IS-IS Designated Intermediate System (DIS) Tasks

On broadcast multi-access networks, a single router is elected as the DIS. There is no backup DIS elected. The DIS is the router that creates the pseudonode and acts on behalf of the pseudonode.

Two major tasks are performed by the DIS:

1. Creating and updating pseudonode LSP for reporting links to all systems on the broadcast subnetwork. See the Pseudenode LSP section for more information.

2. Flooding LSPs over the LAN.

Flooding over the LAN means that the DIS sends periodic complete sequence number protocol data units (CSNPs) (default setting of 10 seconds) summarizing the following information:

LSP ID
Sequence Number
Checksum
Remaining Lifetime

The DIS is responsible for flooding. It creates and floods a new pseudonode LSP for each routing level in which it is participating (Level 1 or Level 2) and for each LAN to which it is connected. A router can be the DIS for all connected LANs or a subset of connected LANs, depending on the IS-IS priority or the Layer 2 address. The DIS will also create and flood a new pseudonode LSP when a neighbor adjacency is established, torn down, or the refresh interval timer expires. The DIS mechanism reduces the amount of flooding on LANs.
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