As a link-state routing protocol, OSPF uses a more structured mechanism to share routing protocols and does not depend on or follow routing-by-rumor as other routing protocols like RIP and IGRP. Routers keep track of the status of links within their respective areas and neighbours and based on the link state SPF algorithm is run and the topology database is created.
OSPF LSA (Link state Advertisements)
LSA stands for “Link State Advertisement” which is used in Link state protocols such as OSPF (Open Shortest Path First) and IS-IS (Intermediate System to Intermediate System). Routers forward and exchange link-state advertisements (LSAs) to ensure the topology database is consistent on each router within an area to calculate the SPF.
LSA contains below information:
- Sharing Router Information in Areas.
- Having Topology Information based on the Link state connected with the neighbor router.
- Synchronize the router Database with all neighbours.
- Calculate the Shortest Path (SPF) to reach the destination. This calculation is typically performed using a shortest-path-first (SPF) algorithm, such as Dijkstra’s algorithm.
- LSA will share when a new adjacency is formed.
- LSA will share when a change occurs to the topology table.
Type of LSAs:
LSA-1: Router LSA
LSA-2: Network LSA
LSA-3: Network Summary LSA
LSA-4: ASBR Summary LSA
LSA-5: External LSA
LSA-6: OSPF in multicast (MOSPF)
LSA-7: NSSA External
LSA-8: External Attribute (if we make a BGP combination with LSA 8 will run, but Cisco not support this LSA)
LSA-9: Opaque
LSA-10: Opaque (Use in MPLS TE)
LSA-11: Opaque
Note: In OSPF only Green Marked LSA are in use. Rest are not in use.
LSA age time is 60 min and after 60 min it will again share. It refreshes Interval is 30 min.
LSA carries a Sequence number, Age timer, and Checksum value. In every 5 min, it will check errors in the checksum.
So LSA has the below info
- Link ID
- Adv Router
- Seq no.
- Ages
- Checksum value
IP version 4 has 11 LSAs
IP version 3 had 9 LSAs
Use to LSA Types with Example
- Router LSA (Type 1) – LSA 1 also called Type-1. This LSA is generated by the Local or same-area routers and flooded to all other routers within the local area.
- Network LSA (Type 2) – Also called Type-2. In the NBMA network after electing DR and BDR, DR (Designated Router} generates Type-2 LSA which contains a list of all BDR and other attached Routers to the Designated Router.
OSPF LSA Type 1 and 2 (Router LSA & Network LSA):
1 Both LSAs work in a Single Area
2 These LSA information can be checked by sh ip OSPF database command
In the above example, LSA 1 and 2 will be generated in Area-1 and Area-2 routers. Below is the output.
Network Summary LSA (Type 3)
- It is also called Type-3 LSA. It is generated by all ABRs.
As in the above example R3 is connected with Area-1 and Area-0, So R3 is ABR. For LSA Type-3 Summary LSA below features:
- ABR contains only other area routes marked with OIA.
- It will maintain 2 databases (one for area 1 and another for other external areas with the best route with the best matric with summary LSA.
- ABR will send a summary route to Area 1 to reach another area via ABR (R3)
- ABR (R3) will also share the cost of the outside network.
- It contains a list of all destination networks within an area.
- Type 3 LSAs are sent between areas to allow inter-area communication to occur
ASBR Summary LSA (Type 4)
- ABRs generates LSA 4. If any Network has any external network like EIGRP, BGP, etc., then ABR generates and shares LSA Type-4 (ASBR Summary LSA) into its local area so that internal routers know how to reach the ASBR network.
External LSA (Type 5)
- ASBRs generate this LSA-5.
- Contains routes to destination networks outside the local Autonomous System.
- Type 5 LSAs can also take the form of a default route to all networks outside the local AS.
- Type 5 LSAs are flooded to all areas.
- LSA 5 came from External AS via redistribution
- ASBR will generate this type 5 LSA containing his RID, Matric of external network
- There are 2 types of matric—E1 and E2, by default cisco sent E2 matric
- Type 2 ((E2) is matric from ASBR to the external network
- Type 1 (E1) will add end-to-end or Source to the external network via ASBR total Matic will calculate
- Type 5 will be identical for the whole OSPF network, it will not change
- ASBR will send his information to every ABR, but other Local area which is connected to ABR locally doesn’t have any information about this External AS and LSA-5. So, it’s ABR’s responsibility to send that ASBR info to their area local routers. So ABR generates the LSA-4 which contains ASBR network information. We can understand this like this- ABR receives LSA-5 from ASBR and then ABR converts that LSA-5 to LSA-4 and shares it with local area Routers.
In the above example, R4 will send LSA-5 to R3 and then R3 will convert LSA-5 to LSA-4 and will share that LSA-5 information with R1 and R2 in Area-1. Same will be done by R5 for Area 2.
Type 7 LSA:
- Same as we understood about LSA-5 and LSA-4 Concept If an ASBR exists within the NSSA area, then ASBR will generate Type 7 LSAs. But other Areas’ router will not understand LSA-7, so here ABR again converts LSA-7 to LSA-5 for other Areas and LSA-5 to LSA-4 for their Local area routers.
Summary
- All routers will generate Router (Type 1) LSAs in the same area
- Designated Routers will generate Network (Type 2) LSAs in the same area.
- Area Border Routers (ABRs) will generate Network Summary (Type 3) LSAs.
- ABRs will also generate ASBR Summary (Type 4) LSAs to share External Routes with Local area routers
- ASBRs will generate External (Type 5) LSAs.
- ASBR exists within the NSSA area, then ASBR will generate NSSA External (Type 7) LSAs.
- ABRs will convert LSA 7 into LSA 5
FAQ
Q. What is LSA Stand for in OSPF?
A. LSA Stands for Link State Advertisement.
Q. What are LSA types?
Q. Below are all LSA types in OSPF:
LSA-1: Router LSA
LSA-2: Network LSA
LSA-3: Network Summary LSA
LSA-4: ASBR Summary LSA
LSA-5: External LSA
LSA-6: OSPF in multicast (MOSPF)
LSA-7: NSSA External
LSA-8: External Attribute (if we make a BGP combination with OSPF then LSA 8 will run, but Cisco not support this LSA)
LSA-9: Opaque
LSA-10: Opaque (Use in MPLS TE)
LSA-11: Opaque
Read more OSPF-Part-1 –https://techblog.kbrosistechnologies.com/what-is-ospf-basics-routing-protocols-link-state-routing-protocol/
Read more OSPF-Part-2- https://techblog.kbrosistechnologies.com/what-is-ospf-dr-bdr-network-type-neighbour-state-routing-protocol/
Read more OSPF-Part-3-https://techblog.kbrosistechnologies.com/ospf-part-3-advance-router-types-abr-asbr-area-0-backbone-area-virtual-link/
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