Introduction to OSPF

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Link-State Database Synchronization

Figure 4-10 illustrates the initial synchronization of the link-state database, which occurs in five steps as detailed in the numbered sequence following the figure.


Figure 4-10  Link-state database synchronization.

The states for link-state database synchronization as illustrated in Figure 4-10 are as follows:

1.  Establish bi-directional (2-way) communication. Accomplished by the discovery of the Hello protocol routers and the election of a DR.
2.  Exstart state. Two neighbor routers form a master/slave relationship and agree upon a starting sequence that will be incremented to ensure LSAs are acknowledged properly and no duplication occurs. Database Description (DD) packets begin.
3.  Exchange state. Database Description (DD) packets continue to flow as the slave router acknowledges the master’s packets. At this step, OSPF is considered operational because the routers can send and receive LSAs.
4.  Loading state. Link-state requests are sent to neighbors asking for recent advertisements that have not yet been discovered. At this stage, the router builds several lists to ensure all links are up-to-date and have been acknowledged properly. Figure 4-11 shows the fields and information contained within the link-state request packet format.


Figure 4-11  Link-state request packet format.

5.  Full state. Neighbor routers are fully adjacent because their link-state databases are fully synchronized.

During the five steps of link-state database synchronization, normal LSAs are not sent. Instead, the routers exchange packets known as Database Description (DD) packets, which are type 2 packets that are used when an adjacency is being initialized and the two routers in question are exchanging and synchronizing their link-state databases. These DD packets contain the contents of the link-state database. Figure 4-12 shows the fields and information contained within each DD packet.


Figure 4-12  Database Description packet format.

Of course, multiple packets might be needed to complete the synchronization and in that case a poll-response procedure is used with one router becoming the master and the other the slave.

LSA Packet Types

Unlike distance vector protocols (RIP or IGRP), OSPF does not actually send its routing table to other routers. Instead, routing tables are derived from the LSA database. Table 4-1 lists and describes the six different types of LSA packets that can be generated by the source router and entered into the destination router’s LSA database.

Table 4-1 LSA packet types.
LSA packet type Description

1 Router Link Advertisements
2 Network Link Advertisements
3 Summary Link Advertisements (ABRs)
4 Summary Link Advertisements (ASBRs)
5 Autonomous Systems (AS) External Link Advertisements
7 Not-So-Stubby Areas (NSSA)

Although there are several different types of LSAs and each has a unique packet structure to reflect the information it contains, they all share a common header as shown in Figure 4-13.


Figure 4-13  Link-state advertisement common header.

The sections that follow provide general descriptions of the six different LSA packet types.

Type 1: Router LSAs

Router LSAs are generated by each router for each area to which it belongs. These packets describe the states of the router’s links to the area and are only flooded within a particular area. The link-state ID is the originating router’s ID. Figure 4-14 shows the layout of each Router LSA packet.


Figure 4-14  Router LSA packet layout.

Type 2: Network LSAs

Network LSAs are generated by Designated Routers (DR) and describe the set of routers attached to a particular network. They are flooded in the area that contains the network. The link-state ID is the IP interface address of the DR. Figure 4-15 shows the layout of each Network LSA packet.


Figure 4-15  Network LSA packet layout.


Notes:  
TOS has been removed from OSPF’s specifications; however, most implementations in the field have yet to see this, so TOS fields will remain for clarity.

Type 3: Summary LSAs for ABRs

Summary LSAs are generated by Area Border Routers (ABRs) and describe inter-area routes to various networks. They can also be used for aggregating routes. The link-state ID is the destination network number. Figure 4-16 shows the layout of each Summary LSA packet.


Figure 4-16  Summary LSA packet (Type 3 and 4) layout.

Type 4: Summary LSAs for ASBRs

Summary LSAs describe links to Autonomous System Border Routers (ASBRs) and are also generated by Area Border Routers (ABRs). The link-state ID is the router ID of the described ASBR. Figure 4-16 (shown previously) illustrates the layout of each packet.

Type 5: Autonomous System External LSAs

Type 5 LSAs are generated by the Autonomous System Border Routers (ASBRs). They describe routes to destinations external to the Autonomous System. They will be flooded everywhere with the exception of stub areas. The link-state ID is the external network number.

Type 7: Not-So-Stubby Area (NSSA)

Type 7 LSAs are generated by Area Border Routers (ABRs). They describe routes within the NSSA. They can be summarized and converted into Type 5 LSAs by the ABRs. After they are converted to Type 5 LSAs, they will be distributed to areas that can support Type 5 LSAs. Refer to RFC 1587 for further details on how this conversion is done.

Link-State Advertisement Operation Example

Now that all six LSAs have been discussed and you understand how they operate within the OSPF functional environment, refer to Figure 4-17 for a visual representation for the operation and interaction between the various types of LSAs within an OSPF network.


Figure 4-17  Link-state advertisement operation.


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OSPF Network Design Solutions
OSPF Network Design Solutions
ISBN: 1578700469
EAN: 2147483647
Year: 1998
Pages: 200
Authors: Tom Thomas

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