clude these Intermediate systems (and no others) in the Link State PDU it generates for the pseudonode representing the broadcast subnetwork.7.2.5 Multiple LSPs for the same systemThe Update process is capable of dividing a single logical LSP into a number of separate PDUs for the purpose of conserving link bandwidth and processing (see 7.3.4).  The Decision Process, on the other hand, shall regard the LSP with LSP Number zero in a special way. If the LSP with LSP Number zero and remaining lifetime > 0, is not present for a particular system then the Decision Process shall not process any LSPs with non-zero LSP Number which may be stored for that system. The following information shall be taken only from the LSP with LSP Number zero. Any values which may be present in other LSPs for that system shall be disregarded by the Decision Process.a)The setting of the LSP Database Overload bit. b)The value of the IS Type field. c)The Area Addresses option.7.2.6 Routeing Algorithm OverviewThe routeing algorithm used by the Decision Process is a shortest path first (SPF) algorithm. Instances of the algorithm are run independently and concurrently by all Intermediate systems in a routeing domain. Intra-Domain routeing of a PDU occurs on a hop-by-hop basis: that is, the algorithm determines only the next hop, not the complete path, that a data PDU will take to reach its destination. To guarantee correct and consistent route computation by every Intermediate system in a routeing domain, this International Standard depends on the following properties: a)All Intermediate systems in the routeing domain converge to using identical topology information; andb)Each Intermediate system in the routeing domain generates the same set of routes from the same input topology and set of metrics.The first property is necessary in order to prevent inconsistent, potentially looping paths. The second property is necessary to meet the goal of determinism stated in 6.6.A system executes the SPF algorithm to find a set of legal paths to a destination system in the routeing domain. The set may consist of:a)a single path of minimum metric sum: these are termed minimum cost paths;b)a set of paths of equal minimum metric sum: these are termed equal minimum cost paths; orc)a set of paths which will get a PDU closer to its destination than the local system: these are called downstream paths.Paths which do not meet the above conditions are illegal and shall not be used.The Decision Process, in determining its paths, also ascertains the identity of the adjacency which lies on the first hop to the destination on each path. These adjacencies are used to form the Forwarding Database,  which the forwarding process uses for relaying PDUs.Separate route calculations are made for each pairing of a level in the routeing hierarchy (i.e. L1 and L2) with a supported routeing metric. Since there are four routeing metrics and two levels some systems may execute multiple instances of the SPF algorithm. For example,-if an IS is a L2 Intermediate system which supports all four metrics and computes minimum cost paths for all metrics, it would execute the SPF calculation eight times.-if an IS is a L1 Intermediate system which supports all four metrics, and additionally computes downstream paths, it would execute the algorithm  4 W (number of neighbours + 1) times.Any implementation of an SPF algorithm meeting both the static and dynamic conformance requirements of clause 12 of this International Standard may be used. Recommended implementations are described in detail in Annex C.7.2.7 Removal of Excess PathsWhen there are more than maximumPathSplits legal paths to a destination, this set shall be pruned until only maximumPathSplits remain. The Intermediate system shall discriminate based upon:NOTE - The precise precedence among the paths is specified in order to meet the goal of determinism defined in 6.6. -adjacency type: Paths associated with End system or level 2 reachable address prefix adjacencies are retained in preference to other adjacencies-metric sum: Paths having a lesser metric sum are retained in preference to paths having a greater metric sum. By metric sum is understood the sum of the metrics along the path to the destination.-neighbour ID: where two or more paths are associated with adjacencies of the same type, an adjacency with a lower neighbour ID is retained in preference to an adjacency with a higher neighbour id.-circuit ID: where two or more paths are associated with adjacencies of the same type, and same neighbour ID, an adjacency with a lower circuit ID is retained in preference to an adjacency with a higher circuit ID, where circuit ID is the value of:7ptPtCircuitID for non-broadcast circuits, 7l1CircuitID for broadcast circuits when running the Level 1 Decision Process, and 7l2CircuitID for broadcast circuits when running the Level 2 Decision Process. -lANAddress: where two or more adjacencies are of the same type, same neighbour ID, and same circuit ID (e.g. a system with multiple LAN adapters on the same circuit) an adjacency with a lower lANAddress is retained in preference to an adjacency with a higher lANAddress.7.2.8 Robustness Checks7.2.8.1 Computing Routes through Overloaded Intermediate systemsThe Decision Process shall not utilise a link to an Intermediate system neighbour from an IS whose LSPs have the LSP Database Overload indication set. Such paths may introduce loops since the overloaded IS does not have a complete routeing information base. The Decision Process shall, however utilise the link to reach End system neighbours since these paths are guaranteed to be non-looping.7.2.8.2 Two-way connectivity checkThe Decision Process shall not utilise a link between two Intermediate Systems unless both ISs report the link. NOTE - the check is not applicable to links to an End System.Reporting the link indicates that it has a defined value for at least the default routeing metric. It is permissible for two endpoints to report different defined values of the same metric for the same link. In this case, routes may be asymmetric. 7.2.9 Construction of a Forwarding DatabaseThe information that is needed in the forwarding database for routeing metric k is the set of adjacencies for each system N.7.2.9.1 Identification of Nearest Level 2 IS by a Level 1 ISLevel 1 Intermediate systems need one additional piece of information per routeing metric: the next hop to the nearest level 2 Intermediate system according to that routeing metric. A level 1 IS shall ascertain the set, R, of attached level 2 Intermediate system(s) for metric k such that the total cost to R for metric k is minimal. If there are more adjacencies in this set than maximum PathSplits, then the IS shall remove excess adjacencies as described in 7.2.7.7.2.9.2 Setting the Attached Flag in Level 2 Intermediate SystemsIf a level 2 Intermediate system discovers, after computing the level 2 routes for metric k, that it cannot reach any other areas using that metric, it shall:-set AttachedFlag for metric k to False;-regenerate its Level 1 LSP with LSP number zero; and-compute the nearest level 2 Intermediate system for metric k for insertion in the appropriate forwarding database, according to the algorithm described in 7.2.9.1 for level 1 Intermediate systems.NOTE - AttachedFlag for each metric k is examined by the Update Process, so that it will report the value in the ATT field of its Link State PDUs. If a level 2 Intermediate system discovers, after computing the level 2 routes for metric k, that it can reach at least one other area using that metric, it shall-set AttachedFlag for metric k to True;-regenerate its Level 1 LSP with LSP number zero; and -set the level 1 forwarding database entry for metric k which corresponds to nearest level 2 Intermediate system to Self.7.2.10 Information for Repairing Partitioned AreasAn area may become partitioned as a result of failure of one or more links in the area. However, if each of the partitions has a connection to the level 2 subdomain, it is possible to repair the partition via the level 2 subdomain, provided that the level 2 subdomain itself is not partitioned. This is illustrated in Figure 4.All the systems A  I, R and P are in the same area n. When the link between D and E is broken, the area be comes partitioned. Within each of the partitions the Partition Designated Level 2 Intermediate system is selected from among the level 2 Intermediate systems in that partition. In the case of partition 1 this is P, and in the case of partition 2 this is R. The level 1 repair path is then established between between these two level 2 Intermediate systems. Note that the repaired link is now between P and R, not between D and E.The Partition Designated Level 2 Intermediate Systems repair the partition by forwarding NPDUs destined for other partitions of the area through the level 2 subdomain. They do this by acting in their capacity as Level 1 Intermediate Systems and advertising in their Level 1 LSPs adjacencies to each Partition Designated Level 2 Intermediate System in the area. This adjacency is known as a Virtual Adjacency or Virtual Link. Thus other Level 1 Intermediate Systems in a partition calculate paths to the other partitions through the Partition Designated Level 2 Intermediate System. A Partition Designated Level 2 Intermediate System forwards the Level 1 NPDUs through the level 2 subdomain by encapsulating them in 8473 Data NPDUs with its Virtual Network Entity Title as the source NSAP and the adjacent Partition Designated Level 2 Intermediate System's Virtual Network Entity Title as the destination NSAP. The following sub-clauses describe this in more detail.7.2.10.1 Partition Detection and Virtual Level 1 Link CreationPartitions of a Level 1 area are detected by the Level 2 Intermediate System(s) operating within the area.  In order to participate in the partition repair process, these Level 2 Intermediate systems must also act as Level 1 Intermediate systems in the area. A partition of a given area exists whenever two or more Level 2 ISs located in that area are reported in the L2 LSPs as being a Partition Designated Level 2 IS. Conversely, when only one Level 2 IS in an area is reported as being the Partition Designated Level 2  IS, then that area is not partitioned.  Partition repair is accomplished by the Partition Designated Level 2 IS.  The election of the Partition Designated Level 2 IS as described in the next subsection must be done before the detection and repair process can begin.In order to repair a partition of a Level 1 area, the Partition designated Level 2 IS creates a Virtual Network Entity to represent the partition.  The Network Entity Title for this virtual network entity shall be constructed from the first listed area address from its Level 2 Link State PDU, and the ID of the Partition Designated Level 2 IS.  The IS shall also construct a virtual link (represented by a new Virtual Adjacency managed object) to each Partition Designated Level 2 IS in the area, with the NET of the partition recorded in the Identifier attribute.  The virtual links are the repair paths for the partition.  They are reported by the Partition Designated Level 2 IS into the entire Level 1 area by adding the ID of each adjacent Partition Designated Level 2 IS to the Intermediate System Neighbours field of its Level 1 Link State PDU.  The Virtual Flag shall be set True for these Intermediate System neighbours.  The metric value for this virtual link shall be the default metric value d(N) obtained from this system's Level 2 PATHS database, where N is the adjacent Partition Designated Level 2 IS via the Level 2 subdomain.An Intermediate System which operates as the Partition Designated Level 2 Intermediate System shall perform the following steps after completing the Level 2 shortest path computation in order to detect partitions in the Level 1 area and create repair paths: a)Examine Level 2 Link State PDUs of all Level 2 Intermediate systems. Search areaAddresses for any address that matches any of the addresses in partition AreaAddresses. If a match is found, and the Partition Designated Level 2 Intermediate system's ID does not equal this system's ID, then inform the level 1 update process at this system of the identity of the  Partition Designated Level 2 Intermediate system, together with the path cost for the default routeing metric to that Intermediate system.b)Continue examining Level 2 LSPs until all Partition Designated Level 2 Intermediate systems in other partitions of this area are found, and inform the Level 1 Update Process of all of the other Partition Designated Level 2 Intermediate systems in other partitions of this area, so that1)Level 1 Link State PDUs can be propagated to all other Partition designated level 2 Intermediate systems for this area (via the level 2 subdomain). 2)All the Partition Designated Level 2 Intermediate systems for other partitions of this area can be reported as adjacencies in this system's Level 1 Link State PDUs.If a partition has healed, the IS shall destroy the associated virtual network entity and virtual link by deleting the Virtual Adjacency.  The Partition Designated Level 2 IS detects a healed partition when another Partition Designated Level 2 IS listed as a virtual link in its Level 1 Link State PDU was not found after running the partition detection and virtual link creation algorithm describe