As we have already discussed OSPF LSA Types in our last post. In this post we will learn the importance of OSPF Type 4 LSA and see which router actually propagates the LSA 4 and what it actually contains.
OSPF Type 4 LSA: Example scenario
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
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 Type 4 LSA 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 type 4 LSA 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
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
Type 4 LSA 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.
Continue Reading:
OSPF DR BDR Election: Explained
ABOUT THE AUTHOR
I am here to share my knowledge and experience in the field of networking with the goal being – “The more you share, the more you learn.”
I am a biotechnologist by qualification and a Network Enthusiast by interest. I developed interest in networking being in the company of a passionate Network Professional, my husband.
I am a strong believer of the fact that “learning is a constant process of discovering yourself.”
– Rashmi Bhardwaj (Author/Editor)
