GRE简介
通用路由选择封装（Generic RoutingEncapsulation，GRE）是网络中通过隧道将通信从一个专用网络传输到另一个专用网络常用到的一个协议。
尽管GRE不提供加密服务，但它提供低开销隧道。本实验通过GRE的RFC展示了GRE的不同实现并比较了其不同之处。
今天我们通过一个简单的实验来了解GRE
实验要求
1、现有一家公司，有两家分公司，这两家分公司分别在不同的地方，通过一个公用网通信
2、为在公用网中建立专用网络，实现公司内部加密通信，使用GRE协议技术实现此目的
实验拓扑
3、在总公司和两家分公司之间分别建立虚拟通信链路，两分公司的通信要通过总公司
具体实验过程
一、配置IP
总公司：
Router>en
Router#conf t
Enter configurationcommands, one per line. End with CNTL/Z.
Router(config)#int f0/1
Router(config-if)#ip add192.168.20.1 255.255.255.0
Router(config-if)#no sh
%LINK-5-CHANGED: InterfaceFastEthernet0/1, changed state to up
%LINEPROTO-5-UPDOWN: Lineprotocol on Interface FastEthernet0/1, changed state to up
Router(config-if)#ex
Router(config)#int f0/0
Router(config-if)#ip add172.1.1.2 255.255.255.252
Router(config-if)#no sh
%LINK-5-CHANGED: InterfaceFastEthernet0/0, changed state to up
Router(config-if)#ex
Router(config)#int loop0
%LINK-5-CHANGED: InterfaceLoopback0, changed state to up
%LINEPROTO-5-UPDOWN: Lineprotocol on Interface Loopback0, changed state to up
Router(config-if)#ip add192.168.0.1 255.255.255.255
Router(config-if)#ex
分公司1：
Router>en
Router#conf t
Enter configurationcommands, one per line. End with CNTL/Z.
Router(config)#int f0/1
Router(config-if)#ip add192.168.36.1 255.255.255.0
Router(config-if)#no sh
%LINK-5-CHANGED: InterfaceFastEthernet0/1, changed state to up
%LINEPROTO-5-UPDOWN: Lineprotocol on Interface FastEthernet0/1, changed state to up
Router(config-if)#exi
Router(config)#int f0/0
Router(config-if)#ip add172.1.2.2 255.255.255.252
Router(config-if)#no sh
%LINK-5-CHANGED: InterfaceFastEthernet0/0, changed state to up
Router(config-if)#ex
Router(config)#int loop 0
%LINK-5-CHANGED: InterfaceLoopback0, changed state to up
%LINEPROTO-5-UPDOWN: Lineprotocol on Interface Loopback0, changed state to up
Router(config-if)#ip add192.168.0.2 255.255.255.255
Router(config-if)#exi
分公司2：
Router>en
Router#conf t
Enter configurationcommands, one per line. End with CNTL/Z.
Router(config)#int f0/1
Router(config-if)#ip add192.168.44.1 255.255.255.0
Router(config-if)#no sh
%LINK-5-CHANGED: InterfaceFastEthernet0/1, changed state to up
%LINEPROTO-5-UPDOWN: Lineprotocol on Interface FastEthernet0/1, changed state to up
Router(config-if)#ex
Router(config)#int f0/0
Router(config-if)#ip add172.1.3.2 255.255.255.252
Router(config-if)#no sh
%LINK-5-CHANGED: InterfaceFastEthernet0/0, changed state to up
Router(config-if)#int loop0
%LINK-5-CHANGED: InterfaceLoopback0, changed state to up
Router(config-if)#
%LINEPROTO-5-UPDOWN: Lineprotocol on Interface Loopback0, changed state to up
Router(config-if)#ip ad192.168.0.3 255.255.255.255
Router(config-if)#ex
Router(config)#
INTERNET（中间路由）:
Router#conf t
Enter configurationcommands, one per line. End with CNTL/Z.
Router(config)#int f0/0
Router(config-if)#ip add172.1.1.1 255.255.255.252
Router(config-if)#no sh
%LINK-5-CHANGED: InterfaceFastEthernet0/0, changed state to up
%LINEPROTO-5-UPDOWN: Lineprotocol on Interface FastEthernet0/0, changed state to up
Router(config-if)#ex
Router(config)#int f0/1
Router(config-if)#ip add172.1.2.1 255.255.255.252
Router(config-if)#no sh
%LINK-5-CHANGED: InterfaceFastEthernet0/1, changed state to up
%LINEPROTO-5-UPDOWN: Lineprotocol on Interface FastEthernet0/1, changed state to up
Router(config-if)#ex
Router(config)#int f1/0
Router(config-if)#ip add172.1.3.1 255.255.255.252
Router(config-if)#no sh
%LINK-5-CHANGED: InterfaceFastEthernet1/0, changed state to up
%LINEPROTO-5-UPDOWN: Lineprotocol on Interface FastEthernet1/0, changed state to up
Router(config-if)#ex
Router(config)#int loop 0
%LINK-5-CHANGED: InterfaceLoopback0, changed state to up
%LINEPROTO-5-UPDOWN: Lineprotocol on Interface Loopback0, changed state to up
Router(config-if)#ip add172.1.0.1 255.255.255.255
Router(config-if)#ex
终端设备IP设置略
二、为3个路由设置NAT和ACL表
总公司：
Router#conf t
Enter configurationcommands, one per line. End with CNTL/Z.
Router(config)#int f0/1
Router(config-if)#ip nat in
Router(config-if)#ip natinside
Router(config-if)#exit
Router(config)#int f0/0
Router(config-if)#ip nat ou
Router(config-if)#ip natoutside
Router(config-if)#exi
Router(config)#ac
Router(config)#access-list100 per
Router(config)#access-list1 permit 192.168.20.0 0.0.0.255
Router(config)#ip natinside sou
Router(config)#ip natinside source list 1 ?
interface Specify interface for global address
pool Name pool of global addresses
Router(config)#ip natinside source list 1 int f0/0
Router(config)#exit
分公司1：
Router>en
Router#conf t
Enter configurationcommands, one per line. End with CNTL/Z.
Router(config)#int f0/1
Router(config-if)#ip natinsi
Router(config-if)#ip natinside
Router(config-if)#ex
Router(config)#int f0/0
Router(config-if)#ip nat o
Router(config-if)#ex
Router(config)#ac
Router(config)#access-list1 per
Router(config)#access-list1 permit 192.168.36.0 0.0.0.255
Router(config)#ip nat in
Router(config)#ip natinside s
Router(config)#ip natinside source list 1 int f0/0
Router(config)#ex
分公司2：
Router#conf t
Enter configurationcommands, one per line. End with CNTL/Z.
Router(config)#int f0/1
Router(config-if)#ip nat i
Router(config-if)#ex
Router(config)#int f0/0
Router(config-if)#ip nat o
Router(config-if)#ex
Router(config)#ac
Router(config)#access-list1 pe
Router(config)#access-list1 permit 192.168.44.0 0.0.0.255
Router(config)#ip nat in
Router(config)#ip natinside so
Router(config)#ip natinside source list 1 int f0/0
Router(config)#ex
三、GRE协议配置
总公司分公司1
总公司配置
Router>en
Router#conf t
Enter configuration commands, one perline. End with CNTL/Z.
Router(config)#int t
Router(config)#int tunnel 0
%LINK-5-CHANGED: Interface Tunnel0, changedstate to up
Router(config-if)#ip add 192.168.1.1255.255.255.252
Router(config-if)#tunnel s
Router(config-if)#tunnel source f0/0
Router(config-if)#tun
Router(config-if)#tunnel d
Router(config-if)#tunnel destination172.1.2.2
Router(config-if)#ex
分公司1配置
Router>en
Router#conf t
Enter configuration commands, one perline. End with CNTL/Z.
Router(config)#int t 0
%LINK-5-CHANGED: Interface Tunnel0, changedstate to up
Router(config-if)#ip add 192.168.1.2255.255.255.252
Router(config-if)#tu
Router(config-if)#tunnel s f0/0
Router(config-if)#tu
Router(config-if)#tunnel d 172.1.1.2
Router(config-if)#ex
总公司分公司2
总公司配置
Router#conf t
Enter configuration commands, one perline. End with CNTL/Z.
Router(config)#int t 1
%LINK-5-CHANGED: Interface Tunnel1, changedstate to up
Router(config-if)#ip add 192.168.1.5255.255.255.252
Router(config-if)#t s f0/0
Router(config-if)#t d 172.1.3.2
Router(config-if)#exi
分公司2配置
Router>en
Router#conf t
Enter configuration commands, one perline. End with CNTL/Z.
Router(config)#int t 0
%LINK-5-CHANGED: Interface Tunnel0, changedstate to up
Router(config-if)#ip ad 192.168.1.6255.255.255.252
Router(config-if)#t s f0/0
Router(config-if)#t d 172.1.1.2
Router(config-if)#ex
四、配置动态路由OSPF
开启OSPF动态路由协议并配置
总公司
Router#conf t
Enter configurationcommands, one per line. End with CNTL/Z.
Router(config)#router ?
bgp Border Gateway Protocol (BGP)
eigrp Enhanced Interior Gateway Routing Protocol (EIGRP)
ospf Open Shortest Path First (OSPF)
rip Routing Information Protocol (RIP)
Router(config)#router ospf?
<1-65535> Process ID
Router(config)#router ospf1
Router(config-router)#net
Router(config-router)#network192.168.20.0 0.0.0.255 area 0
Router(config-router)#network192.168.0.1 0.0.0.0 area 0
Router(config-router)#network192.168.1.0 0.0.0.3 area 0
Router(config-router)#network192.168.1.4 0.0.0.3 area 0
Router(config-router)#exit
Router(config)#ip route0.0.0.0 0.0.0.0 172.1.1.1（默认路由，作为连接入公网的边界路由，要配置默认路由）
%LINEPROTO-5-UPDOWN: Lineprotocol on Interface Tunnel0, changed state to up
%LINEPROTO-5-UPDOWN: Lineprotocol on Interface Tunnel1, changed state to up
Router(config)#
分公司1：
Router#conf t
Enter configurationcommands, one per line. End with CNTL/Z.
Router(config)#router o
% Incomplete command.
Router(config)#router o 1
Router(config-router)#net192.168.36.0 0.0.0.255 a 0
Router(config-router)#net192.168.0.2 0.0.0.0 a 0
Router(config-router)#net192.168.1.0 0.0.0.3 a 0
Router(config-router)#exit
Router(config)#ip route0.0.0.0 0.0.0.0 172.1.2.1
%LINEPROTO-5-UPDOWN: Lineprotocol on Interface Tunnel0, changed state to up
Router(config)#
00:50:26: %OSPF-5-ADJCHG:Process 1, Nbr 192.168.0.1 on Tunnel0 from LOADING to FULL, Loading Done
Router(config)#
分公司2
Router#conf t
Enter configurationcommands, one per line. End with CNTL/Z.
Router(config)#router o 1
Router(config-router)#net 192.168.44.00.0.0.255 a 0
Router(config-router)#net192.168.0.3 0.0.0.0 a 0
Router(config-router)#net192.168.1.4 0.0.0.3 a 0
Router(config-router)#ex
Router(config)#ip route0.0.0.0 0.0.0.0 172.1.3.1
%LINEPROTO-5-UPDOWN: Lineprotocol on Interface Tunnel0, changed state to up
Router(config)#
01:00:16: %OSPF-5-ADJCHG:Process 1, Nbr 192.168.0.1 on Tunnel0 from LOADING to FULL, Loading Done
Router(config)#
INTERNET配置
Router#conf t
Enter configurationcommands, one per line. End with CNTL/Z.
Router(config)#router o 1
Router(config-router)#net172.1.1.0 0.0.0.3 a 0
Router(config-router)#net172.1.2.0 0.0.0.3 a 0
Router(config-router)#net172.1.3.0 0.0.0.3 a 0
Router(config-router)#net172.1.0.1 0.0.0.0 a 0
Router(config-router)#exit
Router(config)#
至此，全部配置完成，在中间配置过程中也可以虽是进行测试，这里我们对结果进行个测试
，结果全部连通，我们来抓包看看协议的工作。
通过以上两个进出包的分析，我们很容易发现，数据包从总公司出去的时候加上了GRE封装，IP协议在这里既做了载荷协议又做了承载协议。并且经过GRE封装之后，可以看到承载协议的IP头源地址和目的地址被转换为我们的通信隧道的源端和目的端。