139
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 40 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). Codes 128 through 132 are defined in RFC 1195.
Codes 222 through 237 are defined in RFC 5120.
Table 12 CLV codes and PDU types
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
No. of Octets
1
1
Length
Code
Length
Value
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