IS-IS Overview 891■ Mode-1: It applies to a network where some routers do not support LSPfragment extension. In this mode, an adjacency is formed between theoriginating system and each virtual system, with the link cost from theoriginating system to each virtual system as 0. Thus, each virtual system acts asa router connected to the originating system in the network, but the virtualsystem is reachable through the originating system only. Therefore, the IS-ISrouters not supporting LSP fragment extension can operate normally withoutmodifying the extended LSP fragments received, but some limitation isimposed on the link state information in the extended LSP fragmentsadvertised by the virtual systems.■ Mode-2: This mode is recommended in a network where all the routerssupport LSP fragment extension. In this mode, all the IS-IS routers in thenetwork know which originating system the LSPs generated by the virtualsystems belong to; therefore, no limitation is imposed on the link stateinformation of the extended LSP fragments advertised by the virtual systems.The operation mode of LSP fragment extension is configured based on area androuting level. Mode-1 is backward-compatible and allows the routers supportingLSP fragment extension and those not supporting this feature to interoperate witheach other, but it restricts the link state information in the extended fragments.Mode-2 does not restrict the link state information in the extended fragments.Mode-2 is recommended in a network where all the routers that are in the samearea and at the same routing level support LSP fragment extension.Dynamic host name mapping mechanismThe dynamic host name mapping mechanism provides the mapping between thehost names and the system IDs for the IS-IS routers. The dynamic host nameinformation is announced in the dynamic host name CLV of an LSP.This mechanism also provides the mapping between a host name and the DIS of abroadcast network, which is announced in a dynamic host name TLV of apseudonode LSP.A host name is intuitively easier to remember than a system ID. After enabling thisfeature on the router, you can see the host names instead of system IDs after usingthe display command.Protocols and Standards ■ ISO 10589 ISO IS-IS Routing Protocol■ ISO 9542 ES-IS Routing Protocol■ ISO 8348/Ad2 Network Services Access Points■ RFC 1195 - Use of OSI IS-IS for Routing in TCP/IP and Dual Environments■ RFC 2763 - Dynamic Hostname Exchange Mechanism for IS-IS■ RFC 2966 - Domain-wide Prefix Distribution with Two-Level IS-IS■ RFC 2973 - IS-IS Mesh Groups■ RFC 3277 - IS-IS Transient Blackhole Avoidance■ RFC 3358 - Optional Checksums in ISIS■ RFC 3373 - Three-Way Handshake for IS-IS Point-to-Point Adjacencies
