UTT Technologies Chapter 12 VPN
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PPTP tunnel establishment.
In addition, on the Router, you can adjust the global PPTP tunnel MTU (i.e., tunnelmtu) to
minimize the fragmentation: if an IP packet exceeds the specified MTU, it will be
fragmented by the original computer before transmission. The following two examples
describe how to calculate PPTP tunnel MTU. Figure 12-3 illustrates the format of the
PPTP packet to be sent over a static IP or DHCP Internet connection; and Figure 12-4
illustrates the format of the PPTP packet to be sent over a PPPoE Internet connection.
Therein, the sizes of standard Ethernet MTU and each encapsulation header
are as
follows:
Ethernet MTU
1500 Bytes
IP Header
20 Bytes
GRE Header
8 Bytes
PPTP Header
30 Bytes (at most)
PPPoE Header
8 Bytes
Figure 12-3 PPTP Packet Format - Static IP/DHCP Internet Connection
Figure 12-4 PPTP Packet Format - PPPoE Internet Connection
Therefore, to avoid fragmentation and reassembly in the PPTP switching path, the PPTP
tunnel MTU should be smaller or equal to 1442 bytes (1500-20-8-30=1442) when the
PPTP packets are sent over a static IP or DHCP Internet connection (see Figure 12-3);
and it must be smaller or equal to 1434 bytes (1442-8=1434) when the PPTP packets are
sent over a PPPoE Internet connection (see Figure 12-4).
On the Router, the PPTP tunnel MTU is 1400 bytes by default. In most cases, please
leave the default value because it can meet most application needs.
12.1.1.6
PPTP Sessions Limit
The Router supports two concurrent PPTP sessions (i.e., tunnels) at most. If there are
already two active PPTP sessions on the Router, the system will reject any request for
creating a new PPTP session and prompt you.
