BGP over GRE cant establish


hey guys
I use gre between vyos and cisco.
but I cant use the tunnel to connect bgp.
The bgp state always stay in openconfirm.
can someone help me ?

Hello @Niko I know that cisco requires defining routing MTU if it is not 1500, check this moment.

Checked BGP connectivity through GRE tunnel between Cisco 7206 and VyOS in our LAB, works properly.

Thank you so much for your answer!
But I have checked mtu,and found no problem.

BGP state still stuck on openconfirm

I believe you should not define 1500 MTU on the tunnel interface. It should be like ip routing mtu or something like this. Better to enable debug on both sides and look through the log

For sure MTU issue,
GRE overhead is (from memory) 24 bytes, I’d always lower by 8 for pppoe somewhere in the pah so set MTU to 1468 on gre tunnel interfaces.
Or some lower safe vale , like 1440
To find real mtu , use ping with DF set. (Cisco can do ping sweep, just enter
and add options one by one

Which version?
In old 1.3/ 1.4 it could be ttl parameter

Thank you so much!It works.

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