3Com S7906E Configuration Manual page 726

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:
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.
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;
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 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.
Dynamic host name mapping mechanism
The dynamic host name mapping mechanism provides the mappings between the host names and the
system IDs for the IS-IS routers. The dynamic host name information 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 a broadcast network,
which is announced in the dynamic host name TLV of a pseudonode LSP.
A host name is easier to remember than a system ID. After enabling this feature on the router, you can
see the host names instead of system IDs using the display command.
BFD
For more information about BFD, refer to BFD Configuration in the High Availability Volume.
Bidirectional forwarding detection (BFD) provides a single mechanism to quickly detect and monitor the
connectivity of links between IS-IS neighbors, thus reducing network convergence time.
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