OSPF次优路由选择的探讨

拓扑为工大瑞普ccnp实验lab17，描述如下：

R1(S1/1)--R2(S1/0)，cost=10；R1(S1/2)-framerelay-R4(S1/2)，cost=2

R2(S1/1)--R3(S1/0)，cost=2，R2(F0/0)--R4(F0/0)，cost=2

R3(S1/1)--R4(S1/0)，cost=5

R1的F0/0口连接网络192.168.1.0/24，cost=1

R1#show run

interface FastEthernet0/0
ip address 192.168.1.1 255.255.255.0

!

interface Serial1/1
description DCE,OSPF area 0,connected to R2's S1/0,ip 172.16.255.2/30
ip address 172.16.255.1 255.255.255.252
ip ospf cost 10
clock rate 9600
!
interface Serial1/2
description FrameRelay,OSPF area 0,connected to R4's S1/2,ip 172.16.255.14/30
ip address 172.16.255.13 255.255.255.252
encapsulation frame-relay
ip ospf network broadcast
ip ospf cost 2
!

router ospf 100
router-id 1.1.1.1
network 172.16.255.0 0.0.0.3 area 0
network 172.16.255.12 0.0.0.3 area 0
network 192.168.1.0 0.0.0.255 area 2
!

////////////////////

R2#show run

interface FastEthernet0/0
description OSPF area 1,connected to Network 172.16.255.16/30
ip address 172.16.255.17 255.255.255.252
ip ospf cost 2
!
interface Serial1/0
description DTE,OSPF area 0,connected to R1's S1/1,ip 172.16.255.1/30
ip address 172.16.255.2 255.255.255.252
ip ospf cost 10
!
interface Serial1/1
description DCE,OSPF area 1,connected to R3's S1/0,ip 172.16.255.6/30
ip address 172.16.255.5 255.255.255.252
ip ospf cost 2
clock rate 9600
!

router ospf 100
router-id 2.2.2.2
network 172.16.255.0 0.0.0.3 area 0
network 172.16.255.4 0.0.0.3 area 1
network 172.16.255.16 0.0.0.3 area 1
!

////////////////////

R3#show run

interface Serial1/0
description DTE,OSPF area 1,connected to R2's S1/1,ip 172.16.255.5/30
ip address 172.16.255.6 255.255.255.252
ip ospf cost 2
!
interface Serial1/1
description DCE,OSPF area 1,connected to R4's S1/0,ip 172.16.255.10/30
ip address 172.16.255.9 255.255.255.252
ip ospf cost 5
clock rate 9600
!

router ospf 100
router-id 3.3.3.3
network 172.16.255.4 0.0.0.3 area 1
network 172.16.255.8 0.0.0.3 area 1

!

////////////////////

R4#show run

interface FastEthernet0/0
description OSPF area 1,connected to Network 172.16.255.16/30
ip address 172.16.255.18 255.255.255.252
ip ospf cost 2
!
interface Serial1/0
description DTE,OSPF area 1,connected to R3's S1/1,ip 172.16.255.9/30
ip address 172.16.255.10 255.255.255.252
ip ospf cost 5
!

interface Serial1/2
description FrameRelay,OSPF area 0,connected to R1's S1/2,ip 172.16.255.13/30
ip address 172.16.255.14 255.255.255.252
encapsulation frame-relay
ip ospf network broadcast
ip ospf cost 2
!

router ospf 100
router-id 4.4.4.4
network 172.16.255.8 0.0.0.3 area 1
network 172.16.255.12 0.0.0.3 area 0
network 172.16.255.16 0.0.0.3 area 1

!

R3#traceroute 192.168.1.1

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 1 172.16.255.5 24 msec 68 msec 12 msec
 2 172.16.255.1 20 msec *  20 msec

R3#show ip route
    172.16.0.0/30 is subnetted, 5 subnets
O IA    172.16.255.0 [110/12] via 172.16.255.5, 00:00:07, Serial1/0
C       172.16.255.4 is directly connected, Serial1/0
C       172.16.255.8 is directly connected, Serial1/1
O IA    172.16.255.12 [110/6] via 172.16.255.5, 00:00:07, Serial1/0
O       172.16.255.16 [110/4] via 172.16.255.5, 00:00:07, Serial1/0
O IA 192.168.1.0/24 [110/7] via 172.16.255.5, 00:00:07, Serial1/0

从R3到达192.168.1.0/24，最少的cost为R3-R2-R4-R1=7，可实际路由为R3-R2-R1=13。为何出现这样的情况?

首先查看R3的ospf database：

R3#show ip ospf data

               Router Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum Link count
2.2.2.2         2.2.2.2         673         0x80000005 0x003C97 3
3.3.3.3         3.3.3.3         615         0x80000003 0x005569 4
4.4.4.4         4.4.4.4         628         0x80000005 0x00951E 3

               Net Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum
172.16.255.17   2.2.2.2         673         0x80000002 0x002C22