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:
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:
Figure 2. OSPF Link State Update and List State Advertisement within an Ethernet frame
Notice that the destination IP address is multicast address 126.96.36.199, 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: