网络路由实验 #
网络拓扑 #
- PC1
- IP 地址:
192.168.1.100/24
- IP 地址:
- PC2
- IP 地址:
192.168.2.100/24
- IP 地址:
- Router
- Cisco 7200路由器
eth0IP地址：192.168.1.1/24，MAC地址ca:01:05:24:00:00eth1IP地址：192.168.2.1/24，MAC地址ca:01:05:24:00:1c
GNS配置与操作 #
PC1
sudo ip addr add 192.168.1.100/24 dev ens4
sudo ip link set ens4 up
sudo ip route add default via 192.168.1.1
# Error: Nexthop has invalid gateway.
# 网卡需要先up，另外不在一个网段也会报错。单纯的配置，不会因为1.1不通而无法配置
PC2
sudo ip addr add 192.168.2.100/24 dev ens4
sudo ip link set ens4 up
sudo ip route add default via 192.168.2.1
Router
R1#show ip interface brief
Interface IP-Address OK? Method Status Protocol
FastEthernet0/0 unassigned YES unset administratively down down
FastEthernet1/0 unassigned YES unset administratively down down
R1#configure terminal
R1(config)#interface FastEthernet0/0
R1(config-if)#ip address 192.168.1.1 255.255.255.0
R1(config-if)#no shutdown
*Feb 27 14:13:46.143: %LINK-3-UPDOWN: Interface FastEthernet0/0, changed state to up
*Feb 27 14:13:47.143: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/0,
changed state to up
R1(config-if)#interface FastEthernet1/0
R1(config-if)#ip address 192.168.2.1 255.255.255.0
R1(config-if)#no shutdown
PC1 ping PC2
debian@debian:~$ ping 192.168.2.100
PING 192.168.2.100 (192.168.2.100) 56(84) bytes of data.
64 bytes from 192.168.2.100: icmp_seq=1 ttl=63 time=52.4 ms
64 bytes from 192.168.2.100: icmp_seq=2 ttl=63 time=16.8 ms
64 bytes from 192.168.2.100: icmp_seq=3 ttl=63 time=17.7 ms
^C
--- 192.168.2.100 ping statistics ---
3 packets transmitted, 3 received, 0% packet loss, time 2005ms
rtt min/avg/max/mdev = 16.758/28.966/52.413/16.583 ms
路由流程 #
PC1 ping PC2 时抓包，分析路由的工作流程如下：
Step 1 确定next hop网关
基于目标地址192.168.2.100和路由表来决定下一跳网关地址
debian@debian:~$ ip route
default via 192.168.1.1 dev ens4
192.168.1.0/24 dev ens4 proto kernel scope link src 192.168.1.100
debian@debian:~$ ip route get 192.168.2.100
192.168.2.100 via 192.168.1.1 dev ens4 src 192.168.1.100 uid 1000
cache
192.168.2.100不在网络192.168.1.0/24内- 最终命中
default默认路由，确定网关地址为192.168.1.1
Step 2 报文发送给网关
IP报文目的IP地址192.168.2.100不变，但对应二层报文的MAC地址为网关地址192.168.1.1的MAC地址ca:01:05:24:00:00
debian@debian:~$ sudo ip -s -s neigh flush all
192.168.1.1 dev ens4 lladdr ca:01:05:24:00:00 used 1215/1215/1180probes 4 STALE
*** Round 1, deleting 1 entries ***
*** Flush is complete after 1 round ***
debian@debian:~$ sudo ip neighbor show
debian@debian:~$ ping 192.168.2.100
PING 192.168.2.100 (192.168.2.100) 56(84) bytes of data.
64 bytes from 192.168.2.100: icmp_seq=1 ttl=63 time=23.4 ms
64 bytes from 192.168.2.100: icmp_seq=2 ttl=63 time=10.9 ms
64 bytes from 192.168.2.100: icmp_seq=3 ttl=63 time=21.3 ms
^C
--- 192.168.2.100 ping statistics ---
3 packets transmitted, 3 received, 0% packet loss, time 2004ms
rtt min/avg/max/mdev = 10.908/18.533/23.371/5.456 ms
Step 3 路由器内部转发
基于目的网络地址选择合适的网卡发出，目的地址192.168.2.100在192.168.2.0/24网络内，从FastEthernet1/0 网卡发出，对应MAC地址为ca:01:05:24:00:1c
R1#show ip route
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route, H - NHRP, l - LISP
a - application route
+ - replicated route, % - next hop override
Gateway of last resort is not set
192.168.1.0/24 is variably subnetted, 2 subnets, 2 masks
C 192.168.1.0/24 is directly connected, FastEthernet0/0
L 192.168.1.1/32 is directly connected, FastEthernet0/0
192.168.2.0/24 is variably subnetted, 2 subnets, 2 masks
C 192.168.2.0/24 is directly connected, FastEthernet1/0
L 192.168.2.1/32 is directly connected, FastEthernet1/0
结论 #
- 跨网络通讯是通过路由器逐跳转发数据包，最终到达目标地址的过程。它类似于快递运输，源 IP 和目标 IP 相当于寄件人和收件人的地址，而路由器（网关）则是不同快递公司的交接站，确保包裹可以从一个区域传送到另一个区域。在这个过程中，每一跳的 MAC 地址都会被重新封装，因为每个网段的链路层地址都是独立的，而 IP 地址始终保持不变，以确保正确传输到最终目的地。