Isis Dis Election
On broadcast media, one IS-IS router becomes the DIS and forms the pseudonode. This election is based on the priority configured on each router's interface. The pseudonode represents a LAN segment, and all routers (nonpseudonodes) form an adjacency with it.
Note that DIS election is preemptive. Adding a router with a higher priority causes it to displace the current DIS and become the DIS itself, c reating a new pseudo node.
Every 3 seconds, the DIS sends out CSNPs listing all the LSPs. Any router can request missing or more-recent LSPs by sending a PSNP. Similarly, it can update the pseudonode by sending more-recent or missing LSPs to the DIS.
Continue reading here: Avoiding Bl ack Holes with ISIS
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