How DIS is elected on ISIS protocol ?

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The default L1 and L2 priority is 64 in Cisco routers, with a value range of 0-127. A value of 0 means the router is ineligible to become the DIS. The router with the highest priority is chosen as the DIS, and in the case of a tie, the router with the numerically highest SNPA becomes the DIS.
DIS is the highest Subnetwork Points of Attachment (SNPA) address on the LAN segment. The SNPA address refers to the data link address, and in this case is the MAC address.
On a LAN, one of the routers elects itself the DIS, based on interface priority (the default is 64). If all interface priorities are the same, the router with the highest subnetwork point of attachment (SNPA) is selected. The SNPA is the MAC address on a LAN, and the local data link connection identifier (DLCI) on a Frame Relay network. If the SNPA is a DLCI and is the same at both sides of a link, the router with the higher system ID becomes the DIS. Every IS-IS router interface is assigned both a L1 priority and a L2
priority in the range of 0 to 127.
The DIS election is preemptive (unlike OSPF). If a new router boots on the LAN with a higher interface priority, the new router becomes the DIS. It purges the old pseudonode LSP and floods a new set of LSPs.

Reference:http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094b42.shtml