need an increased MTU to work ( end to end path between the two hosts).

Hope to help

Giuseppe

[Kgrevemberg]

Figured out my issue. Mostly errors on my end. Took the mtu off the linking interfacea and applied mtu 9000 to the trunks between them. Everything is good now..

[Joseph W. Doherty]

By standards, Ethernet maximum is 1500. Later devices often support non-standard "Jumbo" Ethernet; often can increase size up to about 9K. However, there's much to understand to use this (as noted by Giuseppe), but again, as it's non-standard, be prepared for different vendors and/or devices not supporting the same maximum.