1, DIS is selected based on the highest priority, especially, DIS will be reselected whenever a new router is added into the lan segment. No backup DIS
2,to enable IS-IS on a Junos device, one of the parameters is the network entity title(NET), which is usually set on the lo0 interface, the NET contains the area ID, system ID and selector.
3,IS-IS adjacencies can only form when the level is the same for both peers, and for level 1, the neighbor must be in the same area
4, Link-state PDUs are sent as a response to a link-state request with a PSNP and during the formation of an IS-IS neighbor relationship. They are also sent as a triggered event when network changes occur.