Which of the following parameters are involved in the election of DIS in IS-IS? (Multiple choice)

Question

Options

• APriority
• BRouter ID
• CIP address
• DMAC address

• A
  80% (35)
• B
  14% (6)
• C
  7% (3)

Explanation

In IS-IS, the DIS (Designated Intermediate System) election on broadcast networks first compares interface priority (A) - the router with the highest priority wins. If priorities tie, the election falls to the highest MAC address (D), since IS-IS operates at Layer 2 and uses the SNPA (Subnetwork Point of Attachment) as the tiebreaker.

Router ID (B) is wrong because IS-IS uses a System ID (derived from the NET address), not a "Router ID" concept like OSPF - and the System ID is not a DIS election parameter. IP address (C) is wrong because IS-IS is a Layer 2 protocol; it doesn't rely on IP addresses for neighbor discovery or DIS election, unlike OSPF which uses IP for DR/BDR selection.

Memory tip: "IS-IS DIS = Priority then MAC" - contrast this with OSPF's DR election which uses Priority then Router ID. If you remember OSPF uses the Router ID tiebreaker, IS-IS is the one that drops to the MAC address instead.

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