extended ping with df bit set

Unanswered Question
Mar 6th, 2012

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!

I have this problem too.
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dhalevi Tue, 03/06/2012 - 04:13

hi,

I have followed this procedure on both switches already,

dot1#show system mtu

System MTU size is 1500 bytes

System Jumbo MTU size is 9000 bytes

Routing MTU size is 1500 bytes

dot1#

dot2#show system mtu

System MTU size is 1500 bytes

System Jumbo MTU size is 9000 bytes

Routing MTU size is 1500 bytes

dot2#

nkarpysh Tue, 03/06/2012 - 04:24

Hello,

First of all you need to tell if your ports are L3 or L2. You can see that your system

supports two MTU values;"system mtu jumbo" which stands for

MTU for GigabitEthernet port's L2 link and "system mtu routing" which

stands for MTU of routed packets for routed port.

System afaik does not support fragmentation between such 2 MTU value,

So the routed packets which received by gigabitethernet port will

be dropped if its size exceeds "system mtu routing" and ICMP dest

unreachable will not be sent.

So for further troubleshooting please paste the clear diagram and the ports config.

Nik

dhalevi Tue, 03/06/2012 - 04:34

dot1#sho run int gi1/0/1

Building configuration...

Hi Nikolay,

The interfaces are layer 2.  The only config i did besides the system mtu is I set an ip addess on vlan 1 on each switch and then connected the switches via the gig ports.

Current configuration : 38 bytes

!

interface GigabitEthernet1/0/1

end

dot2#show run int gi2/0/2

Building configuration...

Current configuration : 38 bytes

!

interface GigabitEthernet2/0/2

end

Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge

                  S - Switch, H - Host, I - IGMP, r - Repeater, P - Phone,

                  D - Remote, C - CVTA, M - Two-port Mac Relay

Device ID        Local Intrfce     Holdtme    Capability  Platform  Port ID

dot1             Gig 2/0/2         163              S I   WS-C3750- Gig 1/0/1

dot2#

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Posted March 6, 2012 at 3:11 AM
Updated March 6, 2012 at 3:13 AM
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