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ping with MTU morethan 1500

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?? 

1 ACCEPTED SOLUTION

Accepted Solutions
Cisco Employee

Hi,Normally when DF bit is

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

5 REPLIES
Silver

Hi,are doing ping with do not

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

New Member

I am pinging without DF bit

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

Cisco Employee

Hi,Normally when DF bit is

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

Silver

Assuming you mean packet size

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.

New Member

Thank you all for your reply

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 

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