The discussion of OSPF suboptimal route selection

The topology is Darip CCNP experiment Lab17, described as follows:

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 f0/0 Port Connection Network 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
!

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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
!

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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

!

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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 msec msec msec
2 172.16.255.1 msec * 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

From R3 to 192.168.1.0/24, the minimum cost is r3-r2-r4-r1=7, which can be actually routed to r3-r2-r1=13. Why is there such a situation?

First look at the OSPF database for R3:

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