Cat6K MTU Question

Answered Question
Mar 29th, 2010
User Badges:
  • Gold, 750 points or more

Hi all,


I have two C6Ks on the same subnet and they are connected to each other.


One of the switch configure only global mtu (system jumbomtu 1500)


Another of the switch configure mtu 1521 on the interface as follows.


C6K1:
system jumbomtu 1500

interface GigabitEthernet0/1
ip address 10.1.1.1 255.255.255.0


C6K2:
system jumbomtu 9216 (default)

interface GigabitEthernet0/1
mtu 1521
ip address 10.1.1.2 255.255.255.0


When I try to ping 10.1.1.1 size 1521 on the C6K2, I get a fauiler.


But when I change to mtu 1520 on the C6K2, I get a success.
*ping 10.1.1.1 size 1520


In addision, when I change to the system jumbomtu 1501 on the C6K1, it's success.


What is this 20-byte?


Regards,


Tomoyuki

Correct Answer by Giuseppe Larosa about 7 years 3 months ago

Hello Tomoyuki,

I would suggest to repeat tests using:


set DF bit on packets

you can use the sweep option in extended ping to provide a range of sizes to be explored



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]:
Sweep range of sizes [n]: y
Sweep min size [36]: 1450
Sweep max size [18024]: 1522
Sweep interval [1]:
Type escape sequence to abort.
Sending 365, [1450..1522]-byte ICMP Echos to 10.X.0.Y, timeout is 2 seconds:
Packet sent with the DF bit set


this should help in understanding what is happening.

I think you see the effect of fragmentation on one side


Hope to help

Giuseppe



Cisco will donate $1 to  the Red Cross Haiti fund for every rated post!

https://supportforums.cisco.com/docs/DOC-8727

  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 5 (1 ratings)
Loading.
Correct Answer
Giuseppe Larosa Mon, 03/29/2010 - 08:14
User Badges:
  • Super Silver, 17500 points or more
  • Hall of Fame,

    Founding Member

Hello Tomoyuki,

I would suggest to repeat tests using:


set DF bit on packets

you can use the sweep option in extended ping to provide a range of sizes to be explored



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]:
Sweep range of sizes [n]: y
Sweep min size [36]: 1450
Sweep max size [18024]: 1522
Sweep interval [1]:
Type escape sequence to abort.
Sending 365, [1450..1522]-byte ICMP Echos to 10.X.0.Y, timeout is 2 seconds:
Packet sent with the DF bit set


this should help in understanding what is happening.

I think you see the effect of fragmentation on one side


Hope to help

Giuseppe



Cisco will donate $1 to  the Red Cross Haiti fund for every rated post!

https://supportforums.cisco.com/docs/DOC-8727

t-yamashita Mon, 03/29/2010 - 20:01
User Badges:
  • Gold, 750 points or more

Hello Giuseppe,


Thank you for your response.


Regard,


Tomoyuki

Actions

This Discussion