127
NOTE:
On an IS-IS broadcast network, all routers establish adjacency relationships, but they synchronize their
LSDBs through the DIS.
IS-IS PDUs
PDU
IS-IS PDUs are encapsulated into link layer frames. An IS-IS PDU has 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.
Figure 36 PDU format
Table 4 PDU types
T
e PDU T
e Acron
m
15 Level-1 LAN IS-IS hello PDU L1 LAN IIH
16 Level-2 LAN IS-IS hello PDU L2 LAN IIH
17 Point-to-Point IS-IS hello PDU P2P IIH
18 Level-1 Link State PDU L1 LSP
20 Level-2 Link State PDU L2 LSP
24 Level-1 Complete Sequence Numbers PDU L1 CSNP
25 Level-2 Complete Sequence Numbers PDU L2 CSNP
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.
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