IP in IP or GRE tunnel using iproute

This article was first written in August 2004 for the BeezNest technical
website (http://glasnost.beeznest.org/articles/166).
Setting an IP in IP or GRE tunnel using iproute between two Linux routers is relatively easy. Given the following two routers, each one gateway for its network:

routerA

Public IP: 87.65.43.21 Private network: 192.168.4.0/24 IP on that private network: 192.168.4.4

routerB

Public IP: 123.45.67.8 Private network: 192.168.10.0/24 IP on that private network: 192.168.10.4 To setup routerA, will issue the following commands o