Articles Tagged ‘LSA Types’

Analysis of OSPF Link State Update (LSU) - Link State Advertisement (LSA) Packet Structure. Common LSA Types

Part 4 of our OSPF Routing Protocol Series covers how OSPF uses Link State Advertisement (LSA) to exchange information about the network topology between routers. When a router receives an LSA, it is stored in the Link-State DataBase(LSDB). Once the LSDBs between routers are in sync, OSPF uses the Shortest Path First (SPF) algorithm to calculate the best routes for each network. It is important to understand that LSAs are information about a route that is transported inside Link State Update (LSU) packets.

Each single Link State Update (LSU) packet can contain one or more LSAs inside it and when an LSU is sent between OSPF routers, it floods the LSA information through the network.

It is very important for any network engineer to understand how LSAs are contained within an LSU. We’ll use the example below, where an OSPF router sends an LSU to the OSPF Designated Router (DR) containing LSA information about a new network:

OSPF Link State Update (LSU) packet containing a Link State Advertisement (LSA)

Figure 1. OSPF Link State Multicast Update (LSU) packet containing a Link State Advertisement (LSA)

As shown above, LSAs are contained within LSUs, which are all part of an OSPF packet encapsulated within an Ethernet frame (assuming an Ethernet network).

Our diagram of the LSU/LSA packet structure is confirmed by capturing an OSPF Ethernet frame below. We’ve highlighted each section (LSA, LSU, OSPF Header) using the same colors:

OSPF Link State Update and List State Advertisement within an Ethernet frame

Figure 2. OSPF Link State Update and List State Advertisement within an Ethernet frame

Notice that the destination IP address is multicast address 224.0.0.6, as expected since routers send updates to the Designated Router (DR) using this multicast address. This is also analyzed under the Working Inside a Single Area section in our article How OSPF Protocol Works & Basic Concepts: OSPF Neighbor, Topology & Routing Table, OSPF Areas & Router Roles, Theory & Overview

Most Popular OSPF LSA Types

OSPF currently defines 11 different LSA types, however, despite the large variety of LSAs only around half of them are commonly found in OSPF networks. Table 1 below shows the most popular LSA types, the type of OSPF routers (DR, ABR, ASBR etc) that generate them along with their function and the OSPF areas they affect:

OSPF LSA Types - Purpose and Function of Every OSPF LSA

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:

OSPF LSA Types

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.

LSA Type 1 Packets exchanged between OSPF routers within 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.

 LSA Type 2 Packets exchanged between OSPF DR and neighbor routers

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.

 LSA Type 3 - An OSPF ABR router advertises the summarized route 192.168.2.0/24 to Area 0

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

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