IS-IS LSP fragment extension
When the information in the Protocol Data Unit (PDU) to be advertised by IS-IS is too large, the IS-IS router generates multiple LSP fragments to carry more IS-IS information.
The IS-IS LSP fragment is identified by the LSP Number field in the LSP ID. The length of this field is 1 byte. Therefore, an IS-IS process can generate a maximum of 256 LSP fragments with limited information. As specified in RFC3786, IS-IS can configure virtual SystemID and generate LSP packets of virtual IS-IS to carry routing information.
· Initial system (Originating System): The initial system is a router that runs the IS-IS protocol. A single IS-IS process can advertise LSPs like multiple virtual routers, whereas Originating System refers to the real IS-IS process.
· System ID (Normal System-ID): System ID of the initial system.
Virtual system (Virtual System): The system identified by the additional system ID is used to generate extended LSP fragments. These fragments carry additional system IDs in their LSP IDs.
· Additional system ID (Additional System-ID): System ID of the virtual system, which is allocated by the network manager. Each additional system ID can generate 256 extended LSP fragments.
TLV ·24 (IS Alias ID TLV): Indicates the relationship between the initial system and the virtual system.
In IS-IS, each system ID identifies a system. Each system can generate a maximum of 256 LSP fragments. By adding an additional system ID, you can configure a maximum of 50 virtual systems so that the IS-IS process can generate a maximum of 13056 LSP fragments.
After the fragment extension function is enabled, the system prompts you to restart IS-IS if information is lost due to packet overflow. After the restart, the initial system loads routing information as much as possible. The information that is not installed is sent out in the LSP of the virtual system, and TLV 24 is used to notify other routers of the relationship between the virtual system and the virtual system.
