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公司在国内、日本、美国、德国、新加坡等多地均有业务，中间业务网络用的公司专有 GPN(Global Private Network 中文名是全球私有化网络）链路，目前测试搭建一条备用链路，用于网络冗余和故障切换。
初步选用方案 GRE over IPSec，跑 ospf 路由协议。
一、为何要选 GRE over IPSec：
各个 site 网络比较多，需要使用路由协议进行互联；
IPSEC 不支持组播，即不能传递路由协议，在承载路由协议上不如 GRE 隧道方便；
GRE 隧道不能提供加密保障；
使用 GRE 在两个网关之间搭建一个隧道，运行路由协议及传输正常数据，使用 IPSec 对整个 GRE 隧道进行加密，因此需要把两者进行结合。
二、测试环境：
以中国、日本、美国三地为例，基本网络拓扑如下图，用 Mikrotik RouterOS（简称 ROS）做路由器和防火墙，中间跑 ospf 协议。GPN 链路就相当于一个大二层，能够把中日美三地打通，相当于专线，因此在网络质量上优于直接走大网，做业务主线，这里不多写；主要写一下如何配置 GRE 链路实现备用链路功能，在 GPN 链路中断的时候能够自动切换到备线。
routeros 上配置外网：
中国：101.251.x.x
日本：205.177.x.x
美国：38.83.x.x
三地的内网地址：
中国：10.13.24.0/22
日本：10.13.4.0/22
美国：10.13.12.0/22
三地互联地址 (用 10.13.253.0/24 段做互联地址段)：
中国和日本：10.13.253.0/30
日本和美国：10.13.253.16/30
美国和中国：10.13.253.4/30
GPN 链路网段：
10.13.252.0/24
三、配置
1、三个 ros 的 interface（ether1/2/ 3 分别对应着外网 / 内网 /GPN 网络）：
2、IPSec 配置
中国：
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 | /ip ipsec peer add address=205.177.x.x/32:500 comment="JP Link" auth-method=pre-shared-key secret="mypassword" generate-policy=no exchange-mode=main send-initial-contact=yes nat-traversal=no proposal-check=obey hash-algorithm=sha1 enc-algorithm=3des dh-group=modp1024 lifetime=1d lifebytes=0 dpd-interval=disable-dpd dpd-maximum-failures=5 add address=38.83.x.x/32:500 comment= "USALink" auth-method=pre-shared-key secret="mypassword" generate-policy=no exchange-mode=main send-initial-contact=yes nat-traversal=no proposal-check=obey hash-algorithm=sha1 enc-algorithm=3des dh-group=modp1024 lifetime=1d lifebytes=0 dpd-interval=disable-dpd dpd-maximum-failures=5 /ip ipsec policy add src-address=101.251.x.x/32:any dst-address=205.177.x.x/32:any protocol=all action=encrypt level=require ipsec-protocols=esp tunnel=yes sa-src-address=101.251.x.x sa-dst-address=205.177.x.x proposal=default priority=0 add src-address=101.251.x.x/32:any dst-address=38.83.x.x/32:any protocol=all action=encrypt level=require ipsec-protocols=esp tunnel=yes sa-src-address=101.251.x.x sa-dst-address=38.83.x.x proposal=default priority=0 |
日本：
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 | /ip ipsec peer add address=101.251.x.x/32:500 comment="BJ Link" auth-method=pre-shared-key secret="mypassword" generate-policy=no exchange-mode=main send-initial-contact=yes nat-traversal=no proposal-check=obey hash-algorithm=sha1 enc-algorithm=3des dh-group=modp1024 lifetime=1d lifebytes=0 dpd-interval=disable-dpd dpd-maximum-failures=5 add address=38.83.x.x/32:500 comment= "USA Link" auth-method=pre-shared-key secret="mypassword" generate-policy=no exchange-mode=main send-initial-contact=yes nat-traversal=no proposal-check=obey hash-algorithm=sha1 enc-algorithm=3des dh-group=modp1024 lifetime=1d lifebytes=0 dpd-interval=disable-dpd dpd-maximum-failures=5 /ip ipsec policy add src-address=205.177.x.x/32:any dst-address=101.251.x.x/32:any protocol=all action=encrypt level=require ipsec-protocols=esp tunnel=yes sa-src-address=205.177.x.x sa-dst-address=101.251.x.x proposal=default priority=0 add src-address=205.177.x.x/32:any dst-address=38.83.x.x/32:any protocol=all action=encrypt level=require ipsec-protocols=esp tunnel=yes sa-src-address=205.177.x.x sa-dst-address=38.83.x.x proposal=default priority=0 |
美国：
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 | /ip ipsec peer add address=101.251.x.x/32:500 comment="BJ Link" auth-method=pre-shared-key secret="mypassword" generate-policy=no exchange-mode=main send-initial-contact=yes nat-traversal=no proposal-check=obey hash-algorithm=sha1 enc-algorithm=3des dh-group=modp1024 lifetime=1d lifebytes=0 dpd-interval=disable-dpd dpd-maximum-failures=5 add address=205.177.x.x/32:500 comment= "JP Link" auth-method=pre-shared-key secret="mypassword" generate-policy=no exchange-mode=main send-initial-contact=yes nat-traversal=no proposal-check=obey hash-algorithm=sha1 enc-algorithm=3des dh-group=modp1024 lifetime=1d lifebytes=0 dpd-interval=disable-dpd dpd-maximum-failures=5 /ip ipsec policy add src-address=38.83.x.x/32:any dst-address=101.251.x.x/32:any protocol=all action=encrypt level=require ipsec-protocols=esp tunnel=yes sa-src-address=38.83.x.x sa-dst-address=101.251.x.x proposal=default priority=0 add src-address=38.83.x.x/32:any dst-address=205.177.x.x/32:any protocol=all action=encrypt level=require ipsec-protocols=esp tunnel=yes sa-src-address=38.83.x.x sa-dst-address=205.177.x.x proposal=default priority=0 |
中国示例图：
3、GRE 配置：
中国：
1 2 3 4 5 6 7 8 9 10 | /interface gre add name= gre-tunnel1 local-address=101.251.x.x remote-address=205.177.x.x comment="JP Link" add name= gre-tunnel2 local-address=101.251.x.x remote-address=38.83.x.x comment="USA Link" /ip address add address=10.13.253.1/30 interface=gre-tunnel1 network=10.13.253.0 comment="JP Link"add address=10.13.253.5/30 interface=gre-tunnel2 network=10.13.253.4 comment="USA Link" |
日本：
1 2 3 4 5 6 7 8 9 10 | /interface gre add name= gre-tunnel1 local-address=205.177.x.x remote-address=101.251.x.x comment="BJ Link" add name= gre-tunnel2 local-address=101.251.x.x remote-address=38.83.x.x comment="USA Link" /ip address add address=10.13.253.2/30 interface=gre-tunnel1 network=10.13.253.0 comment="BJ Link"add address=10.13.253.17/30 interface=gre-tunnel2 network=10.13.253.16 comment="USA Link" |
美国：
1 2 3 4 5 6 7 8 9 10 | /interface gre add name= gre-tunnel1 local-address=38.83.x.x remote-address=101.251.x.x comment="BJ Link" add name= gre-tunnel2 local-address=38.83.x.x remote-address=205.177.x.x comment="JP Link" /ip address add address=10.13.253.6/30 interface=gre-tunnel1 network=10.13.253.4 comment="BJ Link"add address=10.13.253.18/30 interface=gre-tunnel2 network=10.13.253.16 comment="JP Link" |
中国示例图：
4、OSPF 配置
把本地的内网地址段、GPN 网段（10.13.252.0/24），GRE 互联地址网段都宣布进去,cost 值 GPN 链路的优先级高，设为 10，GRE Tunnel 的 cost 值设为 100：
中国：
1 2 3 4 5 6 7 8 9 10 11 | /routing ospf> interface add interface=eth2 cost=10 interface add interface=eth3 cost=10 interface add interface=gre-tunnel1 cost=100 interface add interface=gre-tunnel2 cost=100 /routing ospf> network add network=10.13.24.0/22 area=backbone comment="内网" network add network=10.13.252.0/24 area=backbone comment="GPN" network add network=10.13.253.0/30 area=backbone comment="JP Link" network add network=10.13.253.4/30 area=backbone comment="USA Link" |
日本：
1 2 3 4 5 6 7 8 9 10 11 | /routing ospf> interface add interface=eth2 cost=10 interface add interface=eth3 cost=10 interface add interface=gre-tunnel1 cost=100 interface add interface=gre-tunnel2 cost=100 /routing ospf> network add network=10.13.4.0/22 area=backbone comment="内网" network add network=10.13.252.0/24 area=backbone comment="GPN" network add network=10.13.253.0/30 area=backbone comment="BJ Link" network add network=10.13.253.16/30 area=backbone comment="USA Link" |
美国：
1 2 3 4 5 6 7 8 9 10 11 | /routing ospf> interface add interface=eth2 cost=10 interface add interface=eth3 cost=10 interface add interface=gre-tunnel1 cost=100 interface add interface=gre-tunnel2 cost=100 /routing ospf> network add network=10.13.12.0/22 area=backbone comment="内网" network add network=10.13.252.0/24 area=backbone comment="GPN" network add network=10.13.253.4/30 area=backbone comment="BJ Link" network add network=10.13.253.16/30 area=backbone comment="JP Link" |
北京示例图：
四、验证
1、查看 ospf 是否启动成功：
2、down 掉 GPN 的 interface，查看 ospf 的路由是否自动切换到 GRE Tunnel:
测试线路自动切换成功，再把 GPN interface 起来之后，查看路由又自动切换到了 GPN 链路。
success！
五、优化
此文中只选了 3 个 site，备线的 ospf cost 值均设为了 100,；在实际部署时会多于 3 个，ospf 值的设置，可以设置为点对点的延迟值，使在切换到备线时 ospf 选路选的是线路延迟最低的路径，也是最优的路径。比如中日之间大网延迟大概为 55ms, 可以把 GRE 的 ospf cost 设为 55。
本文永久更新链接地址 ：http://www.linuxidc.com/Linux/2016-03/128968.htm
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