Types of LSAs

Unlike distance vector protocols (RIP or IGRP), OSPF does not send its routing table to other routers. Instead, routing tables are derived from the LSA database, as discussed in Chapter 2. OSPF has a variety of router designations and area types. This complexity requires that OSPF communicates information as accurately as possible to achieve optimal routing. OSPF accomplishes this communication through the use of different types of LSAs. Table 3-1 describes the ten different types of LSA packets that can be generated by the source router and entered into the destination router's LSA database. However, note that Cisco has not implemented all the possible OSPF LSAs, specifically the Type 6 multicast LSA, as documented in RFC 1584.

104 Chapter 3: OSPF Communication

Table 3-1 Types of LSAs

LSA Type Number

LSA Description

1

Router link advertisements

2

Network link advertisements

3

ABR summary link advertisements

4

ASBR summary link advertisements

5

Autonomous system external route advertisements

6

Multicast group LSA (not implemented by Cisco)

7

Not-so-stubby area (NSSAs) external

9

Opaque LSA: Link-local scope

10

Opaque LSA: Area-local Scope

11

Opaque LSA: autonomous system scope

The following section provides general descriptions, an operational overview, and the packet format of nine of the LSA packet types. (Type 6 is not discussed.)

Although several different types of LSAs exist and each has a unique structure to reflect the information it contains, they each share a common packet header, as shown in Figure 3-1. After the common header, the specific LSA packet information is then provided for router processing. You learn about the packet structure for each LSA type as it is discussed.

Figure 3-1 Link-State Advertisement Common Header

01234567890123456789012345678901

LS Age

Options

LS Type

Link State ID

Advertising Router

LS Sequence Number

LS Checksum

Length

Type 1: Router LSAs

Router LSAs describe the states of the router's links within the area and are flooded only within an area for which that router is a member. The fact that Type 1 LSAs describe links in the area is a key differentiator between this LSA type and the others. For example, an OSPF ABR router is in two areas and sends router LSAs for the link that resides only in each area. (That is, cross-transmission is not allowed.)

Figure 3-2

When discussing OSPF LSAs, you need to understand that OSPF has two native types of routes:

• Intra-area route—A route found within an OSPF area

• Inter-area route—A route found in a different OSPF area Figure 3-2 shows that Type 1 LSAs contain intra-area information.

Type 1 LSA Operation: Router LSAs Describe the Current State of the Router's Links (Interfaces) to the OSPF Area (Intra-Area)

The link-state ID is the originating router's ID. Figure 3-3 shows the structure of each router LSA packet.

Figure 3-3 Router LSA Packet Layout

01234567890123456789012345678901

01234567890123456789012345678901

0 V E B

0

Number of Links

Link ID

Link Data

Type

#TOS

TOS 0 Metric

TOS=x

0

TOS x Metric

TOS=y

0

TOS y Metric

TOS=z

0

TOS z Metric

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