RouterA:

 

interface Tunnel1
 ip address 1.1.1.1 255.255.255.0
 tunnel source 172.18.1.1
 tunnel destination 172.18.1.2
 tunnel sequence-datagrams
 tunnel checksum

 

 

RouterB:

 

interface Tunnel1
 ip address 1.1.1.2 255.255.255.0
 tunnel source 172.18.1.2
 tunnel destination 172.18.1.1
 tunnel sequence-datagrams
 tunnel checksum