ping with MTU morethan 1500

[rishad ahammed]

when I am ping to google I cannot reach mtu morethan 1500... Is it possible to ping through internet with an MTU more than 1500?? 

[e.ciollaro]

Hi,

are doing ping with do not fragment bit set and size 1500 byte?  If so consider that when you send a packet bigger then max admitted MTU over the path, the packet must be fragmented; if you are using the "do not fragment" bit the packet will be discarded and an ICMP Can't Fragment Error packet is generated (type 3 (destination unreachable), code 4 (fragmentation needed but don't-fragment bit set).

If you specify size 1500 the real size of the packet is more then 1500 due to ICMP header

Bye,

enrico

PS please rate if useful

[rishad ahammed]

I am pinging without DF bit set .... I can ping inside my network with MTU more than 1500 also, but it is not possible wen I am trying to ping some IPs over internet

[Nagendra Kumar Nainar · Accepted solution]

Hi,

Normally when DF bit is not set and if the egress MTU is less, the rotuer will fragment the packet. So in your case, it should still reach your destination.

I did a quick try with size of 1501 and I get reply from google.com.

You can try to ping your ISP nexthop with the size and see if it works. Is it working when you do a ping from your default GW to nexthop/google site?.

-Nagendra

[e.ciollaro]

Assuming you mean packet size (not MTU which is a characteristic of network segment) I suppose some appliance in the network infrastructure will drop fragmented packet but it's strange. Try debug ip icmp to see which is the device that discard your packets.

[rishad ahammed]

Thank you all for your reply ...... actually I found the issue is from our upstream service provider ... it working wen I was trying through a  different service service provider