hello,
I am trying to test the MTU between two 3750 switches I have in the lab. I've set the MTU with the command "system mtu 9000" on both switches and rebooted.
The only connections on the switches are the gig ports connecting the two switches. Each interface is a member of vlan 1.
I am doing an extended ping. I set the datagram size to 2000. When the df bit is set the ping doesn't go through. If the DF bit is not set the ping goes through.
The debug ip icmp shows,
4d00h: ICMP: dst (1.1.1.1): frag. needed and DF set.
Why is fragmentation needed when the MTU is set to 9000?
GigabitEthernet1/0/1 is up, line protocol is up (connected)
Hardware is Gigabit Ethernet, address is 0015.2b7d.0d01 (bia 0015.2b7d.0d01)
MTU 9000 bytes, BW 1000000 Kbit, DLY 10 usec,
GigabitEthernet2/0/2 is up, line protocol is up (connected)
Hardware is Gigabit Ethernet, address is 0017.597f.7082 (bia 0017.597f.7082)
MTU 9000 bytes, BW 1000000 Kbit, DLY 10 usec,
dot2# ping ip
Target IP address: 1.1.1.1
Repeat count [5]:
Datagram size [100]: 2000
Timeout in seconds [2]:
Extended commands [n]: y
Source address or interface:
Type of service [0]:
Set DF bit in IP header? [no]: yes
Validate reply data? [no]: yes
Data pattern [0xABCD]:
Loose, Strict, Record, Timestamp, Verbose[none]: v
Loose, Strict, Record, Timestamp, Verbose[V]:
Sweep range of sizes [n]:
Type escape sequence to abort.
Sending 5, 2000-byte ICMP Echos to 1.1.1.1, timeout is 2 seconds:
Packet sent with the DF bit set
Reply data will be validated
Request 0 timed out
Request 1 timed out
Request 2 timed out
Request 3 timed out
Request 4 timed out
Success rate is 0 percent (0/5)
4d00h: ICMP: dst (1.1.1.1): frag. needed and DF set.
thanks!
