1.拓扑图： 
备注：因为应用原因，需要在linux2上添加一个公网地址，并且在中间路由设备不受控制的情况下，Linux1能访问到linux2上面的公网地址。
2.基本接口配置：
linux1:192.168.10.1/24
linux2:192.168.20.2/24
R1:
interface FastEthernet0/0
ip address 192.168.10.10 255.255.255.0
no shutdown
!
interface FastEthernet0/1
ip address 192.168.20.10 255.255.255.0
no shutdown
3.路由配置：
linux1网关:192.168.10.10
linux2网关:192.168.20.10
R1：只有直连路由
4.Linux2单网卡多地址配置：
[root@Linux1 ~]# cp /etc/sysconfig/network-scripts/ifcfg-eth0 /etc/sysconfig/network-scripts/ifcfg-eth0:0
[root@Linux1 ~]# vi /etc/sysconfig/network-scripts/ifcfg-eth0:0
# Advanced Micro Devices [AMD] 79c970 [PCnet32 LANCE]
DEVICE=eth0:0
BOOTPROTO=none
ONBOOT=yes
HWADDR=00:0c:29:08:48:63
NETMASK=255.255.255.252
IPADDR=202.100.2.2
TYPE=Ethernet
USERCTL=no
IPV6INIT=no
PEERDNS=yes
[root@Linux1 ~]# service network restart
[root@Linux1 ~]# ping 202.100.2.2
PING 202.100.2.2 (202.100.2.2) 56(84) bytes of data.
64 bytes from 202.100.2.2: icmp_seq=1 ttl=64 time=0.124 ms
--- 202.100.2.2 ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 0.124/0.124/0.124/0.000 ms
[root@ams ~]# ping 192.168.10.10
PING 192.168.10.10 (192.168.10.10) 56(84) bytes of data.
64 bytes from 192.168.10.10: icmp_seq=1 ttl=255 time=70.6 ms
--- 192.168.10.10 ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 70.629/70.629/70.629/0.000 ms
5.GRE tunnel配置：
A.确定是否加载了GRE模块
[root@Linux1 ~]# lsmod |grep ip_gre
[root@Linux2 ~]# lsmod |grep ip_gre
B.加载GRE模块
[root@linux1 ~]# uname -an
Linux linux1 2.6.9-78.EL#1 Wed Jul 9 15:27:01 EDT 2008 i686 i686 i386 GNU/Linux
[root@linux1 ~]# insmod /lib/modules/2.6.9-78.EL/kernel/net/ipv4/ip_gre.ko
[root@linux2 ~]# uname -an
Linux linux2 2.6.18-164.el5 #1 SMP Thu Sep 3 03:33:56 EDT 2009 i686 i686 i386 GNU/Linux
[root@linux2 ~]# insmod /lib/modules/2.6.18-164.el5/kernel/net/ipv4/ip_gre.ko
C.GRE tunnel接口配置
Linux1:
ip tunnel add tunnel0 mode gre remote 192.168.20.2 local 192.168.10.1 ttl 255
ip link set tunnel0 up mtu 1400
ip addr add 172.16.1.1/30 dev tunnel0
ip addr add 172.16.1.1/30 peer 172.16.1.2/30 dev tunnel0
ip route add 202.100.2.2/32 dev tunnel0
Linux2:
ip tunnel add tunnel0 mode gre remote 192.168.10.1 local 192.168.20.2 ttl 255
ip link set tunnel0 up mtu 1400
ip addr add 172.16.1.2/30 dev tunnel0
ip addr add 172.16.1.2/30 peer 172.16.1.1/30 dev tunnel0
D.将tunnel配置开机运行：
linux1:
vi /etc/init.d/gre.sh ##内容如下：
insmod /lib/modules/2.6.9-78.EL/kernel/net/ipv4/ip_gre.ko
ip tunnel add tunnel0 mode gre remote 192.168.20.2 local 192.168.10.1 ttl 255
ip link set tunnel0 up mtu 1400
ip addr add 172.16.1.1/30 dev tunnel0
ip addr del 172.16.1.1/30 peer 172.16.1.2/30 dev tunnel0
ip addr add 172.16.1.1/30 peer 172.16.1.2/30 dev tunnel0
ip route add 202.100.2.2/32 dev tunnel0
chmod +x /etc/init.d/gre.sh
echo "/etc/init.d/gre.sh" >> /etc/rc.d/rc.local
linux2:
vi /etc/init.d/gre.sh ##内容如下：
insmod /lib/modules/2.6.18-164.el5/kernel/net/ipv4/ip_gre.ko
ip tunnel add tunnel0 mode gre remote 192.168.10.1 local 192.168.20.2 ttl 255
ip link set tunnel0 up mtu 1400
ip addr add 172.16.1.2/30 dev tunnel0
ip addr del 172.16.1.2/30 peer 172.16.1.1/30 dev tunnel0
ip addr add 172.16.1.2/30 peer 172.16.1.1/30 dev tunnel0
chmod +x /etc/init.d/gre.sh
echo "/etc/init.d/gre.sh" >> /etc/rc.d/rc.local
D.验证GRE接口
[root@Linux1 ~]#ip addr show
1: lo: 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 brd 127.255.255.255 scope host lo
inet6 ::1/128 scope host
valid_lft forever preferred_lft forever
2: eth0: mtu 1500 qdisc pfifo_fast qlen 1000
link/ether 00:0c:29:e4:65:78 brd ff:ff:ff:ff:ff:ff
inet 192.168.10.1/24 brd 192.168.10.255 scope global eth0
inet6 fe80::20c:29ff:fee4:6578/64 scope link
valid_lft forever preferred_lft forever
3: sit0: mtu 1480 qdisc noop
link/sit 0.0.0.0 brd 0.0.0.0
4: gre0: mtu 1476 qdisc noop
link/gre 0.0.0.0 brd 0.0.0.0
5: tunnel0@NONE: mtu 1400 qdisc noqueue
link/gre 192.168.10.1 peer 192.168.20.2
inet 172.16.1.1 peer 172.16.1.2/30 scope global tunnel0
[root@Linux2 ~]#ip addr show
1: lo: 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: mtu 1500 qdisc pfifo_fast qlen 1000
link/ether 00:0c:29:08:48:63 brd ff:ff:ff:ff:ff:ff
inet 192.168.20.2/24 brd 192.168.20.255 scope global eth0
inet 202.100.2.2/30 brd 202.100.2.3 scope global eth0:0
inet6 fe80::20c:29ff:fe08:4863/64 scope link
valid_lft forever preferred_lft forever
3: sit0: mtu 1480 qdisc noop
link/sit 0.0.0.0 brd 0.0.0.0
4: gre0: mtu 1476 qdisc noop
link/gre 0.0.0.0 brd 0.0.0.0
5: tunnel0@NONE: mtu 1400 qdisc noqueue
link/gre 192.168.20.2 peer 192.168.10.1
inet 172.16.1.2 peer 172.16.1.1/30 scope global tunnel0
5.效果测试：
[root@linux1 ~]# ping 202.100.2.2
PING 202.100.2.2 (202.100.2.2) 56(84) bytes of data.
64 bytes from 202.100.2.2: icmp_seq=0 ttl=64 time=82.4 ms
64 bytes from 202.100.2.2: icmp_seq=1 ttl=64 time=48.7 ms
--- 202.100.2.2 ping statistics ---
2 packets transmitted, 2 received, 0% packet loss, time 1002ms
rtt min/avg/max/mdev = 48.784/65.633/82.482/16.849 ms, pipe 2
