Mar 9, 2013

IS-IS DIS Election


Election of the DIS

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 from 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.
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