LAB: OSPF Path Selection with Virtual-links

- Configure the OSPF domain so that traffic from SW3 going to OSPF area51 transits the point-to-point Serial link between R3 and R2.

Current traceroute from SW3

Rack1SW3#traceroute 51.51.51.51

Type escape sequence to abort.Tracing the route to 51.51.51.51

  1 155.1.79.7 0 msec 4 msec 0 msec
  2 155.1.67.6 12 msec 24 msec 4 msec
  3 155.1.146.1 36 msec 24 msec 12 msec
  4 155.1.0.5 36 msec 20 msec 12 msec
  5 155.1.0.2 36 msec 12 msec 16 msec
  6 192.10.1.254 44 msec *  48 msec

changing the OSPF cost of interface s1/3 of R3 towards R2 to cost of 1. the direct route to R2 cannot be used because R3 does not know how to  reach the ASBR for area 51, which is one of the requirements in OSPF. as a result it is not installed int the routing table.

Rack1R3(config)#int s1/3
Rack1R3(config-if)#ip ospf cost 1

Rack1R3#show ip route 51.51.51.51
Routing entry for 51.51.51.51/32
  Known via "ospf 1", distance 110, metric 20, type extern 2, forward metric 39160
  Last update from 155.1.0.5 on Serial1/0, 00:00:16 ago
  Routing Descriptor Blocks:
  * 155.1.0.5, from 192.10.1.254, 00:00:16 ago, via Serial1/0
      Route metric is 20, traffic share count is 1

Rack1R3#traceroute 51.51.51.51

Type escape sequence to abort.
Tracing the route to 51.51.51.51

  1 155.1.0.5 28 msec 20 msec 16 msec
  2 155.1.0.2 16 msec 28 msec 8 msec
  3 192.10.1.254 8 msec *  24 msec

Rack1R3#show ip ospf database external 51.51.51.51

            OSPF Router with ID (150.1.3.3) (Process ID 1)

                Type-5 AS External Link States

  Routing Bit Set on this LSA
  LS age: 1753
  Options: (No TOS-capability, DC)
  LS Type: AS External Link
  Link State ID: 51.51.51.51 (External Network Number )
  Advertising Router: 192.10.1.254
  LS Seq Number: 80000001
  Checksum: 0x9C6C
  Length: 36
  Network Mask: /32
        Metric Type: 2 (Larger than any link state path)
        TOS: 0
        Metric: 20
        Forward Address: 0.0.0.0
        External Route Tag: 0

as confirmed by the output above which has a forward address of 0.0.0.0. this means that R3 needs to lookup the advertising routing for the external LSA.  which is 192.10.1.254

Rack1R3#show ip ospf database asbr-summary 192.10.1.254

            OSPF Router with ID (150.1.3.3) (Process ID 1)

                Summary ASB Link States (Area 0)

  Routing Bit Set on this LSA
  LS age: 1593
  Options: (No TOS-capability, DC, Upward)
  LS Type: Summary Links(AS Boundary Router)
  Link State ID: 192.10.1.254 (AS Boundary Router address)
  Advertising Router: 150.1.2.2
  LS Seq Number: 80000001
  Checksum: 0x8D1B
  Length: 28
  Network Mask: /0
        TOS: 0  Metric: 300
<output omitted>

       Summary ASB Link States (Area 5)

  Adv Router is not-reachable
  LS age: 1615
  Options: (No TOS-capability, DC, Upward)
  LS Type: Summary Links(AS Boundary Router)
  Link State ID: 192.10.1.254 (AS Boundary Router address)
  Advertising Router: 150.1.2.2
  LS Seq Number: 80000001
  Checksum: 0x8D1B
  Length: 28
  Network Mask: /0
        TOS: 0  Metric: 300

  LS age: 1621
  Options: (No TOS-capability, DC, Upward)
  LS Type: Summary Links(AS Boundary Router)
  Link State ID: 192.10.1.254 (AS Boundary Router address)
  Advertising Router: 150.1.3.3
  LS Seq Number: 80000001
  Checksum: 0xD46D
  Length: 28
  Network Mask: /0
        TOS: 0  Metric: 39160

above output shows that R3 has multiple paths to the ASBR of Area 51, but only area 0 routes towards R2 to reach 192.168.10.1.254  are valid. it cannot use the area 5 routes toward R2 because the Adv Router is not-reachable through this path. this means Area 5 cannot be used as a transit area.

by using virtual-link between R3 and R2 via area 5. it will create a direct area 0 logical link between R2 and R3 via area 5.

R2:
router ospf 1
area 5 virtual-link 150.1.3.3
 

R3:
router ospf 1
area 5 virtual-link 150.1.2.2

verfication:

now that R3 has an area 0 route to R2 via the virtual-link through area 5 and the cost of the area 5 link is 1.  R3 chooses this as the shortest path towards the ASBR.  Now R3 advertise a better cost towards area 51 into area 2 than R6 does.

output below shows that SW3 is able to reach the ASBR for 51.51.51.51/32 network which is through 192.10.1.254.

Also is shows that from SW3 the ASBR is reachable bia the Area2 ABR 150.1.3.3 R3 and 1501.6.6 R6.  R3's metric is 301 while R6 is 23030.

Rack1SW3#show ip ospf database external 51.51.51.51

            OSPF Router with ID (150.1.9.9) (Process ID 1)

                Type-5 AS External Link States

  Routing Bit Set on this LSA
  LS age: 96
  Options: (No TOS-capability, DC)
  LS Type: AS External Link
  Link State ID: 51.51.51.51 (External Network Number )
  Advertising Router: 192.10.1.254
  LS Seq Number: 80000002
  Checksum: 0x9A6D
  Length: 36
  Network Mask: /32
        Metric Type: 2 (Larger than any link state path)
        TOS: 0
        Metric: 20
        Forward Address: 0.0.0.0
        External Route Tag: 0

Rack1SW3#show ip ospf database asbr-summary 192.10.1.254   

            OSPF Router with ID (150.1.9.9) (Process ID 1)

                Summary ASB Link States (Area 2)

  Routing Bit Set on this LSA
  LS age: 78
  Options: (No TOS-capability, DC, Upward)
  LS Type: Summary Links(AS Boundary Router)
  Link State ID: 192.10.1.254 (AS Boundary Router address)
  Advertising Router: 150.1.3.3
  LS Seq Number: 80000002
  Checksum: 0x881C
  Length: 28
  Network Mask: /0
        TOS: 0  Metric: 301

  LS age: 1884
  Options: (No TOS-capability, DC, Upward)
  LS Type: Summary Links(AS Boundary Router)
  Link State ID: 192.10.1.254 (AS Boundary Router address)
  Advertising Router: 150.1.6.6
  LS Seq Number: 80000001
  Checksum: 0x601D
  Length: 28
  Network Mask: /0
        TOS: 0  Metric: 23030 


Rack1SW3#traceroute 51.51.51.51                             

Type escape sequence to abort.
Tracing the route to 51.51.51.51

  1 155.1.79.7 0 msec 4 msec 4 msec
  2 155.1.37.3 16 msec 16 msec 4 msec
  3 155.1.23.2 32 msec 12 msec 12 msec
  4 192.10.1.254 28 msec *  36 msec