130
Type PDU Type Acronym
26 Level-1 Partial Sequence Numbers PDU L1 PSNP
27 Level-2 Partial Sequence Numbers PDU L2 PSNP
Hello PDU
IS-to-IS hello (IIH) PDUs are used by routers to establish and maintain neighbor relationships. On
broadcast networks, Level-1 routers use Level-1 LAN IIHs, and Level-2 routers use Level-2 LAN IIHs.
The P2P IIHs are used on point-to-point networks.
LSP
The LSPs carry link state information. LSPs include Level-1 LSPs and Level-2 LSPs. 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.
SNP
A sequence number PDU (SNP) describes the complete or partial LSPs for LSDB synchronization.
SNPs include CSNP and PSNP, which are further divided into Level-1 CSNP, Level-2 CSNP, Level-1
PSNP, and Level-2 PSNP.
A CSNP describes the summary of all LSPs for LSDB synchronization between neighboring routers.
On broadcast networks, CSNPs are sent by the DIS periodically (every 10 seconds by default). On
point-to-point networks, CSNPs are sent only during the first adjacency establishment.
A 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 missing LSPs from a neighbor.
CLV
The variable fields of PDU comprise multiple Code-Length-Value (CLV) triplets.
Figure 38 CLV format
Table 12 shows that different PDUs contain different CLVs. Codes 1 through 10 are defined in ISO
10589 (code 3 and 5 are not shown in the table), and others are defined in RFC 1195.
Table 12 CLV codes and PDU types
CLV Code Name PDU Type
1 Area Addresses IIH, LSP
2 IS Neighbors (LSP) LSP
4 Partition Designated Level 2 IS L2 LSP
6 IS Neighbors (MAC Address) LAN IIH
7 IS Neighbors (SNPA Address) LAN IIH
8 Padding IIH
9 LSP Entries SNP
10 Authentication Information IIH, LSP, SNP
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