s Tunnel Internet address is 172.16.0.1/24 MTU 1514 bytes, BW 9 Kbit, DLY 500000 usec, reliability 255/255, txload 1/255, rxload 1/255 Encapsulation TUNNEL, loopback not set Chapter 12: Tunnels Keepalive not set Tunnel source 10.100.100.100 (Loopbacko), destination 10.200.200.200 Tunnel protocol/transport GRE/IP, key disabled, sequencing disabled Checksumming of packets disabled, fast tunneling enabled Last input never, output never, output hang never Last clearing of "show interface" counters never Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0 Queueing strategy: fifo Output queue :0/0 (size/max) 5 minute input rate 0 bits/sec, 0 packets/sec 5 minute output rate 0 bits/sec, 0 packets/sec 0 packets input, 0 bytes, o no buffer Received 0 broadcasts, o runts, 0 giants, 0 throttles 0 input errors, 0 CRC, o frame, 0 overrun, o ignored, o abort 0 packets output, 0 bytes, 0 underruns 0 output errors, 0 collisions, 0 interface resets 0 output buffer failures, 0 output buffers swapped out Notice that although Router A indicates that the interface is up, because Router D does not yet have a tunnel interface, nothing can be sent across the link. Be careful of this, as you may get confused under pressure. The tunnel network 172.16.0.0/24 is even active in Router A's routing table (it will not be found on Router D at this time): Router-A#sho ip route Gateway of last resort is not set 172.16.0.0/24 is subnetted, 1 subnets 172.16.0.0 is directly connected, Tunnelo 10.0.0.0/8 is variably subnetted, 4 subnets, 2 masks 10.20.20.0/24 [90/3196416] via 192.168.1.2, 04:25:38, Serial0/1 10.10.10.0/24 is directly connected, Etherneto/1 10.100.100.100/32 is directly connected, Loopback0 10.200.200.200/32 [170/3321856] via 192.168.1.2, 00:36:03, Serial0/1 192.168.1.0/24 is directly connected, Serial0/1 192.168.2.0/24 [90/2681856] via 192.168.1.2, 04:25:39, Serial0/1 192.168.3.0/24 [90/3193856] via 192.168.1.2, 04:25:39, Serial0/1 To terminate the tunnel on Router D, we need to add the tunnel interface there. We'll 