H3C Technologies H3C S10500 Series Switches - Manual (Page 161)

146
Number field, allowing a maximum of only 256 fragments to be generated by an IS-IS router, limits the
amount of link information 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
• 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.
• System ID
System ID of the originating system
• 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.
• Virtual system
A virtual system is identified by an additional system ID and generates extended LSP fragments.
• Original LSP
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
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 the following modes:
• 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. 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. The IS-IS routers not supporting LSP fragment extension can operate
properly 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: Applicable to a network where all the routers support LSP fragment extension. In this mode,
all the IS-IS routers know which virtual system belongs to which originating system; 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
information in the extended fragments, and is recommended for an area where all the routers are at the
same routing level and support LSP fragment extension.