As a link-state protocol, IS-IS works by gathering reliable and complete information about the routing environment through the use of special packets known as Link State Protocol Data Units (LSPs). A protocol data unit (PDU) also means a packet. Each router generates an LSP, which captures local link-state information describing connected links, neighbor routers, IP subnets, related metric information, and so forth. Copies of the LSP are distributed to all routers in a specific area through a process referred to as flooding. Ultimately, all routers in an area obtain every other router's LSP and synchronize their databases. Because the area link-state database is used for only intra-area routing (also referred to as Level 1 routing), it is called the Level 1 link-state database. The Level 2 routers interconnected into the backbone similarly maintain a Level 2 link-state database through the exchange of Level 2 LSPs. Best paths though the network are resolved by running the SPF algorithm over the information in the Level 1 and Level 2 databases separately. The sections that follow address the following subtopics:
• Overview of the IS-IS link-state database
• Flooding and database synchronization
• The SPF algorithm and route calculation
The first section provides a high-level overview of the IS-IS link-state database. The next section discusses the flooding process through which database synchronization is achieved, and the last section tops the earlier discussions with an overview of the SPF algorithm, also known as the Dijkstra algorithm.
The operation of a link-state protocol requires each node in an area to have a complete view of the entire area and, using that knowledge, calculate the best paths to each destination in the area, starting with itself. As indicated previously, LSPs are the vehicles for propagating each router's limited view of its immediate surroundings; therefore, the assembly of LSPs by routers to obtain the complete routing picture of the network frequently has been compared to the process of solving a jigsaw puzzle. The solved puzzle represents the entire picture of the network or its complete topology. Each of the unique Level 1 databases represents the state of adjacencies within a specific area, while the Level 2 database represents the interconnections among the various areas in the domain. On Cisco routers, the command show isis database [ detail | level-11 level-2 ] [ lspid ] can be used to view the LSPs in the Level 1 or Level 2 databases. Exercising the detail option of the command displays details about elements in all known LSPs or a specific LSP. Figure 10-11 shows the format of an LSP.
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