Re: MTU size

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James Cameron wrote:

Summary:

The MTU should be left at 1500, or lowered to whatever works.
You have to test with many many IP addresses to find the right MTU.

Detail:

When IP packets are sent over PPP, they grow slightly. When the
packets are then sent over GRE, they grow some more. This growth
makes them too big to fit in the usual 1500 limit.

Normally, when everything is working correctly, the packets are split
into two parts and sent one after the other, and then reassembled at
the other side.

Sometimes, the need to split the packets cannot be determined at the
server or the client, because the need is recognised by a node in the
network between the server and the client. The need is transmitted in
a separate message, called an ICMP Fragmentation Needed. This message
comes from a third-party to either the server or the client.

Further, sometimes this message is blocked, by accident, by policy, by
security. When that happens, the server or the client don't know that
their original packet didn't get through, and so the connection
freezes.

If lowering the MTU fixes the symptom, then it is a reasonable guess
that the above description is the problem.

You can find more on this at
http://en.wikipedia.org/wiki/Path_MTU_Discovery

On Tue, Nov 19, 2013 at 12:31:15PM +0000, Khapare Joshi wrote:

Just curios what MTU size ppp server should be configured ?

I see some of those ppp client has

UP POINTOPOINT RUNNING NOARP MULTICAST MTU:1396 Metric:1

UP POINTOPOINT RUNNING NOARP MULTICAST MTU:1496 Metric:1

Why this is ? should not be 1500 for every client that connect to vpn ?

So the problem is some of my users can connect to my VPN server but they could
not get anything such as website outside my company as an example.

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