MTU mismatch on Server and Switch

[cisco_lite]

If the server is set to 9000 MTU and the GigabitEthernet port on Cat 6500 is set to default (1500), would the traffic between the two drop completely, or it would it be dropped only for frames sent above 1500 or the traffic would flow normally unaffected.

[Giuseppe Larosa]

Hello Cisco_lite,

the server will try to use frames with size bigger then 1500 bytes those frames are discarded by the switch.

if the server can perform any Path mtu discovery it will adjust the sizes of packets so that they can travel.

So the impact depends on the protocol and application.

However, this is not a good scenario:

you should enable jumbo frames or increase system mtu to allow the servers to use bigger frames.

Or you reduce to 1500 bytes the server MTU

Hope to help

Giuseppe

[cisco_lite]

If the server is set to MTU 9000 and the Cat 6500 interface MTU is set to 9216, would there be any communication problem. i.e. switch MTU is bigger than server MTU.

[Giuseppe Larosa]

Hello Cisco_Lite,

>> If the server is set to MTU 9000 and the Cat 6500 interface MTU is set to 9216, would there be any communication problem. i.e. switch MTU is bigger than server MTU.

within the L2 vlan server will be able to use frames of size 9000 bytes with no problem

when going through a routed network the normal size will be used if path mtu discovery works well ( I don't expect routers to be able to support so big packets)

Hope to help

Giuseppe

[Joseph W. Doherty]

"the server will try to use frames with size bigger then 1500 bytes those frames are discarded by the switch."

BTW, although I haven't personally confirmed, but years ago a Cisco SE told me Cisco switches capable of jumbo Ethernet will accept jumbo frame even when configured for 1500 MTU.  However, the switch will not transmit jumbo frames, on another port, unless configured for jumbo MTU.

"if the server can perform any Path mtu discovery it will adjust the sizes of packets so that they can travel."

Also BTW, PMTUD only "engages" at L3 hops, and there are a lot of possible "gotchas" that can preclude it working as it should.

"you should enable jumbo frames or increase system mtu to allow the servers to use bigger frames."

One more BTW, when doing jumbo, ideally you want it supported end-to-end, otherwise you may fragment or drop at L3, or drop at L2.

[morevj]

what if servers are configured with default MTU 1500 and switch is configured with MTU9000 default. Does this impact the traffic. will there be slowness or packet drops.