and TLV fields.An LSP header includes the following: TLV variable-length fields contain the following:
Content 4.3 IS-IS Operation 4.3.3 LSP Headers LSPs are given sequence numbers that allow receiving routers to do the following: If a router reloads, the sequence number is set to 1. The router then receives its previous LSPs from its neighbors. The received LSPs contain the last valid sequence number before the router reloaded. The router records this number and reissues its own LSPs with the next-highest sequence number. Each LSP has a remaining lifetime that is used by the LSP aging process to ensure the removal of outdated and invalid LSPs from the topology table after a suitable time. This process is known as the count to zero operation. The default start value is 1200 seconds. Each LSP includes specific information about networks and stations attached to a router. This information is found in multiple TLV fields that follow the common header of the LSP. TLV is sometimes referred to as Code, Length, Value (CLV). The TLV structure is a flexible way to add data to the LSP and an easy mechanism for adding new data fields that may be required in the future. Figure shows examples of TLVs. You can find documentation on important TLVs in ISO 10589 and RFC 1195.
Content 4.3 IS-IS Operation 4.3.4 Implementing IS-IS in NBMA Networks Network topologies can be divided into two general types: IS-IS supports the following two media representations for its link states: Note
IS-IS has no concept of nonbroadcast multiaccess (NBMA) networks. It is recommended that you use point-to-point links, such as point-to-point subinterfaces, over NBMA networks such as ATM, Frame Relay, or X.25. Cisco IOS software automatically uses broadcast mode for LAN links and multipoint WAN links. It uses point-to-point mode for point-to-point links, such as point-to-point subinterfaces and dialer interfaces. There is no specific support for NBMA networks in IS-IS. When implemented in broadcast mode, Cisco IOS software assumes that the NBMA environment features a full mesh of PVCs. Use the broadcast keyword when creating static maps to map the remote IP address to the local DLCI on a Frame Relay interface, because broadcast mode uses multicast updates that will not be sent unless this keyword is set. When you use multipoint WAN links, such as multipoint Frame Relay interfaces, you must also allow CLNS broadcasts and multicasts. You can do this using the command frame relay map clns dlci-number broadcast (in addition to creating the IP maps). It is highly recommended that you implement NBMA environments, such as Frame Relay, as point-to-point links (using subinterfaces) instead of multipoint links.
Content 4.3 IS-IS Operation 4.3.5 Implementing IS-IS in Broadcast Networks Broadcast networks are LAN interfaces or multipoint WAN interfaces. Note
Broadcast mode is recommended on LAN interfaces only, although it is also the default for multipoint WANs. Separate adjacencies are established for Level 1 and Level 2. If two neighboring routers in the same area run both Level 1 and 2, they establish two adjacencies, one for each level. The router stores the Level 1 and Level 2 adjacencies in separate adjacency tables. On LANs, routers establish the two adjacencies with specific Layer 1 and Layer 2 IIH PDUs. Routers on a LAN establish adjacencies with all other routers on the LAN (unlike OSPF, where routers establish adjacencies only with the DR and backup designated router [BDR]). IIH PDUs announce the area address. Separate IIH packets announce the Level 1 and Level 2 neighbors. Adjacencies form based on the area address communicated in the incoming IIH and the type of router (Level 1 or Level 2). Level 1 routers accept Level 1 IIH PDUs from their own area and establish adjacencies with other routers in their own area. Level 2 routers (or the Level 2 process within any Level 1–2 router) accept only Level 2 IIH PDUs and establish only Level 2 adjacencies. Dijkstra’s algorithm requires a virtual router (a pseudonode), represented by the Designated Intermediate System (DIS), to build a directed graph for broadcast media. Criteria for DIS selection are, first, highest priority (the priority value is configurable) and, second, highest SNPA (on LANs, the SNPA is the MAC address). Cisco router interfaces have a default Level 1 and Level 2 priority of 64. You can configure a priority from 0 to 127 using the isis priority number-value [level-1 | level-2] command. The Level 1 DIS and the Level 2 DIS on a LAN may or may not be the same router, because an interface can have different Level 1 and Level 2 priorities. Unlike OSPF, the DIS process is preemptive. A selected router is not guaranteed to remain the DIS. Any adjacent IS with a higher interface priority automatically takes over the DIS role. Because the IS-IS LSDB is synchronized frequently on a LAN, giving priority to another IS over the DIS is not a significant issue. Unlike OSPF, IS-IS does not use a backup DIS, and routers on a LAN establish adjacencies both with the DIS and with all other routers. In IS-IS, a broadcast link itself is modeled as a pseudonode that connects all attached routers to a star-shaped topology. The pseudonode is represented by the DIS. Rather than having each router connected to the LAN advertise an adjacency with every other router on the LAN, each router (including the DIS) just advertises a single adjacency to the pseudonode. Otherwise, each IS on a broadcast network with n connected ISs would require (n) (n – 1) / 2 adjacency advertisements. Generating LSPs for each adjacency uses considerable overhead in terms of LSDB synchronization. The DIS generates the pseudonode LSPs. A pseudonode LSP details only the adjacent ISs (for example, the ISs connected to that LAN). The pseudonode LSP is used to build the map of the network and to calculate the shortest path first (SPF) tree. The pseudonode LSP is the equivalent of a network link-state advertisement (LSA) in OSPF. In IS-IS, all routers on the LAN establish adjacencies with all other routers and with the DIS. Therefore, if the DIS fails, another router takes over immediately with little or no impact on the topology of the network. There is no backup DIS. Contrast this with OSPF, where the DR and BDR are selected, and the other routers on the LAN establish full adjacencies with the DR and BDR. In case of DR failure, the BDR is promoted to DR, and a new BDR is elected.
Content 4.3 IS-IS Operation 4.3.6 LSP and IIH Levels Level 1 and Level 2 LSP
IS-IS uses a two-level area hierarchy. The link-state information for these two levels is distributed separately, which results in Level 1 LSPs and Level 2 LSPs. Each IS originates its own LSPs (one for Level 1 and one for Level 2). On a LAN, one router (the DIS) sends out LSP