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| 7.3 |  | IS-IS Operation |
| 7.3.8 |  | IS-IS data flow |

In IS-IS, routers may have adjacencies with other routers on point-to-point links. In a LAN environment, routers report their adjacencies to the DIS, which generates an additional LSP, commonly known as the pseudonode LSP. The DIS is responsible for conducting flooding over the LAN and also for maintaining synchronization.

The flow of information within the IS-IS routing function is represented by the IS-IS data-flow diagram. This consists of four processes and a Routing Information Base (RIB). The RIB consists of the link-state database and the forwarding database. The four processes in the IS-IS data-flow diagram are receive, update, decision, and forward.

The receive process is the entry point for all data, including user data, error reports, routing information, and control packets. The receive process passes user data and error reports to the forward process. It passes routing information and control packets, such as Hello messages, LSPs, and sequence number packets, to the update process.

The update process generates local link information that is flooded to adjacent routers. In addition, the update process receives, processes, and forwards link information received from adjacent routers. This process manages the L1 and L2 link-state databases and floods L1 and L2 LSPs throughout an area. Each LSP that resides in the link-state database has a remaining lifetime, a checksum, and a sequence number. The LSP remaining lifetime counts down from 1200 seconds to 0 (zero). Twenty minutes is the MaxAge. The LSP originator must periodically refresh its LSPs to prevent the remaining lifetime from reaching 0 (zero). The refresh interval is 15 minutes, with a random jitter of up to 25 percent. If the remaining lifetime reaches 0 (zero), the expired LSP will be kept in the database for an additional 60 seconds before it is purged. This additional 60 seconds is known as ZeroAgeLifetime. If a router receives an LSP with an incorrect checksum, the router will cause a purge of the LSP. The router does this by setting the remaining lifetime value to 0 (zero), removing the LSP data, and reflooding it. This triggers the LSP originator to send a new LSP. This behavior is different from that of OSPF, where only the originating router can purge an LSP. IS-IS can be configured so that LSPs with incorrect checksums are not purged, but the router that originated the LSP will not know that the LSP was not received.

The decision process runs the shortest path first (SPF) algorithm on the link-state database, and creates the forwarding database. It computes next-hop information and computes sets of equal-cost paths, creating an adjacency set that is used for load balancing. On a Cisco router, IS-IS supports load balancing over and up to six equal-cost paths.

The forward process gets its input from the receive process and uses the forwarding database to forward data packets toward their destination. It also redirects load sharing and generates error reports.

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