rip(routing infomation protocol,路由信息协议) ，是一个纯粹的距离矢量路由选择协议，RIP每隔30s就将自己完整的路由选择表从所有激活的接口上送出。RIP只将跳计数作为判断到达远程网络最佳路径的依据，并且在默认情况下允许最大的跳计数为15.也就是说，16就被认为不可达的。
RIP版本1只使用有类的路由选择，即网络中的所有设备都必须使用相同的子网掩码。这是因为RIP版本1在发送更新数据中不携带子网掩码信息。RIP版本2提供了前缀路由选择信息，并可以在路由更新中传送子网掩码信息，这就是无类的路由选择。
避免路由环路的四种办法：
1，水平分割，只朝一个方向广播数据更新。
2，最大跳数，设置最大跳数为15，则16跳就表示为不可达。
3，路由中毒，当网络出现故障时，通告网络的跳计数为16不可达。
4，保持关闭，保持关闭就是当有接口关闭并打开的时候，路由信息立即更新，然后又关闭，路由信息又立即更新。保持关闭可以阻止太频繁的路由改变。
RIP 例子
router0 路由器配置：
Building configuration... Current configuration : 637 bytes
!
version 12.4
no service timestamps log datetime msec
no service timestamps debug datetime msec
no service password-encryption
!
hostname Router
!
!
!
!
!
!
!
!
no ip cef
no ipv6 cef
!
!
!
!
!
!
!
!
!
!
!
!
spanning-tree mode pvst
!
!
!
!
!
!
interface FastEthernet0/0
ip address 172.16.1.254 255.255.255.0
duplex auto
speed auto
!
interface FastEthernet0/1
ip address 192.168.12.1 255.255.255.0
duplex auto
speed auto
!
interface Vlan1
no ip address
shutdown
!
router rip
network 172.16.0.0
network 192.168.12.0
!
ip classless
!
ip flow-export version 9
!
!
!
!
!
!
!
line con 0
!
line aux 0
!
line vty 0 4
login
!
!
!
end
router1 的配置
Building configuration... Current configuration : bytes
!
version 12.4
no service timestamps log datetime msec
no service timestamps debug datetime msec
no service password-encryption
!
hostname Router
!
!
!
!
!
!
!
!
no ip cef
no ipv6 cef
!
!
!
!
!
!
!
!
!
!
!
!
spanning-tree mode pvst
!
!
!
!
!
!
interface FastEthernet0/
ip address 192.168.23.2 255.255.255.0
duplex auto
speed auto
!
interface FastEthernet0/
ip address 192.168.12.2 255.255.255.0
duplex auto
speed auto
!
interface Vlan1
no ip address
shutdown
!
router rip
network 192.168.12.0
network 192.168.23.0
!
ip classless
!
ip flow-export version
!
!
!
!
!
!
!
line con
!
line aux
!
line vty
login
!
!
!
end
router2的配置
Building configuration... Current configuration : bytes
!
version 12.4
no service timestamps log datetime msec
no service timestamps debug datetime msec
no service password-encryption
!
hostname Router
!
!
!
!
!
!
!
!
no ip cef
no ipv6 cef
!
!
!
!
!
!
!
!
!
!
!
!
spanning-tree mode pvst
!
!
!
!
!
!
interface Loopback1
ip address 172.16.31.1 255.255.255.0
!
interface Loopback2
ip address 172.16.32.1 255.255.255.0
!
interface Loopback3
ip address 172.16.33.1 255.255.255.0
!
interface FastEthernet0/
ip address 192.168.23.3 255.255.255.0
duplex auto
speed auto
!
interface FastEthernet0/
no ip address
duplex auto
speed auto
shutdown
!
interface Vlan1
no ip address
shutdown
!
router rip
network 172.16.0.0
network 192.168.23.0
!
ip classless
!
ip flow-export version
!
!
!
!
!
!
!
line con
!
line aux
!
line vty
login
!
!
!
end
测试：在Router1上ping172.16.1.254
Router#ping 172.16.1.254 Type escape sequence to abort.
Sending , -byte ICMP Echos to 172.16.1.254, timeout is seconds:
..!U!
Success rate is percent (/), round-trip min/avg/max = // ms
Router1上的路由表为：
Router#sh ip rou
Router#sh ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type , N2 - OSPF NSSA external type
E1 - OSPF external type , E2 - OSPF external type , E - EGP
i - IS-IS, L1 - IS-IS level-, L2 - IS-IS level-, ia - IS-IS inter area
* - candidate default, U - per-user static route, o - ODR
P - periodic downloaded static route Gateway of last resort is not set R 172.16.0.0/ [/] via 192.168.12.1, ::, FastEthernet0/
[/] via 192.168.23.3, ::, FastEthernet0/
C 192.168.12.0/ is directly connected, FastEthernet0/
C 192.168.23.0/ is directly connected, FastEthernet0/
到172.16.0.0的有两个路由，可以从192.168.12.1走，也可以从192.168.23.3 走，从而ping 172.16.1.254 可以从两条路线走，从而导致时通时坏。
Router0上的路由表为：
Router#sh ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type , N2 - OSPF NSSA external type
E1 - OSPF external type , E2 - OSPF external type , E - EGP
i - IS-IS, L1 - IS-IS level-, L2 - IS-IS level-, ia - IS-IS inter area
* - candidate default, U - per-user static route, o - ODR
P - periodic downloaded static route Gateway of last resort is not set 172.16.0.0/ is subnetted, subnets
C 172.16.1.0 is directly connected, FastEthernet0/
C 192.168.12.0/ is directly connected, FastEthernet0/
R 192.168.23.0/ [/] via 192.168.12.2, ::, FastEthernet0/
Router0上只有到达192.168.23.0的路由，没有学习到到Router3上的172.16.31.1，172.16.32.1，172.16.33.1 的路由。
Router2上的路由表为：
Router#sh ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type , N2 - OSPF NSSA external type
E1 - OSPF external type , E2 - OSPF external type , E - EGP
i - IS-IS, L1 - IS-IS level-, L2 - IS-IS level-, ia - IS-IS inter area
* - candidate default, U - per-user static route, o - ODR
P - periodic downloaded static route Gateway of last resort is not set 172.16.0.0/ is subnetted, subnets
C 172.16.31.0 is directly connected, Loopback1
C 172.16.32.0 is directly connected, Loopback2
C 172.16.33.0 is directly connected, Loopback3
R 192.168.12.0/ [/] via 192.168.23.2, ::, FastEthernet0/
C 192.168.23.0/ is directly connected, FastEthernet0/
Router2 也没有学习到Router0上的172.16.1.254 的网络。
Router1 上的sh ip protocol如下
Router#sh ip protocols
Routing Protocol is "rip"
Sending updates every seconds, next due in seconds
Invalid after seconds, hold down , flushed after
Outgoing update filter list for all interfaces is not set
Incoming update filter list for all interfaces is not set
Redistributing: rip
Default version control: send version , receive any version
Interface Send Recv Triggered RIP Key-chain
FastEthernet0/
FastEthernet0/
Automatic network summarization is in effect
Maximum path:
Routing for Networks:
192.168.12.0
192.168.23.0
Passive Interface(s):
Routing Information Sources:
Gateway Distance Last Update
192.168.12.1 ::
192.168.23.3 ::
Distance: (default is )
在Router1 上的RIP通过如下：
Router#debug ip rip
RIP protocol debugging is on
Router#RIP: received v1 update from 192.168.12.1 on FastEthernet0/
172.16.0.0 in hops
RIP: received v1 update from 192.168.23.3 on FastEthernet0/
172.16.0.0 in hops
RIP: sending v1 update to 255.255.255.255 via FastEthernet0/ (192.168.12.2)
RIP: build update entries
network 192.168.23.0 metric
RIP: sending v1 update to 255.255.255.255 via FastEthernet0/ (192.168.23.2)
RIP: build update entries
network 192.168.12.0 metric
在Router1上的通过为 收到router0 的172.16.0.0 的通告，也收到Router2的172.16.0.0 的通告。
在Router0 上的RIP通告如下：
Router#debug ip rip
RIP protocol debugging is on
Router#RIP: sending v1 update to 255.255.255.255 via FastEthernet0/ (172.16.1.254)
RIP: build update entries
network 192.168.12.0 metric
network 192.168.23.0 metric
RIP: sending v1 update to 255.255.255.255 via FastEthernet0/ (192.168.12.1)
RIP: build update entries
network 172.16.0.0 metric
RIP: received v1 update from 192.168.12.2 on FastEthernet0/
192.168.23.0 in hops
Router0 上的通过是发送172.16.0.0通告给Router1 路由器。根本没有收到172.16.31.0，172.16.32.0，172.16.33.0 的通告。为什么没有收到路由通告呢？原因是由于172.16.1.254是带有子网掩码的子类，被汇总为172.16.0.0/16了，要让网络正常，只能使用RIP v2.并且必须关闭自动汇总。
自动汇总：
RIP是一个有类路由协议，会在主类网络边界产生自动汇总。
什么是边界呢？当路由器从一个接口向外发送更新包时，如果要传送的路由与路由传播接口IP地址所在网络的主网络号不同，则路由器要把传送的路由汇总成主类网络在发送出去。
在Router0上有直连网络192.168.12.0/24和172.16.1.0/24两个主类网络的边界。
RIPv2将172.16.1.0/24这个网络宣告出去时，通过Fa0/1口。而Fa0/1的IP地址所在的主网络是192.168.12.0/24。
172.16.1.0/24与192.168.12.0/24不是同一个主网络号，所以172.16.1.0/24会汇总为172.16.0.0/16发送给Router1。
Router#debug ip rip
RIP protocol debugging is on
Router#RIP: sending v1 update to 255.255.255.255 via FastEthernet0/ (172.16.1.254)
RIP: build update entries
network 192.168.12.0 metric
network 192.168.23.0 metric
RIP: sending v1 update to 255.255.255.255 via FastEthernet0/ (192.168.12.1)
RIP: build update entries
network 172.16.0.0 metric 1
RIP: received v1 update from 192.168.12.2 on FastEthernet0/
如何让该网络保持正常通信，必须在每个路由器上的rip协议下关闭自动汇总。
命令为：
Router(config)#router rip
Router(config-router)#version 2 Router(config-router)#no auto-summary
重新配置后的路由器的路由表为：
router0 ：
Router#sh ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type , N2 - OSPF NSSA external type
E1 - OSPF external type , E2 - OSPF external type , E - EGP
i - IS-IS, L1 - IS-IS level-, L2 - IS-IS level-, ia - IS-IS inter area
* - candidate default, U - per-user static route, o - ODR
P - periodic downloaded static route Gateway of last resort is not set 172.16.0.0/ is subnetted, subnets
C 172.16.1.0 is directly connected, FastEthernet0/
R 172.16.31.0 [/] via 192.168.12.2, ::, FastEthernet0/
R 172.16.32.0 [/] via 192.168.12.2, ::, FastEthernet0/
R 172.16.33.0 [/] via 192.168.12.2, ::, FastEthernet0/
C 192.168.12.0/ is directly connected, FastEthernet0/
R 192.168.23.0/ [/] via 192.168.12.2, ::, FastEthernet0/
router1：
Router#sh ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type , N2 - OSPF NSSA external type
E1 - OSPF external type , E2 - OSPF external type , E - EGP
i - IS-IS, L1 - IS-IS level-, L2 - IS-IS level-, ia - IS-IS inter area
* - candidate default, U - per-user static route, o - ODR
P - periodic downloaded static route Gateway of last resort is not set 172.16.0.0/ is variably subnetted, subnets, masks
R 172.16.0.0/ [/] via 192.168.12.1, ::, FastEthernet0/
[/] via 192.168.23.3, ::, FastEthernet0/
R 172.16.1.0/ [/] via 192.168.12.1, ::, FastEthernet0/
R 172.16.31.0/ [/] via 192.168.23.3, ::, FastEthernet0/
R 172.16.32.0/ [/] via 192.168.23.3, ::, FastEthernet0/
R 172.16.33.0/ [/] via 192.168.23.3, ::, FastEthernet0/
C 192.168.12.0/ is directly connected, FastEthernet0/
C 192.168.23.0/ is directly connected, FastEthernet0/
router2：
Router#sh ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type , N2 - OSPF NSSA external type
E1 - OSPF external type , E2 - OSPF external type , E - EGP
i - IS-IS, L1 - IS-IS level-, L2 - IS-IS level-, ia - IS-IS inter area
* - candidate default, U - per-user static route, o - ODR
P - periodic downloaded static route Gateway of last resort is not set 172.16.0.0/ is subnetted, subnets
R 172.16.1.0 [/] via 192.168.23.2, ::, FastEthernet0/
C 172.16.31.0 is directly connected, Loopback1
C 172.16.32.0 is directly connected, Loopback2
C 172.16.33.0 is directly connected, Loopback3
R 192.168.12.0/ [/] via 192.168.23.2, ::, FastEthernet0/
C 192.168.23.0/ is directly connected, FastEthernet0/
此时的router1 的路由通告如下：
Router#debug ip rip
RIP protocol debugging is on
Router#RIP: received v2 update from 192.168.12.1 on FastEthernet0/
172.16.1.0/ via 0.0.0.0 in hops
RIP: received v2 update from 192.168.23.3 on FastEthernet0/
172.16.31.0/ via 0.0.0.0 in hops
172.16.32.0/ via 0.0.0.0 in hops
172.16.33.0/ via 0.0.0.0 in hops
RIP: received v2 update from 192.168.12.1 on FastEthernet0/
172.16.1.0/ via 0.0.0.0 in hops
RIP: sending v2 update to 224.0.0.9 via FastEthernet0/ (192.168.12.2)
RIP: build update entries
172.16.31.0/ via 0.0.0.0, metric , tag
172.16.32.0/ via 0.0.0.0, metric , tag
172.16.33.0/ via 0.0.0.0, metric , tag
192.168.23.0/ via 0.0.0.0, metric , tag
RIP: sending v2 update to 224.0.0.9 via FastEthernet0/ (192.168.23.2)
RIP: build update entries
172.16.1.0/ via 0.0.0.0, metric , tag
192.168.12.0/ via 0.0.0.0, metric , tag
router0 的RIP通告为
Router#debug ip rip
RIP protocol debugging is on
Router#RIP: received v2 update from 192.168.12.2 on FastEthernet0/
172.16.31.0/ via 0.0.0.0 in hops
172.16.32.0/ via 0.0.0.0 in hops
172.16.33.0/ via 0.0.0.0 in hops
192.168.23.0/ via 0.0.0.0 in hops
RIP: sending v2 update to 224.0.0.9 via FastEthernet0/ (172.16.1.254)
RIP: build update entries
172.16.31.0/ via 0.0.0.0, metric , tag
172.16.32.0/ via 0.0.0.0, metric , tag
172.16.33.0/ via 0.0.0.0, metric , tag
192.168.12.0/ via 0.0.0.0, metric , tag
192.168.23.0/ via 0.0.0.0, metric , tag
RIP: sending v2 update to 224.0.0.9 via FastEthernet0/ (192.168.12.1)
RIP: build update entries
172.16.1.0/ via 0.0.0.0, metric , tag
注：ip cef 和 no ip cef 的区别是，开启ip cef 可以直接在cef表查找是否有对应项，关闭ip cef则需要通过ARP协议查找mac地址是否对应。
什么时候需要开启自动汇总？而什么时候需要关闭自动汇总？
总结:在RIPv1有类路由的情况下开启自动汇总，RIPv1不支持VLSM，不携带子网掩码，在RIPv2中有子网存在的网络中需要关闭自动汇总，VLSM也必须关闭自动汇总,在路由的两端都出现相同网段的子网。
举个例子：R0：192.168.1.1/26, 192.168.30.1 R1:192.168.30.2/24 ,192.16.1.2/24 R2:192.16.1.3 ,192.168.1.252/26.
关闭自动汇总则能学习到完整的路由，没有关闭自动汇总则被汇总为192.168.1.0/24的网络，则中间路由学习到192.168.1.0 的路由指向为两个路由器。这样会导致路由环路，时断时好。路由表如下：
没关自动汇总路由：
Router#sh ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type , N2 - OSPF NSSA external type
E1 - OSPF external type , E2 - OSPF external type , E - EGP
i - IS-IS, L1 - IS-IS level-, L2 - IS-IS level-, ia - IS-IS inter area
* - candidate default, U - per-user static route, o - ODR
P - periodic downloaded static route Gateway of last resort is not set C 192.16.10.0/ is directly connected, FastEthernet0/
R 192.168.1.0/ [/] via 192.168.30.2, ::, FastEthernet0/
[/] via 192.16.10.2, ::, FastEthernet0/
R 192.168.2.0/ [/] via 192.16.10.2, ::, FastEthernet0/
C 192.168.30.0/ is directly connected, FastEthernet0/
关闭之后的自动汇总：
Router#sh ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type , N2 - OSPF NSSA external type
E1 - OSPF external type , E2 - OSPF external type , E - EGP
i - IS-IS, L1 - IS-IS level-, L2 - IS-IS level-, ia - IS-IS inter area
* - candidate default, U - per-user static route, o - ODR
P - periodic downloaded static route Gateway of last resort is not set C 192.16.10.0/ is directly connected, FastEthernet0/
192.168.1.0/ is subnetted, subnets
R 192.168.1.192 [/] via 192.16.10.2, ::, FastEthernet0/
R 192.168.2.0/ [/] via 192.16.10.2, ::, FastEthernet0/
C 192.168.30.0/ is directly connected, FastEthernet0/
关闭之后网络恢复正常。
有一个问题，就是r0有192.168.1.0的网段，中间连接r1，在连接一个r2，r2上也有192.168.1.0的网段，如果在r1与192.168.1.0的网段通信则会一会掉一会好。
最后补充一下：
子网就是比如A类的10.10.10.1子网掩码是255.255.255.0.（10.10.10.0/24或者/16）
B类的172.16.110.0/24这样。
C类 192.168.1.1/26.（自动汇总为192.168.1.0/24）