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lower MTU switch between higher MTU routers

Hi,

I have layer 2 switch with 1500 interface MTU between routers having having 9000 MTU. Does this layer 2 switch fragment >1500 packets?

Router A (9192 MTU) --------------(1500 MTU) Layer 2 switch (1500 MTU) -------------(9192 MTU) Router B

Also another connectivity is like below:

Router A (9192 MTU) --------------(9192 MTU)  Layer 2 switch  (1500 MTU) --------------(1500 MTU) Router B            how the fragmentation between Routers A and B?

                                                                                                                                                                          

Regards

Siva Kondala Rao

3 REPLIES 3
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Cisco Employee

lower MTU switch between higher MTU routers

Hi Siva,

first scenario is wrong, the intermediate switch will drop traffic above 1500MTU.

Second scenario is fine and fragmentation will occur on L2 switch for frames above 1500 received from Router A going to Router B.

Riccardo

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lower MTU switch between higher MTU routers

Thanks Riccardo, what about fragmentation in below scenario.

Router A (9192 MTU) --------------(1500 MTU)  Layer 2 switch  (9192 MTU) --------------(9192 MTU) Router B

Regards

Siva Kondala Rao

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Re: lower MTU switch between higher MTU routers

Siva, both interface on the same link needs to use the same MTU parameters.

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