2022-07-06 网工进阶（十六）路由策略-关于双点双向重分布的解决方案

双点双向重分布的问题

上图所示的网络中左边为OSPF协议，右边为IS-IS协议。R3与R4是两边的ASBR，这种情况下存在几个问题。

一 次优路径

当R1引入外部路由R7时，R3在OSPF区域学习到的优先级为150，引入IS-IS区域后变成了15，R4因为15的优先级更优，选择了R5作为去往R1的下一跳。

解决方案：在R3与R4上修改OSPF的外部优先级小于IS-IS的优先级15。

二 路由倒灌

如果R1引入的R7开销值大于1，从IS-IS区域引入回来的路由开销值是1，会造成R2选择开销更低的路由，造成环路。

三 虚假环路

在R6上引入的R8，如果R8路由DOWN掉后，由于已经通告到了OSPF区域，又会引入回IS-IS区域，因为R6上的R8路由已经DOWN掉，R5无法将报文发回到R6，会造成虚假环路。

解决方案：使用route-policy针对外部引入的路由在R3和R4上通过加tag的方式进行过滤，同时只允许向一个方向转发。

详细命令

R3配置

[R3]ip ip-prefix 1 index 10 permit 30.1.0.0 24
[R3]ip ip-prefix 2 index 10 permit 40.1.0.0 24[R3]route-policy OI deny node 10
[R3-route-policy]if-match tag 100
[R3]route-policy OI permit node 20
[R3-route-policy]if-match ip-prefix 1
[R3-route-policy]apply tag 200
[R3]route-policy OI deny node 30
[R3-route-policy]if-match tag 500
[R3]route-policy OI permit node 40
[R3-route-policy]if-match ip-prefix 2
[R3-route-policy]apply tag 600
[R3]route-policy OI permit node 50[R3]route-policy IO deny node 10
[R3-route-policy]if-match tag 300
[R3]route-policy IO permit node 20
[R3-route-policy]if-match ip-prefix 1
[R3-route-policy]apply tag 400
[R3]route-policy IO deny node 30
[R3-route-policy]if-match tag 700
[R3]route-policy IO permit node 40
[R3-route-policy]if-match ip-prefix 2
[R3-route-policy]apply tag 800
[R3]route-policy IO permit node 50[R3-ospf-1]preference ase 14
[R3-ospf-1]import-route isis 1 type 1 route-policy OI
[R3-isis-1]import-route ospf 1 inherit-cost route-policy IO

R4配置

[R4]ip ip-prefix 1 index 10 permit 30.1.0.0 24
[R4]ip ip-prefix 2 index 10 permit 40.1.0.0 24[R4]route-policy OI deny node 10 
[R4-route-policy]if-match tag 400
[R4]route-policy OI permit node 20
[R4-route-policy]if-match ip-prefix 1
[R4-route-policy]apply tag 300
[R4]route-policy OI deny node 30
[R4-route-policy]if-match tag 800
[R4]route-policy OI permit node 40
[R4-route-policy]if-match ip-prefix 2
[R4-route-policy]apply tag 700
[R4]route-policy OI permit node 50[R4]route-policy IO deny node 10
[R4-route-policy]if-match tag 200
[R4]route-policy IO permit node 20
[R4-route-policy]if-match ip-prefix 1
[R4-route-policy]apply tag 100
[R4]route-policy IO deny node 30
[R4-route-policy]if-match tag 600
[R4]route-policy IO permit node 40
[R4-route-policy]if-match ip-prefix 2
[R4-route-policy]apply tag 500
[R4]route-policy IO permit node 50[R4-ospf-1]preference ase 14
[R4-ospf-1]import-route isis 1 type 1 route-policy OI
[R4-isis-1]import-route ospf 1 inherit-cost route-policy IO