The ISIS Hello [IIH] PDU in the ISIS operation has two functions:
1 – Discover neighbors to establish the adjacency with them.
2 – Act as acknowledgement to maintain the adjacency and to inform the neighbor if there is any adjacency parameter change.
There are two types of ISIS Hello PDU
1 – LAN Hellos are divided into on the types as per the adjacency type and they are both identical in format.
– LAN Hello PDU Level-1.
– LAN Hello PDU Level-2.
2 – Point-to-Point Hellos are also divided as per the level of adjacency.
– Point-to-Point Level-1
– Point-to-Point Level-2
LAN Hello PDU
ISIS LAN Hello format starts from the 9th octet of the PDU because all PDU types are sharing the 1st 8-Octets, so from the 9th octet; every type of PDU has a unique field .
The first octet of the LAN Hello PDU is [Circuit Type]:
Circuit Type is 8-bits field
– 6-bits are reserved and are always zero.
– 2-bits field are changeable as per the level of ISIS adjacency which needed to be established. If the Hello PDU sent to establish Level-1 adjacency then the 2-bits would be  , If Level-2 it would be  if Level-1/Level2 it would be .
The 2nd Hello PDU field is [Source ID].
– Source ID is the System ID length of the originator router of the [IIH] which is 6-octets with Cisco Implementation.
The 3rd Hello PDU field is [Holding time].
– Holding time field carries the period which the neighbor should wait to hear the next IIH before considering the originator dead.
The 4th Hello PDU field is [ PDU Length]
– PDU Length is the size of the entire PDU in octets
The 5th Hello PDU field is [Priority]
– Priority is a 7-bits field used for DIS election process, this 7-bits are changeable based on the Originator Priority configuration, the value is changeable from 0 to 127 , the router with highest priority in the same broadcast domain will be the DIS (Note: There is no Backup DIS in ISIS).
– The 6th Hello PDU field is [LAN ID].
– LAN ID is the DIS System-ID plus one more octet called the [Pseudonode], it identifies the Broadcast segments from another, as the same router can act as DIS in multiple Broadcast segments.
In addition to the main fields explained above, PDUs have another field(s) called TLVs [ Type Length Value]; this field adds more capabilities to IS-IS based on the type of PDU, the PDU controls the type of TLVs that could be appended
More explanation about the TLVs will follow in upcoming threads.
Point-to-Point Hello PDU
In general Point-to-Point Hello PDU format is Similar to LAN Hello PDU, also point-to-point IIH L1/L2 information could be carried in the same Point-to-Point PDU , in Point-to-Point there is no Priority field and instead of the LAN ID field there is the [Local Circuit ID] field.
– Circuit ID field is 1-Octet ID assigned by the Originator Router and it is unique among the Router Interfaces it self and the Neighbor may have another Circuit ID or the same one.