Testing MTUs Across a Network with Cisco IOS

Normally, packets are fragmented into smaller packets if the original length exceeds the MTU (maximum transmission unit) size of a link. For example, if there is a 6000-byte packet traversing a network and it encounters a link with a 1500 byte MTU, it is segmented into chunks that are sized at 1500 bytes or smaller.

It may be useful to test the MTU size across a network. There is a DF (don’t fragment) bit included in an IP header which if set to one, states that the packet should not be fragmented.

When a router receives a packet with the DF set to one, it knows that it should not fragment it and in turn drops the packet if it is larger than the set MTU size. You can test MTUs across a network by utilizing the Cisco IOS extended ping command with the DF options.

For example, the output of the extended ping command with the options shown below indicates that the MTU set on this link is set at 1500-bytes. You can see that MTUs byte sizes 500, 1000, and 1500 were successful, but an MTU of 2000-bytes failed (timed out).

CORE-LAB#ping
Protocol [ip]:
Target IP address: 10.1.2.18
Repeat count [5]: 1
Datagram size [100]:
Timeout in seconds [2]:
Extended commands [n]: y
Source address or interface:
Type of service [0]:
Set DF bit in IP header? [no]: y
Validate reply data? [no]:
Data pattern [0xABCD]:
Loose, Strict, Record, Timestamp, Verbose[none]:
Number of hops [ 9 ]:
Loose, Strict, Record, Timestamp, Verbose[RV]:
Sweep range of sizes [n]: y
Sweep min size [76]: 500
Sweep max size [18024]: 2000
Sweep interval [1]: 500
Type escape sequence to abort.
Sending 4, [500..2000]-byte ICMP Echos to 10.1.2.18, timeout is 2 seconds:
Packet sent with the DF bit set Packet has IP options: Total option bytes= 39, padded length=40
Reply to request 0 (1 ms) (size 500).
Received packet has options Total option bytes= 40, padded length=40
Reply to request 1 (1 ms) (size 1000).
Received packet has options: Total option bytes= 40, padded length=40
Reply to request 2 (4 ms) (size 1500).
Received packet has options Total option bytes= 40, padded length=40
Request 3 timed out (size 2000)
Success rate is 75 percent (3/4), round-trip min/avg/max = 1/2/4 ms

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