IS-IS: Hello Packets

Hello packets also referred to as IIH (IS-IS Hello) are used to perform tasks such as detecting neighboring routers, establishing and maintaining adjacencies, and electing a Designated IS (DIS).

On broadcast type interfaces IS-IS uses separate L1 and L2 Hello packets.

On point to point interfaces IS-IS uses a single L1L2 packet, also referred to as a point to point hello.

Hellos are sent every 10 seconds by default and can be configured between 1 – 65535 seconds using interface sub commands.

Instead of defining a specific Hold time, its based off a multiplier to determine the amount of time it takes for Hellos to detect a change.  The default hello multiplier value is 3, resulting in a default of 30 seconds hold time.  This can also be changed per interface using interface subcommands.  Timers do not need to match for IS-IS adjacencies to form.

DIS timers are always one third of the configured timers this allows the DIS to more readily inform the areas neighbors that it has gone down.


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