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gre tunnelling issues

Hi all

we have  a  internet connection , but isp is using  a  GRE  tunnelling . They have adjusted the mtu  to 1492  and tcp adjust mss to 1452 . However  some websites are  not accessible , what could be reason , is  it  related to lower MTU  than 1500 ?

2)  as  tcp retransmites  dropped packets , we can concluded  that  is not related  to bandwidht restrictions , i  would apprecaite if some one from a Servcie provider background can give me a start to trouble shoot is issue


gre tunnelling issues


is this an ISP managed router? did they apply these settings over the WAN interface?

kindly perform traceroutes to the unaccessible websites and post it here.

Super Bronze

Re: gre tunnelling issues


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Standard GRE headers use 24 bytes (not just 8).  Try IP MTU 1476 and adjust-mss 1436.  Also, if not enabled, activate PMTUD across the tunnel.


PPoE is generally 8 bytes.  If you have a PPoE connection, decrement above by another 8 bytes.

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