The maximum size of an LSP is 1492 bytes.
Available space = 1492 – 27 (Header) = 1465 bytes for TLVs.
Thus an IS-IS router has theoretically up to 256*1465 of space to pack IP reachability TLVs.
The following calculation enables us to determine the number of IP prefixes that can be advertised in an LSP.
The following constraints are to be considered in the calculation:
The maximum size (maxLSPsize) of an LSP is 1492 bytes.
The LSP header (lspHeadersize) is 27 bytes.
The maximum length of a TLV (maxTLVlength) is 255 bytes.
Each TLV 128 consists of type (1 byte), length (1 byte), and IP prefixes (n x 12 bytes) up to total of 255 bytes. The maximum number of fragments of an LSP (maxLSPfragments) is 256.
The number of fragments is determined from the 1-byte LSP Number field in the LSP identifier.
The first fragment contains other TLVs, and the remaining 255 fragments are packed with only TLV 128.
The actual calculation is as follows:
The total space available for TLVs in an LSP is TLVSpace = maxLSPsize – lspHeadersize = 1492 – 27 = 1465 bytes
The number of TLVs that can fit into TLVSpace is 1465/255 = 5.7, approximately 6
Assuming a 1-byte Type field and 1-byte Length field, overhead for 6 TLVs is 6 x 2 = 12 bytes.
Actual space available for prefixes is 1465 – 12 bytes overhead = 1453 bytes
Number of prefixes, each 12 bytes (address + subnet mask + metric) that can fit into TLVSpace is 1453/12 = 121.08 (approximately 121 IP prefixes per LSP)
Considering that few other TLVs can be generated by the router, the number of IP prefixes that can be supported per IS-IS router is 256 fragments, each containing 121 prefixes, for a total of 30,976 prefixes.