ISIS数据包结构

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)