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1.实验拓扑图

 

1.各个链路所属网段和所属OSPF区域如拓扑所示

2.R4上有个回环接口,地址为10.4.4.4/32,宣告进OSPF AREA 0
3.路由收敛完毕,各个路由器路由学习正常
当网络状况正常时R2的路由表:
O     10.4.4.4 [110/3] via 192.168.25.5, 02:36:40, FastEthernet1/1   能学到10.4.4.4的路由
O     192.168.13.0/24 [110/2] via 192.168.23.3, 02:39:04, FastEthernet0/1
O     192.168.14.0/24 [110/3] via 192.168.25.5, 02:36:45, FastEthernet1/1
192.168.23.0/24 is variably subnetted, 2 subnets, 2 masks
C     192.168.23.0/24 is directly connected, FastEthernet0/1
L     192.168.23.2/32 is directly connected, FastEthernet0/1
192.168.25.0/24 is variably subnetted, 2 subnets, 2 masks
C     192.168.25.0/24 is directly connected, FastEthernet1/1
L     192.168.25.2/32 is directly connected, FastEthernet1/1
O     192.168.45.0/24 [110/2] via 192.168.25.5, 02:37:09, FastEthernet1/1
查看R2上的LSA3,也可以看到10.4.4.4的三类LSA
LS age: 1609
Options: (No TOS-capability, DC, Upward)
LS Type: Summary Links(Network)
Link State ID: 10.4.4.4 (summary Network Number)
Advertising Router: 1.1.1.1
LS Seq Number: 80000005
Checksum: 0x70AC
Length: 28
Network Mask: /32
MTID: 0         Metric: 2
当断开R4和R5之间的链路时(把R4的f0/0接口shutdown)。骨干区域被分割
查看R2的路由表,发现10.4.4.4的路由条目消失
O     192.168.13.0/24 [110/2] via 192.168.23.3, 02:51:28, FastEthernet0/1
192.168.23.0/24 is variably subnetted, 2 subnets, 2 masks
C     192.168.23.0/24 is directly connected, FastEthernet0/1
L     192.168.23.2/32 is directly connected, FastEthernet0/1
192.168.25.0/24 is variably subnetted, 2 subnets, 2 masks
C     192.168.25.0/24 is directly connected, FastEthernet1/1
L     192.168.25.2/32 is directly connected, FastEthernet1/1
O    192.168.45.0/24 [110/2] via 192.168.25.5, 02:49:33, FastEthernet1/1
查看R2的LS3,发现依然有10.4.4.4的3类LSA
LS age: 357
Options: (No TOS-capability, DC, Upward)
LS Type: Summary Links(Network)
Link State ID: 10.4.4.4 (summary Network Number)
Advertising Router: 1.1.1.1
LS Seq Number: 80000006
Checksum: 0x6EAD
Length: 28
Network Mask: /32
MTID: 0         Metric: 2
由此得出问题?
1.为什么R2上没有10.4.4.4的路由条目,但是有10.4.4.4的LSA3?
结论
1.R2自身是ABR从AREA 1收到了一个3类LSA会认为是不合法的LSA,虽然接收,但是不会计算路由
总结:ABR只会根据从骨干区域传来的LSA3计算路由,普通区域传递的LSA3,ABR虽然会接收,但是不会计算路由。
思考:
1.若R2上有个回环接口10.2.2.2/32,被宣告进OSPF的AREA 0,当R2和R5之间的链路断开时(把R2的f1/1端口shutdown),R2上是否有10.4.4.4的路由?
 
 

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