A question about MTU

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Mar 15th, 2007
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Hi all,

my question is:

I have two routers back-to-back connected, I configure on interfaces two different MTU (1500 on one router, 4470 on other router).

Why if I do a ping with a size of 1600 the ping fail?



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mohammedmahmoud Thu, 03/15/2007 - 06:11
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Hi there,

Do u set the DF bit in this ping or not ?


Mohammed Mahmoud.

karukuma Thu, 03/15/2007 - 07:18
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Ping is a 2way process, it sends a echo and waits for a reply from the other end.

In this case the MTU settings allow only one side to pass the packets. The router with MTU 1500 shld drop the packets with 1600mtu hence the ping fails.

massipace Thu, 03/15/2007 - 08:29
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I tried the same test reducing the interface mtu from 1500 to 1000; then I tried to ping this interface from a router with interface mtu 1500 and in this scenario all works properly

According to your explication in this case the icmp pachet >1000 shulde be dropped, but in this case it does not happen.

Does it could be related to the system mtu?

flashsplash Thu, 03/15/2007 - 14:30
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the max mtu for ethernet is 1500 and tokenring 4500 keep that in mind...

ciao flash


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