首先配置各个路由器

routerA:

interface Loopback0
 ip address 1.1.1.1 255.255.255.0
!
interface FastEthernet0/0
 ip address 192.168.1.1 255.255.255.0
 duplex auto
 speed auto
!
router rip
 network 1.0.0.0
 network 192.168.1.0

routerB：

interface FastEthernet0/0
 ip address 192.168.1.2 255.255.255.0
 duplex auto
 speed auto
!
router rip
 network 192.168.1.0

routerC：

interface FastEthernet0/0
 ip address 192.168.1.3 255.255.255.0
 duplex auto
 speed auto
!
router rip
 network 192.168.1.0

现在routerA发送路由更新是默认通过广播

routerA#debug ip rip
RIP protocol debugging is on
*Mar  1 00:14:40.935: RIP: sending v1 update to 255.255.255.255 via Loopback0 (1.1.1.1)
*Mar  1 00:14:40.935: RIP: build update entries
*Mar  1 00:14:40.935:   network 192.168.1.0 metric 1
*Mar  1 00:15:04.927: RIP: sending v1 update to 255.255.255.255 via FastEthernet0/0 (192.168.1.1)
*Mar  1 00:15:04.927: RIP: build update entries
*Mar  1 00:15:04.927:   network 1.0.0.0 metric 1

在routerA上把fa0/0配置成被动接口，则routerA只能接收，不能发送更新。指定routerB为邻居，让routerA发送路由更新到指定的邻居routerB

routerA(config)#router rip
routerA(config-router)#passive-interface fa0/0
routerA(config-router)#neighbor 192.168.1.2

看一下现在routerA的已经用单播发送路由更新到routerB

routerA#
*Mar  1 00:22:05.347: RIP: sending v1 update to 192.168.1.2 via FastEthernet0/0 (192.168.1.1)
*Mar  1 00:22:05.347: RIP: build update entries
*Mar  1 00:22:05.347:   network 1.0.0.0 metric 1