I want to connect Shenzhen company headquarters and Beijing company branch through VPN. How to set PPTP\L2TP?

Simply use GRE:

router

Interface Ethernet 0/ 1

Ipaddress10.2.2.1255.255.0 # intranet interface

Interface Serial0/0

Ipaddress192.168.4.1255.255.0 # public network interface.

Interface tunnel 0

Ip address1.1.1.2 255.255.0 # tunnel interface address (note that the opposite end should be connected to a network segment).

Tunnel Source Serial0/0 # Tunnel Source Address

Tunnel destination 192. 168.4.2 # Tunnel destination address

The Ip route10.1.1.0 255.255.01.1# peer-to-peer intranet route points to the tunnel interface.

router

Interface FastEthernet0/ 1

Ip address10.1.1.1255.255.0.

Interface Serial0/0

Ip address192.168.4.2 255.255.0

Interface tunnel 0

Ip address1.1.1.1255.255.0.

Tunnel source Serial0/0

Tunnel destination192.168.4.1

Ip route10.2.2.0 255.255.01.1.2.