3560 Dropping packets

[Amit Singh2000]

Hello guys

    i need your help to find a way to see if my switch is dropping any packets  bigger than 1500 bytes..

The scenario is like this

Router>>>>>>Switch(3750)>>>>>Router

From router to router i can ping with  byte size of 1500.. Any packets bigger than 1501 get dropped.

On switch i have changed the MTU to 1546 .. Restarted ..Still no luck...

the switch inbetween is a 3750EMI switch but in this case just doing layer 2 switching.. No routing is enabled on the switch..

Is there any way i have see if switch is dropping the packets ? The switch is sitting in a DC..

System MTU size is 1546 bytes
System Jumbo MTU size is 1546 bytes
System Alternate MTU size is 1546 bytes
Routing MTU size is 1500 bytes

Thanks ..

[Nagendra Kumar Nainar]

Hi Amit,

What is the MTU size of the router interface?. From secription, Am I assuming right that the MTU is 1500?.

Are you trying ping test between routers with DF bit set?.

Regards,

Nagendra

[Kishore Chennupati]

Hi Amit,

What router are you using and can you please advise if the connection between the 3750/3560 and router is Fast Eth or Gig eth? If its a fasteth , then you are limited to 1500bytes. But if its a GigE then you can manually change the MTU on the interface to support more than 1500.

Routers like 2610, 2801..they all have fasteth and hence are limited . So, even if you increase the MTU on the 3560 it doesnt make any diff as the limitation is with th router.

To test the end to end MTU between the routers use the df-bit.

I am pasting something here for you which will give you a good idea

R2#ping 10.1.1.1 size 1500  df-bit
Type escape sequence to abort.
Sending 5, 1500-byte ICMP Echos to 10.1.1.1, timeout is 2 seconds:
Packet sent with the DF bit set
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 12/50/72 ms

R2#ping 10.1.1.1 size 1501  df-bit

Type escape sequence to abort.
Sending 5, 1501-byte ICMP Echos to 10.1.1.1, timeout is 2 seconds:
Packet sent with the DF bit set
.
Success rate is 0 percent (0/1)

R2#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
R2(config)#int gi1/0           This is a  Gigeth hence can up the MTU >1500
R2(config-if)#mtu
R2(config-if)#mtu 1520
R2(config-if)#end
R2#
*Feb 21 15:19:46.195: %SYS-5-CONFIG_I: Configured from console by console

R2#ping 10.1.1.1 size 1501  df-bit

Type escape sequence to abort.
Sending 5, 1501-byte ICMP Echos to 10.1.1.1, timeout is 2 seconds:
Packet sent with the DF bit set
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 4/20/48 ms
R2#

R2#ping 10.1.1.1 size 1520  df-bit
Type escape sequence to abort.
Sending 5, 1520-byte ICMP Echos to 10.1.1.1, timeout is 2 seconds:
Packet sent with the DF bit set
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 20/41/64 ms

R2#ping 10.1.1.1 size 1521  df-bit             Since the MTU is only 1520 this will fail

Type escape sequence to abort.
Sending 5, 1521-byte ICMP Echos to 10.1.1.1, timeout is 2 seconds:
Packet sent with the DF bit set
.....
Success rate is 0 percent (0/5)
R2#

HTH,

Regards

Please rate if helpful

[jkilleda]

Hello Amitpal,

3560 Dropping packets
Is there a firewall configured in your network , if yes then check the fragmented packets setting.