The following sections provide additional information on the following packet types:

  • IS-IS packets

  • Hello packets

  • Link-state packets

  • Sequence number packets


IS-IS Packet Fields (Alphabetical Order)

  • ATT— Specifies the attachment bits (flag attachments to other areas).

  • Checksum— Gives the checksum of the contents of the LSP from the source ID to the end.

  • Circuit Type— Defines whether the link is Level 1 and/or Level 2.

  • End LSP— Is the LSP ID of the last LSP in CSNP.

  • Holding Time— Defines how long to wait for a hello from this system before clearing the adjacency.

  • ID Length— Gives the length of the system ID field in an NSAP(NET).

  • Intradomain Routing Protocol Discriminator— Is the network layer protocol identifier

  • IS Type— Defines the type of router, Level 1 or Level 2.

  • LAN ID— Consists of the system ID of the designated intermediate system, plus a unique number as an identifier of the LAN.

  • Length Indicator— Gives the length of the fixed header of the packet, in bytes.

  • Local Circuit ID— Is a unique identifier for a link.

  • LSP ID— Is an identifier for a router’s LSP, consisting of the system ID of the router, a fragment number, and a nonzero octet for the pseudonode number, in case of a pseudonode LSP.

  • Maximum Area Addresses— Specifies the number of areas permitted.

  • OL— Is an LSP overload bit (also represented as LSPDBOL).

  • P— Is the partition repair bit.

  • PDU Length— Gives the length of the packet (PDU), in bytes.

  • PDU Type— Specifies the type of packet.

  • Priority— Shows the priority for a node for DIS arbitration.

  • R— See Reserved.

  • Remaining Lifetime— Specifies the remaining time for an LSP to expire.

  • Reserved— Consists of unspecified fields. Is transmitted as zeros and ignored on receipt.

  • Sequence Number— Is the sequence number of the LSP.

  • Source ID— Is the same as the system identifier (SysID).

  • TLV Fields— Consists of Type (or code), Length, and Value fields. Also known as variable-length fields.

  • Version/Protocol ID Extension— Applies to the IS-IS protocol (defined as 1).


Hello Packets
Figure 10-21. LAN Level 1 Hello Packet Format (PDU Type 15)

Figure 10-22. LAN Level 2 Hello Packets (PDU Type 16)

Figure 10-23. Point-to-Point Hello Packets (PDU Type 17)

Link-State Packets
Figure 10-24. Level 1 Link-State Packets (PDU Type 18)

Figure 10-25. Level 2 Link-State Packets (PDU Type 20)

Sequence Number Packets
Figure 10-26. Complete Sequence Number Packets (PDU Type 24)

Figure 10-27. Level 2 Complete Sequence Number Packets (PDU Type 25)

Figure 10-28. Level 1 Partial Sequence Number Packets (PDU Type 26)

Figure 10-29. Level 2 Partial Sequence Number Packets (PDU Type 27)