1.拓扑图
参考：http://www.time-dms.com.cn/viewthread.asp?tid=1904
2.基本接口配置：
R1：
R1(config)#int e0/0
R1(config-if)#ip add 10.1.1.1 255.255.255.0
R1(config-if)#no sh
R2:
R2(config)#int e0/0
R2(config-if)#ip add 10.1.1.2 255.255.255.0
R2(config-if)#no sh
R2(config)#int e0/1
R2(config-if)#ip add 202.100.1.2 255.255.255.0
R2(config-if)#no sh
R3:
R3(config)#int e0/0
R3(config-if)#ip add 202.100.1.3 255.255.255.0
R3(config-if)#no sh
R3(config)#int e0/1
R3(config-if)#ip add 202.100.2.3 255.255.255.0
R3(config-if)#no sh
C1:
eth0: 202.100.1.10/24
eth1:10.1.2.10/24
PC:
eth0:10.1.2.10/24
3.路由配置：
R1(config)#ip route 0.0.0.0 0.0.0.0 10.1.1.2
R2(config)#ip route 0.0.0.0 0.0.0.0 202.100.1.3
C1:Default GateWay 202.100.2.3
PC：Default GateWay 10.1.2.10
4.GRE配置
R3：
R2(config)#int tun 0
R2(config-if)#ip add 172.16.1.1 255.255.255.252
R2(config-if)#tunnel source 202.100.1.2
R2(config-if)#tun
R2(config-if)#tunnel de
R2(config-if)#tunnel destination 202.100.2.10
R2(config-if)#end
R2(config)#ip route 10.1.2.0 255.255.255.0 tunnel 0
C1：
A.确定是否加载了GRE模块
[root@cluster01 ~]# lsmod |grep ip_gre
B.加载GRE模块
[root@cluster01 ~]# insmod /lib/modules/2.6.18-
2.6.18-159.el5/ 2.6.18-238.9.1.el5/ 2.6.18-92.1.10.el5/
2.6.18-164.el5/ 2.6.18-238.el5/
[root@cluster01 ~]# uname -an
Linux cluster01 2.6.18-238.9.1.el5 #1 SMP Tue Apr 12 18:10:56 EDT 2011 i686 i686 i386 GNU/Linux
[root@cluster01 ~]# insmod
insmod insmod.static
[root@cluster01 ~]# insmod /lib/modules/2.6.18-238.9.1.el5/kernel/net/ipv4/ip
ipcomp.ko ip_gre.ko ipip.ko ipvs/
[root@cluster01 ~]# insmod /lib/modules/2.6.18-238.9.1.el5/kernel/net/ipv4/ip_gre.ko
C.GRE tunnel接口配置
[root@cluster01 ~]# ip tunnel add tunnel0 mode gre remote 202.100.1.2 local 202.100.2.10 ttl 225
[root@cluster01 ~]# ip link set tunnel0 up mtu 1500
[root@cluster01 ~]# ip addr add 172.16.1.2/30 peer 172.16.1.1/30 dev tunnel0
[root@cluster01 ~]# ip route add 10.1.1.0/24 dev tunnel0
D.验证GRE接口
[root@cluster01 ~]# ip addr show
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 16436 qdisc noqueue
link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
inet 127.0.0.1/8 scope host lo
inet6 ::1/128 scope host
valid_lft forever preferred_lft forever
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast qlen 1000
link/ether 00:0c:29:27:7d:b8 brd ff:ff:ff:ff:ff:ff
inet 202.100.2.10/24 brd 202.100.2.255 scope global eth0
inet6 fe80::20c:29ff:fe27:7db8/64 scope link
valid_lft forever preferred_lft forever
3: eth1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast qlen 1000
link/ether 00:0c:29:27:7d:c2 brd ff:ff:ff:ff:ff:ff
inet 10.1.2.10/24 brd 10.1.2.255 scope global eth1
inet6 fe80::20c:29ff:fe27:7dc2/64 scope link
valid_lft forever preferred_lft forever
4: sit0: <NOARP> mtu 1480 qdisc noop
link/sit 0.0.0.0 brd 0.0.0.0
5: gre0: <NOARP> mtu 1476 qdisc noop
link/gre 0.0.0.0 brd 0.0.0.0
6: tunnel0@NONE: <POINTOPOINT,NOARP,UP,LOWER_UP> mtu 1500 qdisc noqueue
link/gre 202.100.2.10 peer 202.100.1.2
inet 172.16.1.2 peer 172.16.1.1/30 scope global tunnel0
[root@cluster01 ~]# ip link show
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 16436 qdisc noqueue
link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast qlen 1000
link/ether 00:0c:29:27:7d:b8 brd ff:ff:ff:ff:ff:ff
3: eth1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast qlen 1000
link/ether 00:0c:29:27:7d:c2 brd ff:ff:ff:ff:ff:ff
4: sit0: <NOARP> mtu 1480 qdisc noop
link/sit 0.0.0.0 brd 0.0.0.0
5: gre0: <NOARP> mtu 1476 qdisc noop
link/gre 0.0.0.0 brd 0.0.0.0
6: tunnel0@NONE: <POINTOPOINT,NOARP,UP,LOWER_UP> mtu 1500 qdisc noqueue
link/gre 202.100.2.10 peer 202.100.1.2
[root@cluster01 ~]# ip route show
172.16.1.0/30 dev tunnel0 proto kernel scope link src 172.16.1.2
202.100.2.0/24 dev eth0 proto kernel scope link src 202.100.2.10
10.1.1.0/24 dev tunnel0 scope link
10.1.2.0/24 dev eth1 proto kernel scope link src 10.1.2.10
169.254.0.0/16 dev eth1 scope link
default via 202.100.2.3 dev eth0
E.开启路由转发
[root@cluster01 ~]# echo 1 > /proc/sys/net/ipv4/ip_forward
