介绍：
本试验测试OSPF路由器收到某个外部网络的LSA 5通告时，如果转发地址不为0.0.0.0，对路径选择的影响。
 
 
转发地址如何影响LSA5选路:
以下拓扑，除了R8，均在Area 0；R3\R4重发布进OSPF的静态路由的下一跳地址为172.16.3.x。
 
注意：R3/R4不将172.16.3.x/24接口运行OSPF
 
Router 3 
interface Ethernet0
 ip address 172.16.3.3 255.255.255.0
 ip ospf cost 15
!
interface Serial0
 ip address 172.16.2.3 255.255.255.0
!
router ospf 7
 redistribute static subnets
 network 172.16.2.0 0.0.0.255 area 0
!
ip route 200.200.200.0 255.255.255.0 172.16.3.8
 
Router 4 
interface Ethernet0
 ip address 172.16.3.4 255.255.255.0
!
interface Serial0
 ip address 172.16.7.4 255.255.255.0
!
router ospf 7
 redistribute static subnets
 network 172.16.7.0 0.0.0.255 area 0
!
ip route 200.200.200.0 255.255.255.0 172.16.3.8
 
因为R3/R4对应静态路由的下一跳接口不运行OSPF，因此产生的LSA 5均将Forwarding Address表示为0.0.0.0
 
Router 5 
router-5# show ip ospf database external
            OSPF Router with ID (172.16.6.5) (Process ID 7)
                Type-5 AS External Link States
  Routing Bit Set on this LSA
  LS age: 1514
  Options: (No TOS-capability, DC)
  LS Type: AS External Link
  Link State ID: 200.200.200.0 (External Network Number )
  Advertising Router: 172.16.3.3
  LS Seq Number: 80000030
  Checksum: 0x93C0
  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: 1922
  Options: (No TOS-capability, DC)
  LS Type: AS External Link
  Link State ID: 200.200.200.0 (External Network Number )
  Advertising Router: 172.16.7.4
  LS Seq Number: 80000027
  Checksum: 0x83D4
  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
 
R5选择到达外部路由，根据到最近ASBR原则，会选择从R3访问外部路由：
 
Router 5 
router-5# show ip ospf border-routers
OSPF Process 7 internal Routing Table
Codes: i - Intra-area route, I - Inter-area route
i 172.16.7.4 [74] via 172.16.6.6, Ethernet0, ASBR, Area 0, SPF 15
i 172.16.3.3 [64] via 172.16.2.3, Serial0, ASBR, Area 0, SPF 15
router-5#
 
Router 5 
router-5# show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
       i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area
       * - candidate default, U - per-user static route, o - ODR
       P - periodic downloaded static route
Gateway of last resort is not set
O E2 200.200.200.0/24 [110/20] via 172.16.2.3, 19:59:25, Serial0
     172.16.0.0/24 is subnetted, 3 subnets
C       172.16.6.0 is directly connected, Ethernet0
O       172.16.7.0 [110/74] via 172.16.6.6, 19:59:25, Ethernet0
C       172.16.2.0 is directly connected, Serial0
router-5#
 
到这一步，看不出有任何异常。
 
接下来，将R3的E0口运行OSPF，会导致R3发布出来的LSA 5携带不为0.0.0.0的转发地址。
 
Router 3 
interface Ethernet0
 ip address 172.16.3.3 255.255.255.0
 ip ospf cost 15
!
router ospf 7
 network 172.16.3.0 0.0.0.255 area 0
!
增加以上粗体部分命令1条
 
在R5查看收到的LSA 5，较之前发生一些变化：
 
Router 5 
router-5# show ip ospf database external
            OSPF Router with ID (172.16.6.5) (Process ID 7)
                Type-5 AS External Link States
  Routing Bit Set on this LSA
  LS age: 270
  Options: (No TOS-capability, DC)
  LS Type: AS External Link
  Link State ID: 200.200.200.0 (External Network Number )
  Advertising Router: 172.16.3.3
  LS Seq Number: 80000033
  Checksum: 0x5138
  Length: 36
  Network Mask: /24
        Metric Type: 2 (Larger than any link state path)
        TOS: 0
        Metric: 20
        Forward Address: 172.16.3.8
        External Route Tag: 0
  Routing Bit Set on this LSA
  LS age: 258
  Options: (No TOS-capability, DC)
  LS Type: AS External Link
  Link State ID: 200.200.200.0 (External Network Number )
  Advertising Router: 172.16.7.4
  LS Seq Number: 80000029
  Checksum: 0x7FD6
  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
router-5#
 
发生变化的根本原因是，重发布的静态路由的下一跳地址172.16.3.8所在网段172.16.3.x/24，R3已经将该网段接口运行了OSPF。
现在查看到ASBR的开销，和之前一致，到R4的开销较大，为74
 
Router 5 
router-5# show ip ospf border-routers
OSPF Process 7 internal Routing Table
Codes: i - Intra-area route, I - Inter-area route
i 172.16.7.4 [74] via 172.16.6.6, Ethernet0, ASBR, Area 0, SPF 15
i 172.16.3.3 [64] via 172.16.2.3, Serial0, ASBR, Area 0, SPF 15
router-5#
 
但如果再进一步查看路由表，则会发现路由器选择到“开销较大"的ASBR
 
Router 5 
router-5# show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
       i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area
       * - candidate default, U - per-user static route, o - ODR
       P - periodic downloaded static route
Gateway of last resort is not set
O E2 200.200.200.0/24 [110/20] via 172.16.6.6, 00:35:14, Ethernet0
     172.16.0.0/24 is subnetted, 4 subnets
C       172.16.6.0 is directly connected, Ethernet0
O       172.16.7.0 [110/74] via 172.16.6.6, 00:35:14, Ethernet0
C       172.16.2.0 is directly connected, Serial0
O       172.16.3.0 [110/79] via 172.16.2.3, 00:35:14, Serial0
 
出现这种情况的根本原因，在于开销一致的情况下(均为OE2,开销=20），OSPF先对比到LSA 5转发地址的开销，而到R3发布的LSA转发地址开销为79：
 
Router 5 
router-5# show ip route 172.16.3.8
Routing entry for 172.16.3.0/24
  Known via "ospf 7", distance 110, metric 79, type intra area
  Last update from 172.16.2.3 on Serial0, 00:30:49 ago
  Routing Descriptor Blocks:
  * 172.16.2.3, from 172.16.3.3, 00:30:49 ago, via Serial0
      Route metric is 79, traffic share count is 1
 
因此到R4的转发地址0.0.0.0的开销74反而显小了。
 
至于0.0.0.0代表什么地址呢？这个地址表示路由器本身，取到这个路由器最近的开销，show ip ospf border-routers，正是查看到这个路由本身最近的开销。而OSPF在进行选路计算时，如果有更详细的信息，转发地址，则直接针对这部分信息计算。
 
 
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