OSPF：LSA Type-5 转发地址对选路影响（实验）
| 1.转发地址影响选路（1） |
| 1.1实验拓扑
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| 1.2实验需求 （1）按拓扑完成基本配置 （2）R2、R3上写一条相同的静态路由，ip route 10.10.10.0 255.255.255.0 31.31.23.100 （3）重分发静态路由进入OSPF进程，观察LSA Type 5以及R1的选路情况 （4）R2将与R3之间网络宣告进OSPF，R3不作改变，观察LSA Type 5以及R1的选路情况 |
| 1.3实验步骤 （1）静态路由 R2(config)#ip route 10.10.10.0 255.255.255.0 31.31.23.100 R2(config)#router ospf 1 R2(config-router)#redistribute static subnets R3做相同配置 R2#show ip ospf database external | include Forward Forward Address: 0.0.0.0 Forward Address: 0.0.0.0 R2上Forward Address有两个输出，这是因为该网络中有两台ASBR携带外部路由，此时转发地址为0.0.0.0
（2）R1选路 R1#show ip ospf border-routers OSPF Process 1 internal Routing Table Codes: i - Intra-area route, I - Inter-area route i 2.2.2.2 [1] via 31.31.12.2, FastEthernet0/0, ASBR, Area 0, SPF 6 i 3.3.3.3 [2] via 31.31.14.4, FastEthernet1/0, ASBR, Area 0, SPF 6 由于通告外部路由的是ASBR，且此时转发地址为0.0.0.0，外部路由类型为Type-2 由上输出可见，R1去往ASBR R2的开销更小
R1#show ip route ospf 10.0.0.0/24 is subnetted, 1 subnets O E2 10.10.10.0 [110/20] via 31.31.12.2, 00:03:29, FastEthernet0/0 31.0.0.0/24 is subnetted, 3 subnets O 31.31.34.0 [110/2] via 31.31.14.4, 00:03:29, FastEthernet1/0 可见此时R1选择R2去往外部网络
（3）R2、R3直连网络参与R2 OSPF进程 R2(config)#router ospf 1 R2(config-router)#network 31.31.23.2 0.0.0.0 a 0
R2#show ip ospf database external
OSPF Router with ID (2.2.2.2) (Process ID 1)
Type-5 AS External Link States
LS age: 15 Options: (No TOS-capability, DC) LS Type: AS External Link Link State ID: 10.10.10.0 (External Network Number ) Advertising Router: 2.2.2.2 LS Seq Number: 80000002 Checksum: 0xB50A Length: 36 Network Mask: /24 Metric Type: 2 (Larger than any link state path) TOS: 0 Metric: 20 Forward Address: 31.31.23.100 External Route Tag: 0
Routing Bit Set on this LSA LS age: 359 Options: (No TOS-capability, DC) LS Type: AS External Link Link State ID: 10.10.10.0 (External Network Number ) Advertising Router: 3.3.3.3 LS Seq Number: 80000001 Checksum: 0x2452 Length: 36 Network Mask: /24 Metric Type: 2 (Larger than any link state path) TOS: 0 Metric: 20 Forward Address: 0.0.0.0 External Route Tag: 0 根据转发地址判定规则，此时由R2通告的路由，其转发地址为静态路由中的下一跳
（4）R1选路 R1#show ip ospf border-routers OSPF Process 1 internal Routing Table Codes: i - Intra-area route, I - Inter-area route i 2.2.2.2 [1] via 31.31.12.2, FastEthernet0/0, ASBR, Area 0, SPF 7 i 3.3.3.3 [2] via 31.31.14.4, FastEthernet1/0, ASBR, Area 0, SPF 7 此时去往ASBR的开销并没有发生变化
R1#show ip route ospf 10.0.0.0/24 is subnetted, 1 subnets O E2 10.10.10.0 [110/20] via 31.31.14.4, 00:04:47, FastEthernet1/0 31.0.0.0/24 is subnetted, 4 subnets O 31.31.23.0 [110/11] via 31.31.12.2, 00:04:47, FastEthernet0/0 O 31.31.34.0 [110/2] via 31.31.14.4, 00:04:47, FastEthernet1/0 此时R1去往外部网络选择R4作为下一跳 |
| 1.4实验结论 当出现转发地址时，当前路由器将以去往转发地址的开销作为参考依据而不再是去往ASBR的开销 |
| 2.转发地址影响选路（2） |
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| 2.1实验拓扑
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| 2.2实验需求 （1）按拓扑完成基本配置 （2）R2上写去往5.5.5.0的静态路由指向R5，R5上写缺省路由指向R2以保证连通性 （3）R1上写一条去往5.5.5.0的静态路由，只指定出口，重分发该路由进入OSPF进程，观察实验现象 （4）静态路由增加下一跳为R2，观察实验现象 （5）R4上写一条静态路由，ip route 31.31.123.0 255.255.255.0 null 0，观察实验现象 |
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| 2.3实验步骤 （1）完成需求（2） R2(config)#ip route 5.5.5.0 255.255.255.0 31.31.25.5
R5(config)#ip route 0.0.0.0 0.0.0.0 31.31.25.2
（2）完成需求（3） R1(config)#ip route 5.5.5.0 255.255.255.0 f0/0 R1(config)#router ospf 1 R1(config-router)#redistribute static subnets
R1#show ip ospf database external | include Forward Forward Address: 0.0.0.0
R4#traceroute 5.5.5.5
Type escape sequence to abort. Tracing the route to 5.5.5.5
1 31.31.34.3 60 msec 32 msec 44 msec 2 31.31.123.1 80 msec 60 msec 88 msec 3 31.31.123.2 68 msec 68 msec 120 msec 4 31.31.25.5 124 msec * 76 msec 此时数据包走了一条次优路径
R3#show ip route ospf 5.0.0.0/24 is subnetted, 1 subnets O E2 5.5.5.0 [110/20] via 31.31.123.1, 00:01:53, FastEthernet0/0 R3上目标网络下一跳为R1
（3）完成需求（4） R1(config)#no ip route 5.5.5.0 255.255.255.0 R1(config)#ip route 5.5.5.0 255.255.255.0 31.31.123.
R1#show ip ospf database external | in Forward Forward Address: 31.31.123.2 此时产生了转发地址
R3#show ip route ospf 5.0.0.0/24 is subnetted, 1 subnets O E2 5.5.5.0 [110/20] via 31.31.123.2, 00:00:57, FastEthernet0/0 此时R3将转发地址作为下一跳
（4）完成需求（5） R4(config)#ip route 31.31.123.0 255.255.255.0 null0
R4#show ip ospf database external | in Forward Forward Address: 31.31.123.2 R4#show ip route static 31.0.0.0/24 is subnetted, 2 subnets S 31.31.123.0 is directly connected, Null0 在R4上，此时转发地址不可达
R4#show ip route ospf 输出为空
此时R4上，该LSA失效，Routing Bit Set on this LSA消息 |
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| 2.4实验结论 （1）转发地址可以避免次优路径问题 （2）当转发地址在本地失效时，对应外部网络也将无效 |
| 3.下游设备 |
| 3.1实验拓扑
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| 3.2实验需求 （1）按拓扑完成基本配置 （2）R1上将RIP路由重分发进OSPF，并将连接RIP域的接口网络宣告进OSPF进程 （3）R2上观察LSA Type 5中的转发地址以及路由学习情况 （4）要求此时R2走最优路径，抵达R3的环回口网络 |
| 3.3实验步骤 （1）完成需求（2） R1(config)#router ospf 1 R1(config-router)#redistribute rip subnets R1(config-router)#network 31.31.123.1 0.0.0.0 a 0
（2）实验现象 R2#show ip ospf database external
OSPF Router with ID (2.2.2.2) (Process ID 1)
Type-5 AS External Link States
LS age: 31 Options: (No TOS-capability, DC) LS Type: AS External Link Link State ID: 3.3.3.0 (External Network Number ) Advertising Router: 1.1.1.1 LS Seq Number: 80000001 Checksum: 0x983E Length: 36 Network Mask: /24 Metric Type: 2 (Larger than any link state path) TOS: 0 Metric: 20 Forward Address: 31.31.123.3 External Route Tag: 0 此时出现转发地址，为31.31.123.3，但是路由比特位未出现，表明该LSA无效
R1#show ip route ospf O 192.168.24.0/24 [110/2] via 192.168.14.4, 00:00:04, FastEthernet0/0 不学习远程网络
（3）去除转发地址 R1(config-router)#no network 31.31.123.1 0.0.0.0 a 0
R2#show ip ospf database external
OSPF Router with ID (2.2.2.2) (Process ID 1)
Type-5 AS External Link States
Routing Bit Set on this LSA LS age: 20 Options: (No TOS-capability, DC) LS Type: AS External Link Link State ID: 3.3.3.0 (External Network Number ) Advertising Router: 1.1.1.1 LS Seq Number: 80000002 Checksum: 0x5B37 Length: 36 Network Mask: /24 Metric Type: 2 (Larger than any link state path) TOS: 0 Metric: 20 Forward Address: 0.0.0.0 External Route Tag: 0
Routing Bit Set on this LSA LS age: 25 Options: (No TOS-capability, DC) LS Type: AS External Link Link State ID: 31.31.123.0 (External Network Number ) Advertising Router: 1.1.1.1 LS Seq Number: 80000001 Checksum: 0x7171 Length: 36 Network Mask: /24 Metric Type: 2 (Larger than any link state path) TOS: 0 Metric: 20 Forward Address: 0.0.0.0 External Route Tag: 0 此时转发地址消失，且还通告了R1、R2、R3间的直连网络
R2#show ip route ospf O 192.168.14.0/24 [110/2] via 192.168.24.4, 00:01:02, FastEthernet0/0 3.0.0.0/24 is subnetted, 1 subnets O E2 3.3.3.0 [110/20] via 192.168.24.4, 00:01:02, FastEthernet0/0 出现远端网络（此时从宏观上来看是一个次优路径）
（4）解决出现的问题 R1(config-router)#network 31.31.123.1 0.0.0.0 a 0 //产生转发地址
R2(config-router)#network 31.31.123.2 0.0.0.0 a 0 //让转发地址所属网络加入到OSPF进程中 R2(config-router)#passive-interface f1/0 //防止R1与R2在RIP域中建立OSPF邻接关系
R2#show ip route ospf O 192.168.14.0/24 [110/2] via 192.168.24.4, 00:10:48, FastEthernet0/0 3.0.0.0/24 is subnetted, 1 subnets O E2 3.3.3.0 [110/20] via 31.31.123.3, 00:00:32, FastEthernet1/0 注意： 这里如果配置R1的接口fa1/0为被动接口，则产生的LSA Type 5其Forward Address将为0.0.0.0 |
| 3.4实验结论 当转发地址非0.0.0.0时，判断转发地址所属子网是否存在于本地OSPF进程中，如果不存在，则判定该LSA无效 如果是转发地址所属子网是下游设备的直连网络，只要参与OSPF进程，即便被配置为Passive，也判定有效 |
本文出自 “Thely” 博客，请务必保留此出处http://thely.blog.51cto.com/2695427/1014735