MTU size compatibility

[bapatsubodh]

Hi,

On the Cisco switch MTU is set to 9000 on all ports. Some hosts are configured for MTU as 9000 and some are default 1500. IT's causing problem in the application communications between hosts. Is there anything on the switch side that needs to be configured other that MTU size to 9000.

Please share the experience.

Thanks!

Subodh

[Giuseppe Larosa · Accepted solution]

Hello Subodh,

is the Cisco switch used for inter vlan routing ( OSI Layer 3) or only as L2 device?

Is MTU set to 9000 on all L2 interfaces and all SVIs ( L3 Vlan interfaces)?

the issuemay arise  because one host H1 can send a packet bigger then standard MTU to an host H2 that has only standard MTU. If all involved network devices support the bigger packet the frame is delivered to H2 where it is dropped

You should verify if an ICMP unreachable packet too big can travel back from H2 to H1 in order to make H1 aware of the reduced MTU of the other endpoint

see

http://www.cisco.com/en/US/tech/tk827/tk369/technologies_white_paper09186a00800d6979.shtml#t4

problems with Path MTU Discovery section

Hope to help

Giuseppe