Comprehensive and Detailed In-Depth Explanation:

In IS-IS (Intermediate System to Intermediate System), each LSP (Link-State PDU) is identified by an LSP ID.

The LSP Number field is 4 bits long, meaning a single IS-IS process can generate up to 16 LSP fragments per system ID.

Since each fragment carries a limited amount of information, a router may exceed this limit if it needs to advertise a large number of routes.

Solution: By adding additional system IDs, an IS-IS router can generate more LSP fragments, thereby overcoming the limitation.

This method is commonly used in large-scale IS-IS networks to ensure scalability and efficient routing information exchange.

???? Reference: HCIP-Datacom Advanced Routing & Switching Technology – IS-IS LSP Fragmentation Mechanism