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IP MTU issue

saqibmehmood
Beginner
Beginner

Hi everyone,

I wanted to ask that when mtu is set to 1400 in GRE path then maximum data size packet allowed is 1376(1400-24).Shouldnt it be 1348(1440-24(gre)-20(IP)-8(icmp)).

3 REPLIES 3

Giuseppe Larosa
Hall of Fame Master Hall of Fame Master
Hall of Fame Master

Hello Sagib,

the value 1376 bytes  means the payload of the GRE packet that is an IP packet including its own IPv4 header.

It is the size of the payload into the GRE packet without looking at what is carried inside.

Hope to help

Giuseppe

So as I understand 1376 packet size include 20 bytes of ip header and if I am pinging then 8 bytes of ICMP..is that so?

Just remember that the value you see over the show ip interface tunnel will show the MTU of the outgoing interface without the GRE header overload ( 24 bytes)

Regards,

Julio Carvajal
Senior Network Security and Core Specialist
CCIE #42930, 2xCCNP, JNCIP-SEC
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