Vyatta VC5 - Advanced VPN Site-to-Site Connections - Part 13 - Configure GRE/IPsec Between a Vyatta Router and a Cisco Router Using IPsec ESP in Tunnel Mode and as GRE Tunnel Endpoints Private IP Addresses from the Loopback Interfaces

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.

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