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.

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s