133
Type PDU
Type Acronym
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
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.
illustrates the hello packet format in broadcast networks, where the blue fields are the common
header.
Figure 55
L1/L2 LAN IIH format
Major fields of the L1/L2 LAN IIH are as follows:
•
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.
•
Source ID
—System ID of the router advertising the hello packet.
•
Holding time
—If no hello packets are received from the neighbor within the holding time, the
neighbor is considered down.
•
PDU length
—Total length of the PDU in bytes.
•
Priority
—DIS priority.
