3Com Switch 4800G 24-Port Configuration Manual page 339

System ID
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The system ID of the originating system.
Additional System ID
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It is the additional virtual system ID configured for the IS-IS router after LSP
fragment extension is enabled. Each additional system ID can generate 256 LSP
fragments. Both the additional system ID and the system ID must be unique in the
entire routing domain.
Virtual System
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Virtual System is identified by the additional system ID and generates extended LSP
fragments.
Original LSP
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It is the LSP generated by the originating system. The system ID in its LSP ID field is
the system ID of the originating system.
Extended LSP
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It is the LSP generated by a virtual system. The system ID in its LSP ID field is the
virtual system ID.
After additional system IDs are configured, an IS-IS router can advertise more link
state information in extended LSP fragments. Each virtual system can be
considered as a virtual router. An extended LSP fragment is advertised by a virtual
system identified by additional system ID.
2 Operation modes
The LSP fragment extension feature operates in two modes on an IS-IS router:
Mode-1: It applies to a network where some routers do not support LSP
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fragment extension. In this mode, adjacency is formed between the originating
system and each virtual system, with the link cost from the originating system
to each virtual system as 0. Thus, each virtual system acts as a router connected
to the originating system in the network, but the virtual system is reachable
through the originating system only. Therefore, the IS-IS routers not supporting
LSP fragment extension can operate normally without modifying the extended
LSP fragments received, but some limitation is imposed on the link state
information in the extended LSP fragments advertised by the virtual systems.
Mode-2: This mode is recommended in a network where all the routers
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support LSP fragment extension. In this mode, all the IS-IS routers in the
network know which originating system the LSPs generated by the virtual
systems belong to; therefore no limitation is imposed on the link state
information of the extended LSP fragments advertised by the virtual systems.
The operation mode of LSP fragment extension is configured based on area and
routing level. Mode-1 allows the routers supporting and not supporting LSP
fragment extension to interoperate with each other, but it restricts the link state
information in the extended fragments. Mode-2 does not restrict the link state
IS-IS Overview 339