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Cant Ping with packets with MTU larger than 1518 Bytes

 
 
wilowolf
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      05-24-2009
Hi,

I have two routers connected connected back to back with an Ethernet
cable. I set MTU size on bother GigaEthernet intefaces to 1524 yet,
when the routers cant ping each with packets of size more than 1518
when set the DF on. Any hint ?
 
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bod43
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      05-24-2009
On 24 May, 05:32, wilowolf <(E-Mail Removed)> wrote:
> Hi,
>
> I have two routers connected connected back to back with an Ethernet
> cable. I set MTU size on bother GigaEthernet intefaces to 1524 yet,
> when the routers cant ping each with packets of size more than 1518
> when set the DF on. Any hint ?


Maybe you have already taken this into account, however:

There is no standard for a ping program.

The size specified as a parameter to the ping command
may not be the "ethernet" packet length.

It may be -
Frame length with CRC - default for ethernet is 1518
Frame length without the CRC - 1514
IP packet total length - 1500
IP packet total length less the IP header
ICMP payload length - ?
Some other number the designer fancied at the time

It may also vary (and does if I recall correctly)
with the exact ping program being used. Cisco
router one being different from Windows one I
suspect.

I always just check with wireshark on a PC.

I seem to recall that on Windows you specify -l 1472
to get a maximal length ethernet packet (however
that may well be incorrect). This I guess will be the ICMP
payload length.

 
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