在中间的routerA上
Router>en
Router#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
Router(config)#int f0/0
	
Router(config-if)#ip address 192.168.2.2 255.255.255.0
Router(config-if)#no shut

Router(config-if)#
%LINK-5-CHANGED: Interface FastEthernet0/0, changed state to up

Router(config-if)#exit
Router(config)#ip route 172.3.3.0 255.255.255.0 192.168.2.1
Router(config)#

在新加的routerC上
Router>en
Router#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
Router(config)#int f0/0
Router(config-if)#ip address 192.168.2.1 255.255.255.0
Router(config-if)#no shut

Router(config-if)#
%LINK-5-CHANGED: Interface FastEthernet0/0, changed state to up

%LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/0, changed state to up

Router(config-if)#exit
Router(config)#int f0/1
Router(config-if)#ip address 172.3.3.1 255.255.255.0
Router(config-if)#no shut

Router(config-if)#
%LINK-5-CHANGED: Interface FastEthernet0/1, changed state to up

%LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/1, changed state to up

Router(config-if)#exit
Router(config)#ip route 0.0.0.0 0.0.0.0 192.168.2.2
Router(config)#


在右边的routeB上
Router>en 
Router#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
Router(config)#ip route 172.3.3.0 255.255.255.0 192.168.1.1
Router(config)#