Our previous article explained the purpose of Link State Update (LSU) packets and examined the Link State Advertisement (LSA) information contained within LSU packets. We also saw the most common LSA packets found in OSPF networks. In this article we’ll be diving deeper to analyse all eleven OSPF LSA Types using network network diagrams and examples to help understand when each LSA type is used and how they keep the OSPF network updated.
LSA Types - Quick Overview
Before we begin, let’s take a quick look at the different type of OSPF LSA packets we’ll cover:
- 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
The LSA payload varies in size according to the LSA type and the information it includes. The diagram below clearly shows how LSAs are contained within LSUs:
Figure 1. LSA Types contained within an OSPF LSU packet
As mentioned, OSPF currently supports 11 types of LSAs. Each LSA is used within specific boundaries of an OSPF network.
OSPF concepts, including router roles such as Designated Router (DR), Area Border Router (ABR), Autonomous System Border Router (ASBR), OSPF Areas and more, are analyzed in great depth in our article OSPF Basic Concepts – OSPF Areas – Router Roles. This article assumes the reader has a good understanding of basic OSPF theory and is comfortable with OSPF concepts.
LSA Type 1 – OSPF Router LSA
LSA Type 1 (Router LSA) packets are sent between routers within the same area of origin and do not leave the area. An OSPF router uses LSA Type 1 packets to describe its own interfaces but also carries information about its neighbors to adjacent routers in the same area.
Figure 2. LSA Type 1 Packets exchanged between OSPF routers within the same area
LSA Type 2 – OSPF Network LSA
LSA Type 2 (Network LSA) packets are generated by the Designated Router (DR) to describe all routers connected to its segment directly. LSA Type 2 packets are flooded between neighbors in the same area of origin and remain within that area.
Figure 3. LSA Type 2 Packets exchanged between OSPF DR and neighbor routers
LSA Type 3 – OSPF Summary LSA
LSA Type 3 (Summary LSA) packets are generated by Area Border Routers (ABR) to summarize its directly connected area, and advertise inter-area router information to other areas the ABR is connected to, with the use of a summary prefix (e.g 192.168.0.0/22). LSA Type 3 packets are flooded to multiple areas throughout the network and help with OSPF’s scalability with the use of summary prefixes.
Figure 4. LSA Type 3 - An OSPF ABR router advertises the summarized route 192.168.2.0/24 to Area 0
Looking at the diagram above, ABR router R2 creates a Type 3 Summary LSA and floods it into Area 0. In a similar way, ABR router R3 creates a Type 3 Summary LSA and floods it into Area 2. Type 3 Summary LSAs appear as O IA entries in the router routing table.
LSA Type 4 – OSPF ASBR Summary LSA