gre tunnelling issues

[mirehteshamali]

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

[johnlloyd_13]

hi,

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.

[Joseph W. Doherty]

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Posting

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.

PS:

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