OSPF：LSA Type-5 转发地址对选路影响
1.转发地址影响选路（1）
1.1实验拓扑
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 31.31.23.100R2(config)#router ospf 1R2(config-router)#redistribute static subnetsR3做相同配置
R2#show ip ospf database external | include Forward Forward Address: 0.0.0.0 Forward Address: 0.0.0.0R2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-routersOSPF Process 1 internal Routing TableCodes: i - Intra-area route, I - Inter-area routei 2.2.2.2 via 31.31.12.2, FastEthernet0/0, ASBR, Area 0, SPF 6i 3.3.3.3 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 subnetsO E2 10.10.10.0 via 31.31.12.2, 00:03:29, FastEthernet0/0 31.0.0.0/24 is subnetted, 3 subnetsO 31.31.34.0 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 1R2(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: 15Options: (No TOS-capability, DC)LS Type: AS External LinkLink State ID: 10.10.10.0 (External Network Number )Advertising Router: 2.2.2.2LS Seq Number: 80000002Checksum: 0xB50ALength: 36Network 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 LSALS age: 359Options: (No TOS-capability, DC)LS Type: AS External LinkLink State ID: 10.10.10.0 (External Network Number )Advertising Router: 3.3.3.3LS Seq Number: 80000001Checksum: 0x2452Length: 36Network 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-routersOSPF Process 1 internal Routing TableCodes: i - Intra-area route, I - Inter-area routei 2.2.2.2 via 31.31.12.2, FastEthernet0/0, ASBR, Area 0, SPF 7i 3.3.3.3 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 subnetsO E2 10.10.10.0 via 31.31.14.4, 00:04:47, FastEthernet1/0 31.0.0.0/24 is subnetted, 4 subnetsO 31.31.23.0 via 31.31.12.2, 00:04:47, FastEthernet0/0O 31.31.34.0 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）
2.1实验拓扑
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
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 f0/0R1(config)#router ospf 1R1(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 msec2 31.31.123.1 80 msec 60 msec 88 msec3 31.31.123.2 68 msec 68 msec 120 msec4 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 subnetsO E2 5.5.5.0 via 31.31.123.1, 00:01:53, FastEthernet0/0R3上目标网络下一跳为R1
（3）完成需求（4）R1(config)#no ip route 5.5.5.0 255.255.255.0R1(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 subnetsO E2 5.5.5.0 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: 31.31.123.2R4#show ip route static 31.0.0.0/24 is subnetted, 2 subnetsS 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
R4#show ip ospf database external
OSPF Router with ID (4.4.4.4) (Process ID 1)
Type-5 AS External Link States
LS age: 308Options: (No TOS-capability, DC)LS Type: AS External LinkLink State ID: 5.5.5.0 (External Network Number )Advertising Router: 1.1.1.1LS Seq Number: 80000001Checksum: 0x428FLength: 36Network Mask: /24 Metric Type: 2 (Larger than any link state path) TOS: 0 Metric: 20 Forward Address: 31.31.123.2 External Route Tag: 0R3#show ip ospf database external
OSPF Router with ID (3.3.3.3) (Process ID 1)
Type-5 AS External Link States
Routing Bit Set on this LSALS age: 317Options: (No TOS-capability, DC)LS Type: AS External LinkLink State ID: 5.5.5.0 (External Network Number )Advertising Router: 1.1.1.1LS Seq Number: 80000001Checksum: 0x428FLength: 36Network Mask: /24 Metric Type: 2 (Larger than any link state path) TOS: 0 Metric: 20 Forward Address: 31.31.123.2 External Route Tag: 0
此时R4上，该LSA失效，Routing Bit Set on this LSA消息
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
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 1R1(config-router)#redistribute rip subnetsR1(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: 31Options: (No TOS-capability, DC)LS Type: AS External LinkLink State ID: 3.3.3.0 (External Network Number )Advertising Router: 1.1.1.1LS Seq Number: 80000001Checksum: 0x983ELength: 36Network 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 ospfO 192.168.24.0/24 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 LSALS age: 20Options: (No TOS-capability, DC)LS Type: AS External LinkLink State ID: 3.3.3.0 (External Network Number )Advertising Router: 1.1.1.1LS Seq Number: 80000002Checksum: 0x5B37Length: 36Network 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 LSALS age: 25Options: (No TOS-capability, DC)LS Type: AS External LinkLink State ID: 31.31.123.0 (External Network Number )Advertising Router: 1.1.1.1LS Seq Number: 80000001Checksum: 0x7171Length: 36Network 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 ospfO 192.168.14.0/24 via 192.168.24.4, 00:01:02, FastEthernet0/0 3.0.0.0/24 is subnetted, 1 subnetsO E2 3.3.3.0 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 ospfO 192.168.14.0/24 via 192.168.24.4, 00:10:48, FastEthernet0/0 3.0.0.0/24 is subnetted, 1 subnetsO E2 3.3.3.0 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
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