eality, the tunnel is a logical construct composed of many intermediary devices that are not visible. Specifically, the tunnel hides the fact that Routers B and C are in the path. GRE Tunnels and Routing Protocols The introduction of a routing protocol across a GRE tunnel can cause some interesting problems. Take, for example, our network, now altered as shown in Figure 12-3. This time, we have the links between the routers updating routes using RIPv2. The other interfaces on Router A and Router D are included in RIP using the redistribute con nected command. EIGRP is running on all interfaces on Routers A and D with the exception of the serial links, which are running RIPv2. While this may look a bit odd, consider the possibility that the Routers B and C are owned and operated by a service provider. We cannot control them, and they only run RIPv2. We run EIGRP on our routers (A and D) and want to route between them using EIGRP. GRE Tunnels and Routing Protocols 10.10.10.0/24 10.20.20.0/24 192.168.2.0/24 Router B Router C 192.168.1.0/24 RIP v2 192.168.3.0/24 Router A EIGRP 100 Router D 10.100.100.100/32 10.200.200.200/32 Tunnel 172.16.0.0/24 Figure 12-3. Recursive routing example Here are the pertinent configurations for Router A and Router D (remember, in this scenario, Routers B and C are beyond our control): Router A: interface Loopback0 ip address 10.100.100.100 255.255.255.255 interface Tunnelo ip address 172.16.0.1 255.255.255.0 tunnel source Loopback0 tunnel destination 10.200.200.200 interface Etherneto/o ip address 10.10.10.1 255.255.255.0 interface Serial0/1 ip address 192.168.1.1 255.255.255.0 router eigrp 100 network 10.10.10.0 0.0.0.255 network 10.100.100.0 0.0.0.255 network 172.16.0.00.0.0.255 no auto-summary router rip version 2 redistribute connected passive-interface Etherneto/o passive-interface Loopback0 passive-interface Tunnelo network 192.168.1.0 no auto-summary Chapter 12: Tunnels Router D: interface Loopback0 ip address 10.200.200.200 255.255.255.255 interface Tunnelo ip address 172.16.0.2