Routing path between OSPF areas(about LSA Type3)

[parazhang]

Following is my ospf networking topo.
R8
/ |
area2 R7 |10.0.0.12/30
/ |
/ |
------------------R1 R3---------------
/ |____|
area0 /
R2



In my lab,I ran R1,R2,R3 in OSPF area 0
R1,R3,R7,R8 in OSPF area 2,
the line between R1 and R3 is belong to area0

all routers connect together using same bandwidth/speed serial lines,R1 and R3 connect both area0 and area2.

From the topo,we can see intuitively that from R2 to destination net 10.0.0.12,path R2-R1-R3 should be the shortest way,and also we can prove this by using the following command:

R2#show ip ospf database summary 10.0.0.12

OSPF Router with ID (10.0.0.6) (Process ID 1)

Summary Net Link States (Area 0)

LS age: 1694
Options: (No TOS-capability, DC, Upward)
LS Type: Summary Links(Network)
Link State ID: 10.0.0.12 (summary Network Number)
Advertising Router: 10.0.0.9
LS Seq Number: 80000001
Checksum: 0x73E2
Length: 28
Network Mask: /30
TOS: 0 Metric: 192

Routing Bit Set on this LSA
LS age: 94
Options: (No TOS-capability, DC, Upward)
LS Type: Summary Links(Network)
Link State ID: 10.0.0.12 (summary Network Number)
Advertising Router: 10.0.0.13
LS Seq Number: 80000002
Checksum: 0x547D
Length: 28
Network Mask: /30
TOS: 0 Metric: 64

R2#

but in fact,we I ping or traceroute from R2 to 10.0.0.14,the path packets went through is R2-R1-R7-R8,So I am confused about this,Could some guys answer this question or give me some useful links?

Thanks

Best Regards!

 

[UnaBomber]

OSPF will take the lowest cost route to a destination. So I would assume that the path the packet took was the lowest cost. If there is a tie in the cost it will load ballance with up to 6 different route entries.

It is hard to see from your diagram how they are all connected up (I know its difficult to draw those things in a post)

UnaBomber
ccnp mcse2k

 

[parazhang]

Following is the output by some commands:
R2#traceroute 10.0.0.14

Type escape sequence to abort.
Tracing the route to 10.0.0.14

1 10.0.0.5 188 msec 48 msec 84 msec
2 10.0.0.10 132 msec 120 msec 108 msec
3 172.16.0.2 212 msec * 124 msec
R2#sho ip route
...

Gateway of last resort is not set

172.16.0.0/24 is subnetted, 1 subnets
O IA 172.16.0.0 [110/192] via 10.0.0.5, 00:00:49, Serial1/3
10.0.0.0/30 is subnetted, 4 subnets
O IA 10.0.0.8 [110/128] via 10.0.0.5, 00:00:49, Serial1/3
O IA 10.0.0.12 [110/192] via 10.0.0.5, 00:00:47, Serial1/3
O 10.0.0.0 [110/128] via 10.0.0.5, 00:01:04, Serial1/3
C 10.0.0.4 is directly connected, Serial1/3
R2#

R1#show ip route
...

Gateway of last resort is not set

172.16.0.0/24 is subnetted, 1 subnets
O 172.16.0.0 [110/128] via 10.0.0.10, 00:01:21, Serial1/2
10.0.0.0/30 is subnetted, 4 subnets
C 10.0.0.8 is directly connected, Serial1/2
O 10.0.0.12 [110/192] via 10.0.0.10, 00:01:21, Serial1/2
C 10.0.0.0 is directly connected, Serial1/1
C 10.0.0.4 is directly connected, Serial1/0
R1#


R3#sho ip route
...

Gateway of last resort is not set

172.16.0.0/24 is subnetted, 1 subnets
O 172.16.0.0 [110/128] via 10.0.0.14, 00:01:41, Serial1/0
10.0.0.0/30 is subnetted, 4 subnets
O 10.0.0.8 [110/192] via 10.0.0.14, 00:01:41, Serial1/0
C 10.0.0.12 is directly connected, Serial1/0
C 10.0.0.0 is directly connected, Serial1/1
O 10.0.0.4 [110/128] via 10.0.0.1, 00:01:51, Serial1/1
R3#


R2#show ip ospf database summary

OSPF Router with ID (10.0.0.6) (Process ID 1)

Summary Net Link States (Area 0)

Routing Bit Set on this LSA
LS age: 374
Options: (No TOS-capability, DC, Upward)
LS Type: Summary Links(Network)
Link State ID: 10.0.0.8 (summary Network Number)
Advertising Router: 10.0.0.9
LS Seq Number: 80000001
Checksum: 0x9644
Length: 28
Network Mask: /30
TOS: 0 Metric: 64

LS age: 337
Options: (No TOS-capability, DC, Upward)
LS Type: Summary Links(Network)
Link State ID: 10.0.0.8 (summary Network Number)
Advertising Router: 10.0.0.13
LS Seq Number: 80000001
Checksum: 0x83D2
Length: 28
Network Mask: /30
TOS: 0 Metric: 192

LS age: 336
Options: (No TOS-capability, DC, Upward)
LS Type: Summary Links(Network)
Link State ID: 10.0.0.12 (summary Network Number)
Advertising Router: 10.0.0.9
LS Seq Number: 80000001
Checksum: 0x73E2
Length: 28
Network Mask: /30
TOS: 0 Metric: 192

Routing Bit Set on this LSA
LS age: 364
Options: (No TOS-capability, DC, Upward)
LS Type: Summary Links(Network)
Link State ID: 10.0.0.12 (summary Network Number)
Advertising Router: 10.0.0.13
LS Seq Number: 80000001
Checksum: 0x567C
Length: 28
Network Mask: /30
TOS: 0 Metric: 64

Routing Bit Set on this LSA
LS age: 350
Options: (No TOS-capability, DC, Upward)
LS Type: Summary Links(Network)
Link State ID: 172.16.0.0 (summary Network Number)
Advertising Router: 10.0.0.9
LS Seq Number: 80000001
Checksum: 0x7874
Length: 28
Network Mask: /24
TOS: 0 Metric: 128

LS age: 344
Options: (No TOS-capability, DC, Upward)
LS Type: Summary Links(Network)
Link State ID: 172.16.0.0 (summary Network Number)
Advertising Router: 10.0.0.13
LS Seq Number: 80000001
Checksum: 0x6088
Length: 28
Network Mask: /24
TOS: 0 Metric: 128

 

[leedsit]

Hi,

Can you post your configs of R1 and R3 + also perform a show ip ospf neighbors on your R1 + R3

LEEroy
MCNE6,CCNP,CWNA,CCSA,Project+

 

[leedsit]

P.S what version of code do you have on R1???

I had an issue on a Cisco 7200 series once where Type 3 Routes ( Inter ) was favoured over ( intra ) even though the admin distance for those types were the same.

Lee.

LEEroy
MCNE6,CCNP,CWNA,CCSA,Project+

 

[castlesmadeofsand]

10.0.0.14 and 10.0.0.15 should be 10.0.0.13 and 10.0.0.14, because with the /30, that would make 10.0.0.15 a broadcast address, right? Or am I stoned? Please excuse me while I kiss the sky...

Jimi

 

[parazhang]

yes,you are not stoned!

It should be .13,hehe,I am stoned!

 

[castlesmadeofsand]

Did it work?

 

[parazhang]

.13 or .15 is only the typo error

as you saw,I still have the problem about the routing path selection!

thanks!

 

[KiscoKid]

By the way it's not technically correct to assume that OSPF takes the lowest cost route in all situations.

In reality, the order of preference for OSPF routes is:

intra-area routes, O
interarea routes, O IA
external routes type 1, O E1
external routes type 2, O E2

The costs only become a factor when 2 routes are of the same LSA type, i.e. O IA compared to O IA