关于OSPF 5 类LSA转发地址分析
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关于OSPF 5 类LSA转发地址分析：
如上图所示：
本次分析主要是分析何时5类的转发地址为0，何时又不为0！
在RT1上设有缺省路由，并下发给邻居。
（1）、将RT1上的F2/0口通告进入OSPF进程，但是没有设为被动接口。
RT1#show ip ospf database external
OSPF Router with ID (192.168.0.1) (Process ID 1)
Type-5 AS External Link States
LS age: 5
Options: (No TOS-capability, DC)
LS Type: AS External Link
Link State ID: 0.0.0.0 (External Network Number )
Advertising Router: 192.168.0.1
LS Seq Number: 80000002
Checksum: 0xFDD3
Length: 36
Network Mask: /0
Metric Type: 2 (Larger than any link state path)
TOS: 0
Metric: 1
Forward Address: 192.168.2.10
External Route Tag: 1
RT3#show ip ospf database external
OSPF Router with ID (192.168.0.3) (Process ID 1)
Type-5 AS External Link States
Routing Bit Set on this LSA
LS age: 118
Options: (No TOS-capability, DC)
LS Type: AS External Link
Link State ID: 0.0.0.0 (External Network Number )
Advertising Router: 192.168.0.1
LS Seq Number: 80000002
Checksum: 0xFDD3
Length: 36
Network Mask: /0
Metric Type: 2 (Larger than any link state path)
TOS: 0
Metric: 1
Forward Address: 192.168.2.10
External Route Tag: 1
此时的转发地址为RT1缺省路由的下一跳。
（2）将RT1上的F2/0口设为被动接口。
RT1#show ip ospf database external
OSPF Router with ID (192.168.0.1) (Process ID 1)
Type-5 AS External Link States
LS age: 3
Options: (No TOS-capability, DC)
LS Type: AS External Link
Link State ID: 0.0.0.0 (External Network Number )
Advertising Router: 192.168.0.1
LS Seq Number: 80000003
Checksum: 0x2521
Length: 36
Network Mask: /0
Metric Type: 2 (Larger than any link state path)
TOS: 0
Metric: 1
Forward Address: 0.0.0.0
External Route Tag: 1
此时的转发地址为0.0.0.0。
（3）、如下图所示：
RT6上对于NSSA区域内的LSA7进行转换成LSA5,接下来我们查看一下，此时的LSA5转发地址为多少呢？
RT6#show ip ospf database external
OSPF Router with ID (6.6.6.6) (Process ID 1)
Type-5 AS External Link States
LS age: 120
Options: (No TOS-capability, DC)
LS Type: AS External Link
Link State ID: 4.4.4.4 (External Network Number )
Advertising Router: 6.6.6.6
LS Seq Number: 80000001
Checksum: 0x38D6
Length: 36
Network Mask: /32
Metric Type: 1 (Comparable directly to link state metric)
TOS: 0
Metric: 1000
Forward Address: 10.2.5.1
External Route Tag: 0
LS age: 127
Options: (No TOS-capability, DC)
LS Type: AS External Link
Link State ID: 172.16.4.0 (External Network Number )
Advertising Router: 6.6.6.6
LS Seq Number: 80000001
Checksum: 0x3F1F
Length: 36
Network Mask: /24
Metric Type: 1 (Comparable directly to link state metric)
TOS: 0
Metric: 1000
Forward Address: 10.2.5.1
External Route Tag: 0
RT6#
此处的RT2的lo0并没有通告进ospf,接下来将lo0通告进来看看有什么样的变化！
RT6#show ip ospf database external
OSPF Router with ID (6.6.6.6) (Process ID 1)
Type-5 AS External Link States
LS age: 19
Options: (No TOS-capability, DC)
LS Type: AS External Link
Link State ID: 4.4.4.4 (External Network Number )
Advertising Router: 6.6.6.6
LS Seq Number: 80000001
Checksum: 0xC652
Length: 36
Network Mask: /32
Metric Type: 1 (Comparable directly to link state metric)
TOS: 0
Metric: 1000
Forward Address: 2.2.2.2
External Route Tag: 0
LS age: 22
Options: (No TOS-capability, DC)
LS Type: AS External Link
Link State ID: 172.16.4.0 (External Network Number )
Advertising Router: 6.6.6.6
LS Seq Number: 80000001
Checksum: 0xCD9A
Length: 36
Network Mask: /24
Metric Type: 1 (Comparable directly to link state metric)
TOS: 0
Metric: 1000
Forward Address: 2.2.2.2
External Route Tag: 0
RT6#
此时发现转发地址都是2.2.2.2,再看一下LSA7,是否有何相似之处：
RT6#show ip ospf database nssa-external
OSPF Router with ID (6.6.6.6) (Process ID 1)
Type-7 AS External Link States (Area 2)
Routing Bit Set on this LSA
LS age: 35
Options: (No TOS-capability, Type 7/5 translation, DC)
LS Type: AS External Link
Link State ID: 4.4.4.4 (External Network Number )
Advertising Router: 2.2.2.2
LS Seq Number: 80000001
Checksum: 0xAA74
Length: 36
Network Mask: /32
Metric Type: 1 (Comparable directly to link state metric)
TOS: 0
Metric: 1000
Forward Address: 2.2.2.2
External Route Tag: 0
Routing Bit Set on this LSA
LS age: 37
Options: (No TOS-capability, Type 7/5 translation, DC)
