In the previous parts we configured GRE or IPIP tunnels, protected or not by IPsec, only between Vyatta routers.
In this article we will establish a GRE tunnel between a Vyatta router(VC5) and a Cisco router(3725 IOS 12.4-7), and use IPsec ESP tunnel mode to protect the GRE tunnel, thus to obtain a VPN. We will also run OSPF(multicast) over the GRE tunnel to dynamically discover the networks behind the two routers.
A pre-shared key will be used for IKE MM authentication.
The network diagram:
I've decided to put the commands used to configure the two routers in a table, to have them side-by-side.