rectly connected, Loopback0 10.200.200.200/32 [170/3321856] via 192.168.1.2, 00:00:52, Serial0/1 192.168.1.0/24 is directly connected, Serial0/1 192.168.2.0/24 [90/2681856] via 192.168.1.2, 03:50:27, Serial0/1 192.168.3.0/24 [90/3193856] via 192.168.1.2, 03:50:27, Serial0/1 Now that the loopback addresses are visible in the routing table, it's time to create the tunnel. The process is simple. We'll begin by creating the virtual interfaces for each side of the tunnel. Tunnel interfaces are numbered like all interfaces in IOS, with the first being tunnel 0: Router-A(config)# int tunnel ? <0-2147483647> Tunnel interface number Router-A(config)# int tunnel 0 Router-A(config-if)# 23:23:39: %LINEPROTO-5-UPDOWN: Line protocol on Interface Tunnelo, changed state to Tunnels must have existing routable IP addresses as their beginning and end points, and these must be configured on both routers. The source of the tunnel is the local side, and the destination is the remote side (from the viewpoint of the router being configured): Router-A(config-if)# ip address 172.16.0.1 255.255.255.0 Router-A(config-if)# tunnel source loopback 0 Router-A(config-if)# tunnel destination 10.200.200.200 23:25:15: %LINEPROTO-5-UPDOWN: Line protocol on Interface Tunnelo, changed state to GRE Tunnels As soon as we add the destination IP address to the tunnel, the tunnel interface comes up on Router A: Router-A# sho int tuo Tunnelo is up, line protocol is up Hardware is 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 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 queu