H3C Technologies H3C S7500E Series Switches - Manual (Page 160)

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LSP fragment extension
IS-IS advertises link state information by flooding LSPs. One LSP carries a limited amount of link state
information; therefore, IS-IS fragments LSPs. Each LSP fragment is uniquely identified by a
combination of the System ID, Pseudonode ID (0 for a common LSP or a non-zero value for a
Pseudonode LSP), and LSP Number (LSP fragment number) of the node or pseudo node that
generated the LSP. The one-byte LSP Number field, allowing a maximum of only 256 fragments to be
generated by an IS-IS router, limits the amount of link information that the IS-IS router can advertise.
The LSP fragment extension feature allows an IS-IS router to generate more LSP fragments. Up to 50
additional virtual systems can be configured on the router, and each virtual system is capable of
generating 256 LSP fragments to enable the IS-IS router to generate up to 13056 LSP fragments.
1) Terms
z Originating System
It is the router actually running IS-IS. After LSP fragment extension is enabled, additional virtual
systems can be configured for the router. Originating system is the actual IS-IS process that originally
runs.
z System ID: System ID of the originating system.
z Additional System ID
Additional virtual system IDs are 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.
z Virtual System
A virtual system is identified by an additional system ID and generates extended LSP fragments.
z Original LSP
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.
z Extended LSP
Extended LSPs are generated by virtual systems. 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 a virtual router. An extended LSP
fragment is advertised by a virtual system identified by an additional system ID.
2) Operation modes
The LSP fragment extension feature operates in two modes:
z Mode-1: Applicable to a network where some routers do not support LSP fragment extension. In
this mode, adjacencies are formed between the originating system and virtual systems, 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 systems are 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.