当R1上没有将172.16.1.0网段发布到OSPF中时(no network 172.16.1.0 0.0.0.255 area 0)在R4上查看路由:
R4#show ip route ospf
O E1 10.2.0.0/24 [110/1010] via 10.0.1.2, 00:03:21, Ethernet0/0
O E1 10.3.0.0/24 [110/1010] via 10.0.1.2, 00:03:21, Ethernet0/0
查看5类LSA:
LS age: 271
Options: (No TOS-capability, DC)
LS Type: AS External Link
Link State ID: 10.2.0.0 (External Network Number )
Advertising Router: 1.1.1.1
LS Seq Number: 80000003
Checksum: 0x6610
Length: 36
Network Mask: /24
Metric Type: 1 (Comparable directly to link state metric)
TOS: 0
Metric: 1000
Forward Address: 0.0.0.0 //转发地址为0.0.0.0
External Route Tag: 0
由此可发现但转发地址为0.0.0.0时,外部路由开销的计算就是到达ASBR的开销+外部开销=10+1000=1010
当R1上将172.16.1.0网段发布到OSPF中时(network 172.16.1.0 0.0.0.255 area 0)在R4上查看路由:
R4#show ip route ospf
O E1 10.2.0.0/24 [110/1020] via 10.0.1.2, 00:00:16, Ethernet0/0
O E1 10.3.0.0/24 [110/1020] via 10.0.1.2, 00:00:16, Ethernet0/0
LS age: 172
Options: (No TOS-capability, DC)
LS Type: AS External Link
Link State ID: 10.2.0.0 (External Network Number )
Advertising Router: 1.1.1.1
LS Seq Number: 80000004
Checksum: 0x6610
Length: 36
Network Mask: /24
Metric Type: 1 (Comparable directly to link state metric)
TOS: 0
Metric: 1000
Forward Address: 172.16.1.2 ///转发地址为直连外部路由下跳接口IP External Route Tag: 0
LS age: 2796
Options: (No TOS-capability, DC)
LS Type: AS External Link
Link State ID: 10.3.0.0 (External Network Number )
Advertising Router: 2.2.2.2
LS Seq Number: 80000002
Checksum: 0x4E24
Length: 36
Network Mask: /24
Metric Type: 1 (Comparable directly to link state metric)
TOS: 0
Metric: 1000
Forward Address: 172.16.1.3 //转发地址为直连外部路由下跳接口IP
External Route Tag: 0Link connected to: a Stub Network
1类LSA:
(Link ID) Network/subnet number: 172.16.1.0
(Link Data) Network Mask: 255.255.255.0
Number of TOS metrics: 0
TOS 0 Metrics: 10
由此可发现但转发地址不为0时,外部路由开销的计算就是到达转发地址的开销+外部开销=10+10+1000=1020
在R1上配置被动接口(R1(config-router)#passive-interface e0/1):
Routing Bit Set on this LSA
LS age: 19
Options: (No TOS-capability, DC)
LS Type: AS External Link
Link State ID: 10.2.0.0 (External Network Number )
Advertising Router: 1.1.1.1
LS Seq Number: 80000005
Checksum: 0xF26
Length: 36
Network Mask: /24
Metric Type: 1 (Comparable directly to link state metric)
TOS: 0Metric: 1000
Forward Address: 0.0.0.0 //5类LSA的转发地址全变为0.0.0.0
External Route Tag: 0
大家思考如果是七类LSA的转发地址会怎么样?
区域2是NSSA区域,RIP通过重发布到OSPF中,那么RT2产生的七类LSA的转发地址是怎么样的,经RT6做7转5后,转发地址会有什么变化?
如果172.16.24.0/30的网段没有发布到OSPF中,7类LSA的转发地址会像5类那样变为0.0.0.0吗?如果是的话,在RT6上作7转5后,转发地址还是0.0.0.0,那这表示是到达RT6还是RT2这个ASBR呢?所以7类LSA的转发地址不可能为0.0.0.0,7类转5时,不会改变转发地址。
当172.16.24.0/24没有发布到OSPF中,转发地址为Router-id.
LS age: 324
Options: (No TOS-capability, Type 7/5 translation, DC)
LS Type: AS External Link
Link State ID: 172.16.4.0 (External Network Number )
Advertising Router: 2.2.2.2
LS Seq Number: 80000001
Checksum: 0xB1BC
Length: 36
Network Mask: /24
Metric Type: 1 (Comparable directly to link state metric)
TOS: 0
Metric: 1000
Forward Address: 2.2.2.2 //转发地址不为0
External Route Tag: 0
我重新指定Router-id,重启OSPF进程:
R2(config-router)#do show ip ospf
Routing Process "ospf 1" with ID 10.10.10.10 //router-id为10.10.10.10
Supports only single TOS(TOS0) routes
Supports opaque LSA
Link State ID: 172.16.4.0 (External Network Number )
Advertising Router: 2.2.2.2
LS Seq Number: 80000002
Checksum: 0x410C
Length: 36
Network Mask: /24
Metric Type: 1 (Comparable directly to link state metric)
TOS: 0
Metric: 1000
Forward Address: 10.10.10.10 //转发地址为10.10.10.10
External Route Tag: 0
在RT6上经过7类转5类的LSA的转发地址不会改变:
LS age: 12
Options: (No TOS-capability, DC)
LS Type: AS External Link //5类LSA
Link State ID: 172.16.4.0 (External Network Number )
Advertising Router: 6.6.6.6 //通告路由器
LS Seq Number: 80000001
Checksum: 0x5FE8
Length: 36
Network Mask: /24
Metric Type: 1 (Comparable directly to link state metric)
TOS: 0
Metric: 1000
Forward Address: 10.10.10.10 //转发地址不变
其它的跟5类LSA的转发地址一样,大家可以去验证一下,我就不写了!
最后总结一下:
5类LSA若同时满足以下三个条件:
a)引入的这条外部路由,其对应的出接口启用了OSPF
b)引入的这条外部路由,其对应的出接口未设置为passive-interface
c)引入的这条外部路由,其对应的出接口的OSPF网络类型为broadcast
则产生的Type 5 LSA,其FA地址等于该引入的外部路由的下一条地址,反之为0.0.0.0(ASBR)
7类LSA的转发地址不会为0.0.0.0,满足以上三个条件为外部路由的下一条地址,反之为Router-id。