功能简介：

GRE（Generic Routing Encapsulation，通用路由封装）协议是对某些网络层协议（如IP和IPX）的数据报文进行封装，使这些被封装的数据报文能够在另一个网络层协议（如IP）中传输。封装后的数据报文在网络中传输的路径，称为GRE隧道。GRE隧道是一个虚拟的点到点的连接，其两端的设备分别对数据报进行封装及解封装。

一、组网需求

交换机Switch A和交换机Swich B之间通过Internet相连。运行IP协议的私有网络的两个子网Group 1和Group 2，通过在两台交换机之间使用GRE建立隧道实现互联。

二、组网拓扑

三、配置要点

GRE 隧道配置包括以下几个配置要点：

1、配置隧道接口编号

2、配置隧道模式（本例以配置GRE IP模式为例）

3、配置隧道的源地址

4、配置隧道目的地址

5、配置隧道路由

注意：如果隧道两端Tunnel接口的地址不在同一个网段，则必须配置通过隧道到达对端的转发路由，以便需要进行封装的报文能正常转发。用户可以配置静态路由，也可以配置动态路由，在Tunnel的两端都要进行此项配置。对两个或两个以上使用同种封装协议的Tunnel接口，不能同时配置完全相同的源地址和目的地址。配置Tunnel接口的源端地址时，若采用配置源接口形式，则Tunnel的源地址取的是源接口的主IP地址。

四、配置步骤

说明：已经配置SwitchA和SwitchB之间IPv4报文路由可达。

1）在SwitchA的配置

连接IPv4外部网络的接口

SwitchA>enable

SwitchA#config ter

SwitchA#configureterminal

SwitchA(config)#interface GigabitEthernet 2/1

SwitchA(config-if)#ip address 2.2.2.1 255.255.255.0

连接IPv4内部网络的接口

SwitchA#configure terminal

SwitchA(config)#interface GigabitEthernet 2/2

SwitchA(config-if)#ip address 1.1.1.1 255.255.255.0

配置GRE IP隧道接口

SwitchA#configure terminal

SwitchA(config)#interface Tunnel 100

SwitchA(config-if-Tunnel 100)#tunnel mode gre ip

SwitchA(config-if-Tunnel 100)#ip address 5.5.5.4 255.255.255.0

SwitchA(config-if-Tunnel 100)#tunnel source 2.2.2.1

SwitchA(config-if-Tunnel 100)#tunnel destination 2.2.2.2

配置进隧道的路由

SwitchA#configureterminal

SwitchA(config)#ip route 3.3.3.0 tunnel 100

2）在SwitchB的配置

连接IPv4外部网络的接口

SwitchB#configure terminal

SwitchB(config)#interface GigabitEthernet 2/1

SwitchB(config-if)#ip address 2.2.2.2 255.255.255.0

连接IPv4内部网络的接口

SwitchB#configure terminal

SwitchB(config)#interface GigabitEthernet 2/2

SwitchB(config-if)#ip address 3.3.3.1 255.255.255.0

配置GRE IP隧道接口

SwitchB#configure terminal

SwitchB(config)#interface Tunnel 100

SwitchB(config-if-Tunnel 100)#tunnel mode gre ip

SwitchB(config-if-Tunnel 100)#ip address 5.5.5.5 255.255.255.0

SwitchB(config-if-Tunnel 100)#tunnel source 2.2.2.2

SwitchB(config-if-Tunnel 100)#tunnel destination 2.2.2.1

配置进隧道的路由

SwitchB#configure terminal

SwitchB(config)#ip route 1.1.1.0 tunnel 100

五、显示验证

1、完成上述配置之后，分别查看SwitchA和SwitchB的Tunnel接口状态如下：

SwitchA#show interface tunnel 100

Index(dec):9 (hex):9

Tunnel 100 is UP , line protocol is UP

Hardware is Tunnel

Interface address is: 5.5.5.4/24

Interface IPv6 address is:

No IPv6 address

MTU 1476 bytes, BW 9 Kbit

Encapsulation protocol is Tunnel, loopback not set

Keepalive interval is 10 sec ,retries 0.

Carrier delay is 2 sec

Tunnel attributes:

Tunnel source 2.2.2.1, destination 2.2.2.2, routable

Tunnel TOS/Traffic Class not set, Tunnel TTL 254

Tunnel config nested limit is 4, current nested number is 0

Tunnel protocol/transport is greip

Tunnel transport VPN is no set

Key disabled, Sequencing disabled

Checksumming of packets disabled

RX packets

Drop reason(Down: 0, Checksum error: 0, sequence error: 0, routing: 0)

TX packets

Drop reason(Too big: 0, Payload Type error: 0, Nested-limit: 0)

Rxload is 1/255, Txload is 1/255

10 seconds input rate 0 bits/sec, 0 packets/sec

10 seconds output rate 0 bits/sec, 0 packets/sec

0 packets input, 0 bytes, 0 no buffer, 0 dropped

Received 0 broadcasts, 0 runts, 0 giants

0 input errors, 0 CRC, 0 frame, 0 overrun, 0 abort

0 packets output, 0 bytes, 0 underruns , 0 dropped

0 output errors, 0 collisions, 0 interface resets

SwitchB#show interface tunnel 100

Index(dec):9 (hex):9

Tunnel 100 is UP , line protocol is UP

Hardware is Tunnel

Interface address is: 5.5.5.5/24

Interface IPv6 address is:

No IPv6 address

MTU 1476 bytes, BW 9 Kbit

Encapsulation protocol is Tunnel, loopback not set

Keepalive interval is 10 sec ,retries 0.

Carrier delay is 2 sec

Tunnel attributes:

Tunnel source 2.2.2.2, destination 2.2.2.1, routable

Tunnel TOS/Traffic Class not set, Tunnel TTL 254

Tunnel config nested limit is 4, current nested number is 0

Tunnel protocol/transport is greip

Tunnel transport VPN is no set

Key disabled, Sequencing disabled

Checksumming of packets disabled

RX packets

Drop reason(Down: 0, Checksum error: 0, sequence error: 0, routing: 0)

TX packets

Drop reason(Too big: 0, Payload Type error: 0, Nested-limit: 0)

Rxload is 1/255, Txload is 1/255

10 seconds input rate 0 bits/sec, 0 packets/sec

10 seconds output rate 0 bits/sec, 0 packets/sec

0 packets input, 0 bytes, 0 no buffer, 0 dropped

Received 0 broadcasts, 0 runts, 0 giants

0 input errors, 0 CRC, 0 frame, 0 overrun, 0 abort

0 packets output, 0 bytes, 0 underruns , 0 dropped

0 output errors, 0 collisions, 0 interface resets

2、从Switch A上可以Ping通对端的接口的IPv4地址。

SwitchA#ping2.2.2.2

Sending 5, 100-byte ICMP Echoes to 2.2.2.2, timeout is 2 seconds:

< press Ctrl+C to break >

!!!!!

Success rate is 100 percent (5/5), round-trip min/avg/max = 1/2/10ms