ow the path from Router A to the network 10.20.20.0/24 on Router D will change with the addition of a GRE tunnel. EIGRP 100 10.10.10.0/24 10.20.20.0/24 192.168.1.0/24 192.168.2.0/24 192.168.3.0/24 Router A Router B Router C Router D Figure 12-1. Simple network Given the network in Figure 12-1, the routing table on Router A looks like this: Router-A# sho ip route Gateway of last resort is not set 10.0.0.0/24 is subnetted, 2 subnets 10.20.20.0 [90/3196416] via 192.168.1.2, 03:39:06, Serial0/1 10.10.10.0 is directly connected, Etherneto/1 192.168.1.0/24 is directly connected, Serial0/1 192.168.2.0/24 [90/2681856] via 192.168.1.2, 03:39:06, Serial0/1 192.168.3.0/24 [90/3193856] via 192.168.1.2, 03:39:06, Serial0/1 All routes except the connected routes are available through Serial0/1. Now we're going to add a tunnel between Router A and Router D. Because we prefer to link tun- nels to loopback interfaces, we will add one to each router. Figure 12-2 shows the network as we will create it. EIGRP 100 10.10.10.0/24 10.20.20.0/24 192.168.1.0/24 192.168.2.0/24 192.168.3.0/24 Router A Router B Router C Router D 10.100.100.100/32 10.200.200.200/32 Tunnel 172.16.0.0/24 Figure 12-2. Simple network with a tunnel Chapter 12: Tunnels First, we will add the loopback interfaces on Router A: Router-A# conf t Enter configuration commands, one per line. End with CNTL/Z. Router-A(config)# int lo 0 Router-A(config-if)# ip address 10.100.100.100 255.255.255.255 Next, we will add the loopback interfaces on Router D: Router-D# conf t Enter configuration commands, one per line. End with CNTL/Z. Router-D(config)# int lo 0 Router-D(config-if)# ip address 10.200.200.200 255.255.255.255 Because we are redistributing connected interfaces in EIGRP, they are now both visi- ble in Router A's routing table: Router-A# sho ip route Gateway of last resort is not set 10.0.0.0/8 is variably subnetted, 4 subnets, 2 masks 10.20.20.0/24 [90/3196416] via 192.168.1.2, 03:50:27, Serial0/1 10.10.10.0/24 is directly connected, Etherneto/1 10.100.100.100/32 is di