In the following chapter the structure of the different IS-IS hello packets will be described. Further more the amount of IS-IS packets will be treated. Hello packets will be used to set up and manage the neighbourhood.

In principle we can distinguish IS-IS hello (IIH) PDUs into three types of packets.

We distinguish between

• IS-IS point-to-point hello PDUs
• IS-IS hello PDUs LAN level 1
• IS-IS hello PDUs LAN Level 2

Table 4: IS-IS hello PDU LAN L1L2

Table 5: IS-IS hello point-to-point PDU

Meaning of the hello PDU-header fields:

Table 6: Description of the Hello-PDU-header fields

The hello PDUs are used in ES-IS to communicate between an End-System and an Intermediate-System. These hello packets guarantee the establishment of a connection and the capability of being reached between a host and an Intermediate-System. Hello packets use special multicast network addresses to prevent other systems in a network from getting overloaded. These addresses are “all hosts” and “all routers”.

It can be distinguished between ISH and ESH:

• With ISH packets the Intermediate-System transmits messages only to hosts in the same subnetwork. ISHs contain a physical address and the identity number of the IS.
• With ESH packets the host transmits messages only to Intermediate-Systems in the same subnetwork. ESHs contain a NSAP-Address, a physical address and the duration of validity of the information.

Hello PDUs are transmitted at regular intervals. As a consequence the network is always up-to-date.

Sources:

Abe Martey, IS-IS Network Design Solution, Kap. 3

CCNP 1 Ver. 3.1 Kap.7