Configuration Example Packet Transit Discussion

What follows is a discussion of how the source packet transits the network to its destination using the tunnel:

1. The packet is sourced at PC 1 with a source IPv6 address 2111::2 and a destination IPv6 address 2333::2. These are the original packet header source and destination addresses and will be unchanged when the original packet header is encapsulated into the outer tunnel header.
2. The packet is transmitted to Router 1 using VLAN 11.
3. The packet ingresses Router 1 on VLAN 11 at IPv6 address 2111::1. At this point a standard route table lookup occurs.
4. The route table lookup determines that the best next hop route is using a tunnel that is sourced from loopback 1 using IP address 88.88.88.1 as the source address. The original packet header and payload is encapsulated into the outer tunnel header that has a source address of 88.88.88.1 and a destination address of 99.99.99.1. In this case, the GRE tunnel is functioning as an IPv6 over IPv4 tunnel.
5. The route table lookup determines that the underlying interface for the tunnel is VLAN 50, because a static route exists for VLAN 50 specifying the tunnel destination address 99.99.99.1 as its route destination.
6. The tunnel encapsulated packet is transmitted to the tunnel destination address 99.99.99.1 as a logical single hop from the point of view of the original encapsulated packet header. At the tunnel destination, the outer tunnel header is removed and routing lookup determines the next hop, based upon the best next hop to the destination address of the original packet header.
7. The packet is routed, using a standard route lookup, however many hops required to get to the packet destination.
8. A returning packet that is routed over the tunnel will use the tunnel underlying interface from the point of view of Router 2 when transiting the tunnel. In this case, the initial underlying interface for the tunnel is VLAN 50.