e :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, 0 no buffer Received 0 broadcasts, 0 runts, 0 giants, 0 throttles 0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 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 Note that although Router A shows the interface to be 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, Ethernet0/1 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 use the same commands that we did on Router A, but reverse the source and destination: Router-D(config)# int tu 0 23:45:13: %LINEPROTO-5-UPDOWN: Line protocol on Interface Tunnelo, changed state to Chapter 12: Tunnels Router-D(config-if)# ip address 172.16.0.2 255.255.255.0 Router-D(config-if)# tunnel source lo 0 Router-D(config-if)# tunnel destination 10.100.100.100 Router-D(config-if)# 23:47:06: %LINEPROTO-5-UPDOWN: Line protocol on Interface Tunnelo, changed state to Now we have a live link between these routers, which appear to be directly con- nected. However, the Ethernet network on Router D, which was k