HP MSR Series Configuration Manual page 137

LSP fragment extension
IS-IS advertises link state information by flooding LSPs. Because one LSP carries a limited amount of link
state information, 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 1-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 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.
Terms:
•
Originating system—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 IS-IS
process that originally runs.
System ID—The 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.
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 correctly 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.
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