OSPF Type 4 LSA Importance

In this post we will learn the importance of LSA 4 and see which router actually propagates the LSA 4 and what it actually contains. To achieve this we will make use of the topology below –

  • In the above topology R1 and R2 form neighborship in Area 10.
  • R1 and R3 form OSPF neighborship in Area 0.
  • R3 –R4 –R5 form neighborship in Area 20.
  • R5 form EIGRP neighborship with EIGRP. Hence R5 router acts as the ASBR in our topology since it is connected to a different routing domain as well.
  • R3 is the ABR for Area 20 and Area 0.
  • R2 is the ABR for Area 0 and Area 10.

The LSA for the external network 9.9.56.0 is originated by the ASBR router R5 as LSA 5 in the area 20. ASBR, router 5 also generates a LSA 1 in Area 20 for its own router ID. This LSA 1 when reaches the ABR for the Area 20 is converted to LSA type 4 and propagated in Area 0 and Area 10 both.

 

Output

R5#sh ip ospf database

OSPF Router with ID (5.5.5.5) (Process ID 1)

Router Link States (Area 20)

Link ID         ADV Router      Age         Seq#       Checksum Link count

4.4.4.4         4.4.4.4         31          0x80000003 0x006484 3

5.5.5.5         5.5.5.5         35          0x80000002 0x00245B 1  # LSA 1 for ASBR router ID.

9.9.34.3        9.9.34.3        36          0x80000002 0x0069EA 1

 

Net Link States (Area 20)

Link ID         ADV Router      Age         Seq#       Checksum

9.9.34.3        9.9.34.3        37          0x80000001 0x002F55

9.9.45.5        5.5.5.5         35          0x80000001 0x007746

 

Summary Net Link States (Area 20)

Link ID         ADV Router      Age         Seq#       Checksum

2.2.2.2         9.9.34.3        27          0x80000001 0x00B04A

2.2.2.3         9.9.34.3        27          0x80000001 0x00A653

9.9.12.0        9.9.34.3        27          0x80000001 0x009C49

9.9.13.0        9.9.34.3        67          0x80000001 0x00875E

 

Type-5 AS External Link States

Link ID         ADV Router      Age         Seq#       Checksum Tag

9.9.56.0        5.5.5.5         65          0x80000001 0x00053D 0   # LSA 5 for external network.

Notice for the external network the Link ID is external network and the Adv. Router is the ASBR router ID which in our case is 5.5.5.5.Now R3 propagates this LSA 5 from R5 into area 0 and there onwards router R1 propagates this LSA into area 10.The Adv. router ID in this LSA 5 is the router ID of the ASBR which is 5.5.5.5.In order to reach the external network each router must also know how to reach the Adv. Router 5.5.5.5. This is where the LSA 4 comes into picture. R5 generated as LSA 1 for its own router ID 5.5.5.5 in area 20 but R3 propagates this LSA 1 as LSA type 4 into the other areas i.e. area 0 and area 10. Hence the router in areas 0 and 10 get to know how they can reach the ASBR and hence finally get to the external network 9.9.56.0.

R1#sh ip ospf database

OSPF Router with ID (9.9.13.1) (Process ID 1)

Router Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum Link count

9.9.13.1        9.9.13.1        337         0x80000002 0x001995 1

9.9.34.3        9.9.34.3        338         0x80000002 0x0099E4 1

 

Net Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum

9.9.13.3        9.9.34.3        338         0x80000001 0x00D5B3

 

Summary Net Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum

2.2.2.2         9.9.13.1        372         0x80000001 0x0046CC

2.2.2.3         9.9.13.1        372         0x80000001 0x003CD5

4.4.4.4         9.9.34.3        343         0x80000001 0x004AA9

9.9.12.0        9.9.13.1        372         0x80000001 0x0032CB

9.9.34.0        9.9.34.3        378         0x80000001 0x009F31

9.9.45.0        9.9.34.3        343         0x80000001 0x003094

 

Summary ASB Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum

5.5.5.5         9.9.34.3        343         0x80000001 0x000EE0   #LSA 4 in Area 0

 

Router Link States (Area 10)

Link ID         ADV Router      Age         Seq#       Checksum Link count

2.2.2.3         2.2.2.3         375         0x80000002 0x00FF7E 4

9.9.13.1        9.9.13.1        374         0x80000002 0x001B7F 2

 

Summary Net Link States (Area 10)

Link ID         ADV Router      Age         Seq#       Checksum

4.4.4.4         9.9.13.1        333         0x80000001 0x00F316

9.9.13.0        9.9.13.1        372         0x80000001 0x0027D5

9.9.34.0        9.9.13.1        333         0x80000001 0x00499D

9.9.45.0        9.9.13.1        333         0x80000001 0x00D901

 

Summary ASB Link States (Area 10)

Link ID         ADV Router      Age         Seq#       Checksum

5.5.5.5         9.9.13.1        333         0x80000001 0x00B74D   #LSA 4 in Area 10

 

Type-5 AS External Link States

Link ID         ADV Router      Age         Seq#       Checksum Tag

9.9.56.0        5.5.5.5         379         0x80000001 0x00053D 0

 

Conclusion

LSA 4 is generated by the ABR. Link ID in that is the router ID of ASBR and Adv. Router is the ASBR itself. LSA 4 is required to tell the routers in other areas how to reach the ASBR to get to external network eventually.

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