OSPF LSAs and the Link State Database (LSDB)

As stated in the previous section, OSPF uses several types of Link State Advertisements. Each LSA begins with a standard 20-byte LSA header. This header contains the following fields:

• Link State Age
• Options
• Link State Type
• Link State ID
• Advertising Router
• Link State Sequence Number
• Link State Checksum
• Length

The 2-byte Link State Age field states the time (in seconds) since the LSA was originated. The maximum age of the LSA is 3600 seconds, which means that if the age reaches 3600 seconds, the LSA is removed from the database. To avoid this, the LSA is refreshed every 1800 seconds.

The 1-byte Options field contains the same options as those in the OSPF Hello packet.

The 1-byte Link State Type field represents the types of LSAs. These different LSA packet types are described in detail in the following sections.

The 4-byte Link State ID field identifies the portion of the network that is being described by the LSA. The contents of this field depend upon the advertisement’s LS type.

The 4-byte Advertising Router field represents the router ID of the router originating the LSA.

The 1-byte Link State Sequence Number field detects old or duplicate Link State Advertisements. Successive instances of an LSA are given successive Link State Sequence Numbers. The first sequence number 0x80000000 is reserved; therefore, the actual first sequence number is always 0x80000001. This value is incremented as packets are sent. The maximum sequence number is 0x7FFFFFFF.

The 2-byte Link State Checksum field performs the Fletcher checksum of the complete contents of the LSA, including the LSA header. The Link State Age field is not included in the checksum. The checksum is performed because Link State Advertisements can be corrupted while being stored in memory due to router software or hardware issues or during flooding due to Physical Layer errors, for example.

NOTE: The checksum is performed at the time the LSA is generated or is received. In addition, the checksum is performed at every CheckAge interval, which is 10 minutes. If this field has a value of 0, then it means that the checksum has not been performed.

The 2-byte Length field is the final field and includes the length (in bytes) of the LSA. This includes the 20-byte LSA header. Figure 39.13 below illustrates the LSA header:

Figure 39.13 – Link State Advertisement Header

While OSPF supports 11 different types of Link State Advertisements, only LSA Types 1, 2, and 3, which are used to calculate internal routes, and LSA Types 4, 5, and 7, which are used to calculate external routes, are within the scope of the CCNA exam requirements. Because there is really no need to go into great detail on the other LSAs for the CCNA exam, these LSAs will not be described further in this guide. However, a brief outline and a printable guide are provided at www.in60days.com.

In Cisco IOS software, the show ip ospf database command is used to view the contents of the Link State Database. This command, when used without any keywords, prints out a summary of LSAs in all areas to which the router is connected. The command supports several keywords that provide greater granularity in allowing network administrators to restrict output only to specific types of LSAs, LSAs advertised by the local router, or even LSAs advertised by other routers within the OSPF domain.

While illustrating the output of the usage of each keyword is unrealistic, the following section describes the different LSAs and the common keywords used in conjunction with the show ip ospf database command to view detailed information on these LSAs. The keywords supported by this command are illustrated in the following output:

adv-router        Advertising Router link states

asbr-summary      ASBR Summary link states

database-summary  Summary of database

external          External link states

network           Network link states

nssa-external     NSSA External link states

opaque-area       Opaque Area link states

opaque-as         Opaque AS link states

opaque-link       Opaque Link-Local link states

router            Router link states

self-originate    Self-originated link states

summary           Network Summary link states

|                 Output modifiers

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Router Link State Advertisements (Type 1)

Type 1 LSAs are generated by each router for each area to which it belongs. The Router LSA lists the originating router’s router ID (RID). Each individual router will generate a Type 1 LSA for the area in which it resides. The Router LSAs are the first LSA types printed in the output of the show ip ospf database command.

Network Link State Advertisements (Type 2)

OSPF uses the Network Link State Advertisement (Type 2 LSA) to advertise the routers on the Multi-Access segment. This LSA is generated by the DR and is flooded only within the area. Because the other non-DR/BDRs do not establish adjacencies with each other, the Network LSA allows those routers to know about the other routers on the Multi-Access segment.

Network Summary Link State Advertisements (Type 3)

The Network (Type 3) LSA is a summary of destinations outside of the local area but within the OSPF domain. In other words, this LSA advertises both inter-area and intra-area routing information. The Network Summary LSA does not carry any topological information. Instead, the only information contained in the LSA is an IP prefix. Type 3 LSAs are generated by ABRs and are flooded to all adjacent areas. By default, each Type 3 LSA matches a single Router or Network LSA on a one-for-one basis. In other words, a Type 3 LSA exists for each individual Type 1 and Type 2 LSA. Special attention must be paid to how these LSAs are propagated in relation to the OSPF backbone. This propagation or flooding is performed as follows:

• Network Summary (Type 3) LSAs are advertised from a non-backbone area to the OSPF backbone for intra-area routes (i.e., for Type 1 and Type 2 LSAs).
• Network Summary (Type 3) LSAs are advertised from the OSPF backbone to other non- backbone areas for both intra-area (i.e., Area 0 Type 1 and Type 2 LSAs) and inter-area  routes (i.e., for the Type 3 LSAs flooded into the backbone by other ABRs).

The next three Link State Advertisements, Type 4, Type 5, and Type 7, are used in external route calculation. Type 4 and Type 5 LSAs will be described in the following sections. Type 7 LSAs will be described later in this module when we discuss the different types of OSPF areas.

ASBR Summary Link State Advertisements (Type 4)

The Type 4 LSA describes information regarding the Autonomous System Boundary Router (ASBR). This LSA contains the same packet format as the Type 3 LSA and performs the same basic functionality, with some notable differences. Like the Type 3 LSA, the Type 4 LSA is generated by the ABR. For both LSAs, the Advertising Router field contains the RID of the ABR that generated the Summary LSA. However, the Type 4 LSA is created by the ABR for each ASBR reachable by a Router LSA. The ABR then injects the Type 4 LSA into the appropriate area. This LSA provides reachability information on the ASBR itself. The key differences between the Type 3 and Type 4 LSAs that you should be familiar with are listed below in Table 39.2:

Table 39.2 –  Type 3 and Type 4 Summary LSAs

AS External Link State Advertisements (Type 5)

The External Link State Advertisement is used to describe destinations that are external to the autonomous system. In other words, Type 5 LSAs provide the network information necessary to reach the external networks. In addition to external routes, the default route for an OSPF routing domain can also be injected as a Type 5 Link State Advertisement.