Examkin

Question

Difficulty: MediumOSPFv2 Network Types and DR/BDR Selection

Four Cisco routers (Alpha, Beta, Gamma, and Delta) are connected to the same Ethernet broadcast domain and simultaneously boot and establish OSPFv2 adjacencies. The routers are configured with the following OSPF interface priorities and Router IDs:

- Router Alpha: Priority = 2, Router ID = 1.1.1.1
- Router Beta: Priority = 2, Router ID = 2.2.2.2
- Router Gamma: Priority = 1, Router ID = 10.10.10.10
- Router Delta: Priority = 0, Router ID = 192.168.1.1

Arrange the routers in order of their resulting OSPF roles on the shared segment, starting from the Designated Router (DR) at the top, followed by the Backup Designated Router (BDR), the eligible DROTHER router, and ending with the router completely excluded from election eligibility.

  1. 1Router Beta (Interface Priority: 2, Router ID: 2.2.2.2)
  2. 2Router Alpha (Interface Priority: 2, Router ID: 1.1.1.1)
  3. 3Router Gamma (Interface Priority: 1, Router ID: 10.10.10.10)
  4. 4Router Delta (Interface Priority: 0, Router ID: 192.168.1.1)

Answer

The correct order of routers from DR to election-ineligible is: Router Beta (DR), Router Alpha (BDR), Router Gamma (DROTHER), and Router Delta (Ineligible).
In OSPFv2 multiaccess networks, DR and BDR election precedence is determined first by highest interface priority. Router Alpha and Router Beta tie with the highest priority (2), so their Router IDs break the tie: Router Beta (2.2.2.2) becomes DR and Router Alpha (1.1.1.1) becomes BDR. Router Gamma has a lower priority (1) and becomes an eligible DROTHER. Router Delta has a priority of 0, which makes it ineligible to participate in DR/BDR selection regardless of its high Router ID.

Step-by-Step Solution

1
Evaluate interface priority values for all routers.
Routers Alpha and Beta have priority 2, Router Gamma has priority 1, and Router Delta has priority 0.
OSPFv2 uses interface priority as the primary criterion for DR/BDR election.
2
Identify election eligibility.
Router Delta (priority 0) is marked ineligible for DR/BDR roles and placed at the bottom of election hierarchy.
An interface priority of 0 explicitly prevents a router from becoming a DR or BDR.
3
Determine DR and BDR roles among highest priority routers using Router ID as a tie-breaker.
Router Beta (2.2.2.2>1.1.1.12.2.2.2 > 1.1.1.1) is elected DR, and Router Alpha becomes BDR.
When priority values are equal, the router with the highest Router ID wins the DR election, and the next highest becomes BDR.
4
Place remaining eligible routers into DROTHER status.
Router Gamma (priority 1) assumes the DROTHER role.
Eligible routers with non-zero priority that do not win DR or BDR election remain DROTHERs.

Key Concept

OSPFv2 DR/BDR election precedence prioritizes non-zero interface priority first (highest wins; 0 is ineligible), followed by highest Router ID as a tie-breaker.
Rate this question

Topics

OSPFv2 Network Types and DR/BDR Selection

More questions from this topic