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00.基础阶段/01.网络基础/03.路由原理.md
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00.基础阶段/01.网络基础/03.路由原理.md
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# 03.路由原理
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## 1. 路由原理
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```mermaid
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graph LR
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A[路由器收到数据] --> B[查找路由表]
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B-->C[存在匹配条目]
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B-->D[不存在匹配条目]
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C-->E[匹配条目中有没有出接口]
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E-->F[存在出接口直接转发]
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E-->G[不存在出接口,根据下一跳地址再次查找路由表]
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D-->H[存在默认路由,就从默认路由标识的出口转发]
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H-->G
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D-->I[不存在默认路由,就丢弃]
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```
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```
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R1#show ip route # 查看路由表
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Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
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D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
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N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
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E1 - OSPF external type 1, E2 - OSPF external type 2
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i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
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ia - IS-IS inter area, * - candidate default, U - per-user static route
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o - ODR, P - periodic downloaded static route, H - NHRP, l - LISP
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a - application route
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+ - replicated route, % - next hop override, p - overrides from PfR
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# 以上是路由表的提示信息，标识路由条目中出现的代码意思
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Gateway of last resort is not set
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# 上面这条是表示这个路由器没有默认的路由，当下面的条目与数据中目的地址一个也对不上的时候，就丢弃
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# 如果有默认路由，当下面的条目与数据中目的地址一个也对不上的时候，就从默认的路由转发
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192.168.12.0/24 is variably subnetted, 2 subnets, 2 masks # 为了方便查看，路由表会按照ABC类IP地址进行分类，这个是分类的标题
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C 192.168.12.0/24 is directly connected, Ethernet0/0 # 前面的C，可以对照上面的codes,是直连的条目，后面的意思，可以直接从e0/0接口转发
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L 192.168.12.1/32 is directly connected, Ethernet0/0
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S 192.168.23.0/24 [1/0] via 192.168.12.2 # 前面的S，表示静态，后面并没有表示出接口，只标识了下一跳IP地址，这样就需要再次查找路由表，称为递归查找
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```
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### 1.1 路由匹配条件
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路由表中出现了`192.168.12.0/24`说明：
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1. 数据的目的IP地址的二进制，至少前24位要和`192.168.12.0`二进制前24位完全一致。
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2. 这边的`/24`不一定和子网掩码是一样的，因为数据包中并不携带子网掩码信息。
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### 1.2 路由匹配原则
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### 1.3 最长匹配原则
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```
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R2#sh ip route
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172.16.0.0/16 is variably subnetted, 2 subnets, 2 masks
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S 172.16.0.0/16 [1/0] via 192.168.12.1
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S 172.16.3.0/24 [1/0] via 192.168.23.3
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192.168.12.0/24 is variably subnetted, 2 subnets, 2 masks
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C 192.168.12.0/24 is directly connected, Ethernet0/0
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L 192.168.12.2/32 is directly connected, Ethernet0/0
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192.168.23.0/24 is variably subnetted, 2 subnets, 2 masks
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C 192.168.23.0/24 is directly connected, Ethernet0/1
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L 192.168.23.2/32 is directly connected, Ethernet0/1
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R2#ping 172.16.1.1
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Type escape sequence to abort.
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Sending 5, 100-byte ICMP Echos to 172.16.1.1, timeout is 2 seconds:
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!!!!!
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Success rate is 100 percent (5/5), round-trip min/avg/max = 1/4/5 ms
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R2#ping 172.16.3.3
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Type escape sequence to abort.
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Sending 5, 100-byte ICMP Echos to 172.16.3.3, timeout is 2 seconds:
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!!!!!
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Success rate is 100 percent (5/5), round-trip min/avg/max = 1/3/6 ms
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```
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`ping 172.16.1.1`的时候，由于只能与第3行的条目匹配，所以数据发给了R1,最终能用通信
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`ping 172.16.3.3`的时候，由于第3，4行条目都匹配，路由器会优选匹配最长的条目，也就是第4行，最终能通信
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### 1.4 默认路由
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当路由条目的匹配长度为0的时候，那么所有的IP地址都是可以匹配的上的，这个路由条目被称为缺省路由，或者默认路由
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```
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R1(config)#ip route 0.0.0.0 0.0.0.0 192.168.12.2
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R1#sh ip route
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Gateway of last resort is 192.168.12.2 to network 0.0.0.0
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S* 0.0.0.0/0 [1/0] via 192.168.12.2 # 默认路由在路由表中会带上*号
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172.16.0.0/16 is variably subnetted, 2 subnets, 2 masks
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C 172.16.1.0/24 is directly connected, Loopback0
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L 172.16.1.1/32 is directly connected, Loopback0
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192.168.12.0/24 is variably subnetted, 2 subnets, 2 masks
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C 192.168.12.0/24 is directly connected, Ethernet0/0
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L 192.168.12.1/32 is directly connected, Ethernet0/0
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S 192.168.23.0/24 [1/0] via 192.168.12.2
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R1#ping 172.16.3.3
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Type escape sequence to abort.
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Sending 5, 100-byte ICMP Echos to 172.16.3.3, timeout is 2 seconds:
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!!!!!
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Success rate is 100 percent (5/5), round-trip min/avg/max = 1/1/1 ms
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```
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在`ping 172.16.3.3`的时候，可以和路由表中的默认路由匹配的上。
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在`ping 192.168.23.3`的时候，根据最长匹配原则，会和第11行匹配上
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### 1.5 路由条目的来源
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* 直连接口
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* 必须要是双`up`状态,才能生效
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* 静态路由
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* 必须出接口或者下一跳IP地址有效
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* 动态路由学习
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* 通过各种路由协议，让路由器互相分享路由信息
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## 2. 环回接口
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环回接口是软件模拟出来的接口，用于测试，或者管理
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环回接口正常情况下是不会`down`，只要操作系统可以正常开机，那么环回接口就可以正常`up`
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一台设备往往可以启动很多很多环回接口，用于模拟网络
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```
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R1(config)#interface loopback 0
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R1(config-if)#ip add 172.16.1.1 255.255.255.0
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R1#show ip interface brief # 查看接口的IP地址和状态
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Interface IP-Address OK? Method Status Protocol
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Ethernet0/0 192.168.12.1 YES manual up up
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Ethernet0/1 unassigned YES unset administratively down down
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Ethernet0/2 unassigned YES unset administratively down down
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Ethernet0/3 unassigned YES unset administratively down down
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Loopback0 172.16.1.1 YES manual up up
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```
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