 255.255.255.0 tunnel source Loopback0 tunnel destination 10.100.100.100 interface Etherneto/o ip address 10.20.20.1 255.255.255.0 interface Serial0/1 ip address 192.168.3.2 255.255.255.0 router eigrp 100 network 10.20.20.0 0.0.0.255 network 10.200.200.0 0.0.0.255 network 172.16.0.0 0.0.0.255 no auto-summary router rip version 2 redistribute connected passive-interface FastEtherneto/o passive-interface Loopback0 passive-interface Tunnelo network 192.168.3.0 no auto-summary Everything looks fine, and in fact the tunnel comes up right away, but shortly after it does, we start seeing these errors on the console: 1d01h: %LINEPROTO-5-UPDOWN: Line protocol on Interface Tunnelo, changed state to up 1d01h: %TUN-5-RECURDOWN: Tunnelo temporarily disabled due to recursive routing 1d01h: %LINEPROTO-5-UPDOWN: Line protocol on Interface Tunnelo, changed state to down The error message Tunnelo temporarily disabled due to recursive routing is a result of the destination of the tunnel being learned through the tunnel itself. With the tunnel manually shut down on Router A, the loopback interface on Router D is known through RIP, as expected: 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 [120/3] via 192.168.1.2, 00:00:07, 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 [120/3] via 192.168.1.2, 00:00:07, Serial0/1 192.168.1.0/24 is directly connected, Serial0/1 192.168.2.0/24 [120/1] via 192.168.1.2, 00:00:07, Serial0/1 192.168.3.0/24 [120/2] via 192.168.1.2, 00:00:07, Serial0/1 GRE Tunnels and Routing Protocols Once we bring the tunnel up and EIGRP starts working, the remote loopback becomes known through the tunnel: 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 [90/297246976] via 172.16.0.2, 00:00:04, Tunnelo 10.10.10.0/24