IS-IS packets are encapsulated into link layer frames. The Protocol Data Unit (PDU) consists of two parts, the headers and the variable length fields. The headers comprise the PDU common header and the PDU specific header. All PDUs have the same PDU common header. The specific headers vary by PDU type.
Major fields of the PDU common header are as follows:
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Intradomain routing protocol discriminator—Set to 0x83.
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Length indicator—Length of the PDU header in bytes, including both common and specific headers.
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Version/Protocol ID extension—Set to 1(0x01).
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ID length—Length of the NSAP address and NET ID.
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R (Reserved)—Set to 0.
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PDU type—See Table 4.
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Version—Set to 1(0x01).
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Maximum area address—Maximum number of area addresses supported.
Hello packets are used by routers to establish and maintain neighbor relationships. A hello packet is also an IS-to-IS hello PDU (IIH). For broadcast networks, the Level-1 routers use the Level-1 LAN IIHs; and the Level-2 routers use the Level-2 LAN IIHs. The P2P IIHs are used on point-to-point networks.
Figure 55 illustrates the hello packet format in broadcast networks, where the blue fields are the common header.
Major fields of the L1/L2 LAN IIH are as follows:
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Reserved/Circuit type—The first six bits are reserved with a value of 0. The last two bits indicate the router type. Here, 00 means reserved, 01 indicates L1, 10 indicates L2, and 11 indicates L1/2.
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Source ID—System ID of the router advertising the hello packet.
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Holding time—If no hello packets are received from the neighbor within the holding time, the neighbor is considered down.
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PDU length—Total length of the PDU in bytes.
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Priority—DIS priority.
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LAN ID—Includes the system ID and a one-byte pseudonode ID.
Figure 56 shows the hello packet format on the point-to-point networks.
Instead of the priority and LAN ID fields in the LAN IIH, the P2P IIH has a Local Circuit ID field.
The Link State PDU (LSP) carries link state information. LSP involves two types: Level-1 LSP and Level-2 LSP. The Level-2 LSPs are sent by the Level-2 routers, and the Level-1 LSPs are sent by the Level-1 routers. The Level-1-2 router can send both types of LSPs.
The two types of LSPs have the same format.
Major fields of the L1/L2 LSP are as follows:
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PDU length—Total length of the PDU in bytes.
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Remaining lifetime—LSP remaining lifetime in seconds.
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LSP ID—Consists of the system ID, the pseudonode ID (one byte) and the LSP fragment number (one byte).
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Sequence number—LSP sequence number.
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Checksum—LSP checksum.
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P (Partition Repair)—Partition bit that is only for L2 LSPs; it indicates whether the router supports partition repair.
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ATT (Attachment)—Attach bit that is generated by a L1/L1 router for L1 LSPs only; it indicates that the router generating the LSP is connected to multiple areas.
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OL (LSDB Overload)—Indicates that the LSDB is not complete because the router has run out of memory. Other routers will not send packets to the overloaded router, except packets destined to the networks directly connected to the router. For example, in Figure 58, Router A forwards packets to Router C through Router B. Once other routers know the OL field of LSPs from Router B is set to 1, Router A will send packets to Router C via Router D and Router E, but still send to Router B packets destined to the network directly connected to Router B.
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IS type—Type of the router generating the LSP.
A sequence number PDU (SNP) acknowledges the latest received LSPs. It is similar to an Acknowledge packet, but more efficient.
SNP involves Complete SNP (CSNP) and Partial SNP (PSNP), which are further divided into Level-1 CSNP, Level-2 CSNP, Level-1 PSNP and Level-2 PSNP.
CSNP covers the summary of all LSPs in the LSDB to synchronize the LSDB between neighboring routers. On broadcast networks, CSNP is sent by the DIS periodically (10s by default). On point-to-point networks, CSNP is only sent during the adjacency establishment.
The CSNP packet format is shown in Figure 59.
PSNP only contains the sequence numbers of one or multiple latest received LSPs. It can acknowledge multiple LSPs at one time. When LSDBs are not synchronized, a PSNP is used to request new LSPs from neighbors.
The variable fields of PDU comprise multiple Code-Length-Value (CLV) triplets.
Table 5 shows that different PDUs contain different CLVs. Code 1 to 10 of CLV are defined in ISO 10589 (code 3 and 5 are not shown in the table), and others are defined in RFC 1195.
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