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Query of interface mtu size issue.

syjeon
Level 1
Level 1

Hi Alls.

we've got an issue of internet slowness from branch office. HQ internet access has no issue. Between HQ and branch is connected with GRE.

However, recently, branch office has been asked us to check the internet connectivity. below is GRE tunnel status.

Tunnel10 is up, line protocol is up

  Hardware is Tunnel

  Description: Tunnel to ----

  Internet address is x.x.x.x/30

  MTU 1514 bytes, BW 9 Kbit, DLY 500000 usec,

As above, MTU is 1514. mean that if the packet goes via HQ from branch office, they must encapulated GRE header. and the packet size is 1538.

in this worse, the packet is fragmentated, right?

So, we have to pre-define MTU and MSS size. this is my knowledge from before.

Please, let me know if you have better troubleshooting guide regarding GRE MTU issue.

Thanks you.

1 Reply 1

Vivek Ganapathi
Level 4
Level 4

Hello,

Try to set the command on the tunnel interface ip tcp adjust-mss 1460.

I believe this may fix your issue. I use the MTU ping test to determine the max MTU size on a link. Below is the link with the steps.

http://help.expedient.net/broadband/mtu_ping_test.shtml

Thanks

Vivek

Message was edited by: Vivek Ganapathi

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