[原文档]Forwarding Address对LSA5的路径选择影响
[color=#990000]介绍：[/color][size=5] [/size][size=3]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。 [/size]
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[size=3]转发地址如何影响LSA5选路:[/size][/color]
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[size=3]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[color=#ff0000]172.16.3.x。[/color][/size]
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[size=3][img=346,344]http://www.one-tom.com/bbs/uploads/20070415_044149_type5_lsa_01.gif[/img][/size]
[size=3][color=#000000]注意：R3/R4不将[color=#ff0000]172.16.3.x/24[/color]接口运行OSPF[/color][/size]
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[table=60%][tr]Router 3
[/tr][tr][td]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[/td][/tr][/table]
[table=60%][tr]Router 4
[/tr][tr][td]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[/td][/tr][/table]
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[size=3]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[/size]
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[table=60%][tr]Router 5
[/tr][tr][td]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: [color=#ff0000]0.0.0.0[/color] 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:[color=#ff0000] 0.0.0.0[/color] External Route Tag: 0[/td][/tr][/table]
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[size=3]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：[/size]
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[table=60%][tr]Router 5
[/tr][tr][td]router-5# show ip ospf border-routersOSPF Process 7 internal Routing TableCodes: i - Intra-area route, I - Inter-area routei 172.16.7.4 [74] via 172.16.6.6, Ethernet0, ASBR, Area 0, SPF 15i 172.16.3.3 [64] via 172.16.2.3, Serial0, ASBR, Area 0, SPF 15router-5#[/td][/tr][/table]
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[table=60%][tr]Router 5
[/tr][tr][td]router-5# [url=http://www.cisco.com/en/US/products/sw/iosswrel/ps5207/products_command_reference_chapter09186a00801d2de0.html#wp1138038][font=新宋体][size=3][color=#0000ff]show ip route[/color][/size][/font][/url]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 routeGateway of last resort is not setO E2 200.200.200.0/24 [110/20] via [color=#ff0000]172.16.2.3[/color], 19:59:25, Serial0 172.16.0.0/24 is subnetted, 3 subnetsC 172.16.6.0 is directly connected, Ethernet0O 172.16.7.0 [110/74] via 172.16.6.6, 19:59:25, Ethernet0C 172.16.2.0 is directly connected, Serial0router-5#[/td][/tr][/table]
[size=3][color=#990000]到这一步，看不出有任何异常。[/color][/size]
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[size=3][color=#990000]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。[/color][/size]
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[size=3][color=#990000][table=60%][tr][color=#000000]Router 3[/color]
[/tr][tr][td][color=#000000]interface Ethernet0 ip address 172.16.3.3 255.255.255.0 ip ospf cost 15!router ospf 7[color=#ff0000][b] network 172.16.3.0 0.0.0.255 area 0[/b][/color]!增加以上粗体部分命令1条[/color][/td][/tr][/table]
在R5查看收到的LSA 5，较之前发生一些变化：
[table=60%][tr][color=#000000]Router 5[/color]
[/tr][tr][td][color=#000000]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 [color=#ff0000][b] Forward Address: 172.16.3.8[/b][/color] 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: 0router-5#[/color][/td][/tr][/table]
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[color=#990000]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。
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[color=#990000]现在查看到ASBR的开销，和之前一致，到R4的开销较大，为74[/color]
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[color=#ff0000][table=60%][tr][color=#000000]Router 5[/color]
[/tr][tr][td][color=#000000]router-5# show ip ospf border-routersOSPF Process 7 internal Routing TableCodes: i - Intra-area route, I - Inter-area routei 172.16.7.4 [74] via 172.16.6.6, Ethernet0, ASBR, Area 0, SPF 15i 172.16.3.3 [64] via 172.16.2.3, Serial0, ASBR, Area 0, SPF 15router-5#[/color][/td][/tr][/table][/color]
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[color=#990000]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[/color]
[color=#ff0000][table=60%][tr][color=#000000]Router 5[/color]
[/tr][tr][td][color=#000000]router-5# show ip routeCodes: 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 routeGateway of last resort is not setO E2 [color=#ff0000]200.200.200.0/24 [110/20] via 172.16.6.6[/color], 00:35:14, Ethernet0 172.16.0.0/24 is subnetted, 4 subnetsC 172.16.6.0 is directly connected, Ethernet0O 172.16.7.0 [110/74] via 172.16.6.6, 00:35:14, Ethernet0C 172.16.2.0 is directly connected, Serial0O 172.16.3.0 [110/79] via 172.16.2.3, 00:35:14, Serial0[/color][/td][/tr][/table]
[color=#990000]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：[/color]
[table=60%][tr][color=#000000]Router 5[/color]
[/tr][tr][td][color=#000000]router-5# show ip route 172.16.3.8Routing 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 [color=#ff0000]Route metric is 79[/color], traffic share count is 1[/color][/td][/tr][/table]
[/color][color=#ff0000][/color][color=#990000][/color]
[color=#990000]因此到R4的转发地址0.0.0.0的开销74反而显小了。[/color]
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[color=#990000]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[/color]
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