MTU size on tunnel interface

[bauer.juergen]

hi netpros,

we are running a few GRE tunnels - so far without any problems.

now we have the first issue with large packets getting dropped. we changed the mtu size on the tunnel interface:

interface Tunnel41

ip address 192.168.a.b 255.255.255.252

ip mtu 1300

tunnel source Loopback2

tunnel destination 192.168.x.y

but a sh int t41 shows mtu size of 1514!?

Tunnel41 is up, line protocol is up

Hardware is Tunnel

Internet address is 192.168.a.b/30

MTU 1514 bytes

...

...

...

Is this just a display error or is it no error at all?

any help appreciated :-)

thanks and best regards,

juergen

[bauer.juergen]

As far as I understand the document below it does clear the df-bit - if you dont use tunnel path-mtu-discovery, which we dont.

"Remember that without the tunnel path-mtu-discovery command configured, the DF bit would always be cleared in the GRE IP header. This allows the GRE IP packet to be fragmented even though the encapsulated data IP header had the DF bit set, which normally wouldn't allow the packet to be fragmented."

http://www.cisco.com/en/US/tech/tk827/tk369/technologies_white_paper09186a00800d6979.shtml#more

With the one branch that is not working, it seems that the isp is not using mtu size of 1500 bytes. so we have to wait for feedback about the used mtu size.