Redistribution issues


对于多路由AS之间的具有多个边界路由器的充分发,要小心因两边AS的AD不同而带来的路由环路。当边界上有两个或以上路由器时,做路由重分发,若从AD低的一端发布到高AD的一端,可以放心操作,不会出现环路;但是若从高AD向低AD一方发布,就要在所有边界路由器的低AD一端接口上做distribute-list 列表,过了掉那个高AD的路由信息。

此例子中,145.10.128.0/28 & 150.10.0.0/16从自己的RIP发布到OSPF中时,因为只有一个边界路由器R6,所以不存在环路风险,不需要考量AD差。当OSPF收到这个外部重分发的路由信息时,它将此信息标记为外部路由信息并设为外部AD110(可改),当发布到EIGRP时,因EIGRP的外部重分发路由信息具有外部路由AD170,高于OSPF的这个外部AD110,所以不会发生分发环路。当EIGRP将此外部AD170路由发布到RIP的AD120区域时,就会发生环路,此时就需要在路由AD低的一端(RIP)的路由器接口上设DISTRIBUTE-LIST过滤此路由信息,强制这个低AD区域不接受这些重分发信息。

 

145.10.192.0/18:

Route filters are required at the interfaces F0/1 of R1 and R2.

145.10.0.0/17:

No need of route filter.

145.10.128.0/18 and 150.10.0.0/16:

Route filters are required at the interface F0/1 of R3 and R4.

0.0.0.0/0:

Route filters are required at the interfaces F0/1 of R1, R2, R3 and R4.

Configurations at R1 and R2                                            Configurations at R3 and R4

access-list 10 deny 145.10.192.0 0.0.63.255                access-list 10 deny 145.10.128.0 0.0.63.255

access-list 10 deny 0.0.0.0 0.0.0.0                               access-list 10 deny 150.10.0.0 0.0.255.255

access-list 10 permit any any                                         access-list 10 deny 0.0.0.0 0.0.0.0

router ospf 100                                                                 access-list 10 permit any any

…                                                                                          router rip

distribute-list 10 f0/1 in                                                   …distribute-list 10 f0/1 in

Configuration at R5                                                        Configuration at R6

ip route 145.10.0.0 255.255.0.0 null0                        ip route 145.10.0.0 255.255.0.0 null0

router rip                                                                          router rip

…                                                                                          …

redistribute static …                                                      redistribute static …

The reason of why using null0 in R5 and R6 to propagate summary route.

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