255.255 GRE Tunnels 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 visible 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 directly 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: %LINEPROTO0-5-UPDOWN: Line protocol on Interface Tunnelo, changed state to Tunnels must have existing routable IP addresses as their beginning and endpoints, 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 touter-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 up 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 i