A Designated IS is an IS-IS router that is responsible for gathering and distributing link state information to other Level 1 or Level 2 ISs within the same broadcast network (LAN). The Level 1 and Level 2 Designated ISs within a broadcast network are independent, although the same device can be a Level 1 Designated IS and a Level 2 Designated IS at the same time.
The Designated IS is elected based on the priority of each IS in the broadcast network. When an IS becomes operational, it sends a Level 1 or Level 2 Hello PDU to advertise itself to other ISs. If the IS is configured to be both a Level 1 and a Level 2 IS, the IS sends a separate advertisement for each level:
If the Designated IS becomes unavailable, for example in the case of a reboot, the IS with the next highest priority becomes the new IS. If two or more ISs have the highest priority, the IS with the highest MAC address becomes the Designated IS.
The priority is an interface parameter. Each interface that is enabled for IS-IS can have a different priority.
The figure below shows an example of the results of Designated IS elections. For simplicity, this example shows four of the five routers in Intermediate systems and end systems, with the same domain and areas.
Designated IS election has the following results in this network topology:
In this example, the IS-IS priorities for the IS-IS interfaces in broadcast network 1 have been changed by an administrator. The priorities for the interfaces in the other broadcast networks are still set to the default (64). When there is a tie, IS-IS selects the interface with the highest MAC address.