OSPF LSA Types and their Functions
In this blog, we will understand the OSPF LSA types and how OSPF uses (LSAs) are exchanged, including how the information is exchanged in the network topology between routers. A router’s Link-State Database stores LSAs that are received (LSDB). The Shortest Path First (SPF) algorithm is used by OSPF to determine the optimum routes for each network once the LSDBs between routers are in sync. It is crucial to realise that LSAs are route-related data sent inside Link State Update (LSU) packets. Therefore, it is essential for any network engineer to comprehend how LSAs are kept within an LSU. Let’s dig deep into OSPF LSA types, but before that, we must have a basic understanding of Link State Advertisement (LSA). LSA stands for “Link State Advertisement” in OSPF (Open Shortest Path First). In OSPF Protocol, routers exchange information about the network topology by sharing LSAs. LSAs are packets containing details about the router’s interfaces, neighboring routers, and network links. Each router in an OSPF network generates LSAs and floods them to all other routers within the same OSPF area. LSAs allow routers to build a synchronized view of the network, calculate shortest paths, and maintain routing tables for efficient packet forwarding. Here is a quick overview of OSPF LSA types that we will be discussing further – To create multiple paths throughout the network, OSPF uses LSA exchanges in the following activities: LSAs are the building blocks of the OSPF LSDB. Individually, LSAs act as database records, and when combined, they describe the entire topology of an OSPF area. There are several types of OSPF network LSAs, but not all are in use. Let’s discuss each OSPF LSA type one by one. IRs generate these types of LSA. They carry the information of all directly connected routes, and it always stays within the area. You will find the router-id of the originating router in the link-state id field. Once routing is done on the OSPF router, it generates the Type-1 LSA; you can use this command to verify the LSA. #SHOW IP OSPF DATABASE And you’ll get an LSA Type-1 like this in the database table of OSPF. The DR generates it on a multi-access network segment and always stays within the area when the neighborship is formed between two routers. Same you can verify in the database table of OSF. Summary LSA Using a summary prefix, the ABRs produce it into all other areas in order to summarise the area to which it is directly connected and to promote inter-area router information to other areas to which it is connected. Summary LSAs are further classified into 2 types: Routes to destinations in other areas within the OSPF network, i.e., inter-area destinations (O-IA). Basically, when multi-areas in OSPF are configured then, only this LSA type is generated in OSPF. LSA Type 5 (ASBR External LSA) packets are created by the ASBR in order to advertise externally redistributed routes into the OSPF’s AS. They describe the routes to the destinations external to the OSPF network. These LSAs are forwarded to all the connected and redistributed areas. The ASBR generates LSA Type 5 (ASBR External LSA) packets to advertise external redistributed routes into the OSPF’s AS. They describe the routes to the destinations external to the OSPF network. These LSAs are forwarded to all the connected and redistributed areas. Not supported and not used by OSPF. Cisco iOS doesn’t support this LSA type. They use (PIM) Protocol independent Multicast. It is produced by an ASBR inside an NSSA to characterize routes that have been redistributed there. As LSA 7 exits the NSSA, it is converted into LSA 5. NSSA area routers will be type 7 LSAs, but for other areas (other than NSSA), ABRs will convert them into type 5, which are easily understandable and acceptable in these areas. LSA Type 8 carries information about external routes, including their destination network addresses, path costs, and external route tags. When an OSPF router receives routing information from an external routing protocol, such as BGP (Border Gateway Protocol), it generates an LSA Type 5 called External LSA. The remaining 3 LSA types are opaque. We have also mentioned some information about them. OSPF LSA Type 10, known as OSPF Area Scope Opaque LSA, is used to carry opaque data specific to a particular OSPF area, enabling the transmission of custom information within that specific area. OSPF LSA Type 11, referred to as OSPF AS (Autonomous System) Scope Opaque LSA, is utilized to transmit opaque data throughout the entire OSPF Autonomous System, allowing the exchange of custom information across the entire OSPF domain. These are the various OSPF LSA types. Now let’s get a hold of another important topic OSPF LSA Header. LINK STATE ADVERTISEMENT (LSA) PACKET STRUCTURE (WITHIN A LINK STATE UPDATE – LSU) All the data required to exchange network information inside an OSPF network is contained in the header of each LSA packet. This graphic will allow us to understand what data is contained in an LSA packet. The 20-byte LSA header section contains the following fields: LSAs, or Link State Advertisements, are multicast messages exchanged among routers within the OSPF domain. Internal routers utilize them to notify the DR/BDR routers about any modifications or updates. There are 11 LSA types which are – Router LSA, Network LSA, Summary LSA, Summary ASBR LSA, Autonomous system external LSA, Multicast OSPF LSA, Not-so-stubby area LSA, External attribute LSA, and 3 Opaque LSA Types. Type 3 LSA in OSPF (Open Shortest Path First) is known as the Network Summary LSA. It is responsible for advertising summarized network information to routers within the same OSPF area, enabling efficient routing and reducing the size of the routing tables. OSPF LSA Type 4, OSPF ASBR Summary LSA, is used to advertise the summary information of an Autonomous System Boundary Router (ASBR) within an OSPF area. It provides a summarized view of the ASBR’s external routes, enabling efficient routing decisions within the OSPF domain. In this blog, we have discussed everything related to OSPF LSA types. OSPF is a part of our CCNA and CCNP ENCOR training. For a more detailed explanation, you can check out the OSPF BGP training. In conclusion, OSPF (Open Shortest Path First) utilizes various Link State Advertisements (LSAs) to exchange critical routing information within the OSPF domain. Each LSA type serves a specific purpose, such as advertising network topology, summarizing routes, or carrying custom opaque data. Understanding the different OSPF LSA types is essential for network administrators to effectively design, optimize, and maintain OSPF-based networks, enabling efficient routing and seamless connectivity across the entire OSPF infrastructure. You can also check out – Difference between OSPF and EIGRPIntroduction
What is LSA in OSPF?
OSPF LSA Types – Quick Overview
LSA Type Name LSA Type 1 OSPF Router LSA LSA Type 2 OSPF Network LSA LSA Type 3 OSPF Summary LSA LSA Type 4 OSPF ASBR Summary LSA LSA Type 5 OSPF ASBR External LSA LSA Type 6 OSPF Group Membership LSA LSA Type 7 OSPF Not So Stubby Area (NSSA) External LSA LSA Type 8 OSPF External Attributes LSA (OSPFv2) / Link Local LSA (OSPFv3) LSA Type 9 OSPF Link Scope Opaque (OSPFv2) / Intra Area Prefix LSA (OSPFv3) LSA Type 10 OSPF Area Scope Opaque LSA LSA Type 11 OSPF AS (Autonomous System) Scope Opaque LSA Functions of LSAs in OSPF
OSPF LSA Types
OSPF LSA Type 1 – OSPF Router LSA
OSPF LSA Type 2 – Network LSA
OSPF LSA Type 3 – Summary LSA
OSPF LSA Type 4 – ASBR Summary LSA
OSPF LSA Type 5 – AS External LSA
OSPF LSA Type 6 – Multicast OSPF LSA
OSPF LSA Type 7 – NSSA LSA
LSA Types Advertising Router Contents of LSA Type – I (Router LSA) OSPF router Topology and routing info. Type – II (Net LSA) DR Topology and routing info. Type – III (Summary LSA) ABR Inter-area routing information Type – IV (ASBR Sum. LSA) ABR ASBR’s router-ID Type – V (AS External LSA) ASBR AS External routing information Type – VI (Multicast LSA) – – Type – VII (NSSA-LSA) ASBR External routing information OSPF LSA Type 8 – External attributes LSA
OSPF LSA Type 9 – OSPF Link Scope Opaque (OSPFv2)
OSPF LSA Type 9, also known as OSPF Link Scope Opaque, is used in OSPFv2 to carry non-standardized or proprietary information within the OSPF domain. LSA Type 9 enables OSPF routers to exchange opaque data, allowing for custom extensions and additional functionality beyond the standard OSPF protocol.OSPF LSA Type 10 – OSPF Area Scope Opaque LSA
OSPF LSA Type 11 – OSPF AS (Autonomous System) Scope Opaque LSA
OSPF LSA Header
Frequently Asked Questions
Q1 – What is a LSA in OSPF?
Q2 – What are the LSA types in OSPF?
Q3 – What is Type 3 LSA in OSPF?
Q4 – What is OSPF Type 4 LSA?
Conclusion