How does the routing within a virtual private network?

I have 2 sites with their own internet connection, and there is a router on both ends that use VPN endpoints.  Both sites use 192.168.x.0 24 IP on their local network.

When I ping from a computer on my LAN to a machine on the other LAN, how is that routing happens?  I don't see all the entries in the routing table.  And Setup on both devices is very simple and does not all IP except each other static external IP.  So, how is my router knows that when I ping 192.168.40.15 192.168.100.3 it's time to use the VPN to the other network?  When I run a tracert on that same IP address, it shows just 3 entries, my internal gateway, the other networks external IP and 192.168.100.3.  When I run a tracert address IP external, complete list of hops.

So how is this working?  Obviously, the two cases have the same jumps, but how my router knows that the other router is the endpoint for the traffic directed to the subnet 192.168.100.0/24?

The way that routers identify it is interesting to be incrypted (traffic) through the crypto ACL that you set up and apply to the card encryption.  When that traffic gets into the router, the router checks the routing table and sees that it has no route to the 192.168.100.0 network if it sends traffic using the default route.  When the traffic between, or cross is perhaps a better term, is the external interface ACL crypto and the router begins to take steps to encrypt the traffic and send it on the VPN tunnel.

The crypto ACL must be configured at both ends of the tunnel and be eachother mirror images.  So if one side has the ACL:

access-list 101 permit ip 192.168.40.0 0.0.0.255 192.168.100.0 0.0.0.255

then the other side will have to be:

access-list 101 permit ip 192.168.100.0 0.0.0.255 192.168.40.0 0.0.0.255

So to summarize, routing takes place, but it is in the form of the default route.  While traffic is about to leave the external interface it is mapped to the crypto ACL and if a match is found, the traffic is encrypted and sent over the VPN.

I hope that the explanation is understandable.

--

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Tags: Cisco Security

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