R1, R2, R3, and R4 run IS-S. The DIS Priority of their interfaces is as shown in the figure. If the devices are started at the same time, () is elected as DIS.

Question

Options

• AR4
• BR3
• CR2
• DR1

• A
  71% (17)
• B
  4% (1)
• C
  8% (2)
• D
  17% (4)

Explanation

In IS-IS, the DIS (Designated Intermediate System) is elected based on the highest DIS priority configured on the interface - R4 has the highest priority value among all four routers as shown in the figure, so it wins the election. R1, R2, and R3 are incorrect because their interface DIS priorities are lower than R4's; when all routers start simultaneously, the one with the strictly highest priority value is elected without any further tiebreaking needed. Note that unlike OSPF, a DIS priority of 0 in IS-IS does not disqualify a router from becoming DIS - it simply means that router has the lowest possible priority and will lose to any competitor with a higher value. If two routers share the same highest priority, IS-IS breaks the tie using the higher SNPA (MAC address), but that scenario does not apply here.

Memory tip: Think "highest priority wins" - IS-IS DIS election always crowns the router with the biggest priority number on that interface, and there is no non-eligible state equivalent to OSPF's priority-0 exclusion rule.

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