Four Cisco routers (R1, R2, R3, and R4) are booted simultaneously on a shared broadcast mu… — Cisco CCNA

Four Cisco routers (R1, R2, R3, and R4) are booted simultaneously on a shared broadcast multiaccess Ethernet segment running OSPFv2 in Area 0. No explicit OSPF router ID is configured under the `router ospf` process on any router.

The interface and active IP configurations are as follows:
- R1: GigabitEthernet0/0 IP $192.168.1.1/24$ , Priority = $10$ ; Loopback0 IP = $10.1.1.1/32$
- R2: GigabitEthernet0/0 IP $192.168.1.2/24$ , Priority = $0$ ; Loopback0 IP = $172.16.1.1/32$
- R3: GigabitEthernet0/0 IP $192.168.1.3/24$ , Priority = $10$ ; Loopback0 IP = $10.2.2.2/32$
- R4: GigabitEthernet0/0 IP $192.168.1.4/24$ , Priority = $20$ ; Loopback0 IP = $10.0.0.1/32$

Order the routers in sequence from highest to lowest precedence during the initial Designated Router (DR) and Backup Designated Router (BDR) election, starting with the router selected as DR down to the completely ineligible router.

1Router R4 (Priority = 20, Loopback0 = 10.0.0.1)
2Router R3 (Priority = 10, Loopback0 = 10.2.2.2)
3Router R1 (Priority = 10, Loopback0 = 10.1.1.1)
4Router R2 (Priority = 0, Loopback0 = 172.16.1.1)

Answer

The correct sequence from highest election precedence to lowest is: Router R4 (DR), Router R3 (BDR), Router R1 (DROTHER), and Router R2 (Ineligible).

The election evaluates interface priority first. Router R4 has the highest interface priority (20) and is elected DR. Router R2 has a priority of 0, making it strictly ineligible for DR/BDR election despite having the highest Loopback IP address. Between Router R1 and Router R3 (both having priority 10), Router R3 wins the BDR election because its loopback address (10.2.2.2) forms a higher Router ID than Router R1 (10.1.1.1). Router R1 becomes a DROTHER.

Step-by-Step Solution

Determine initial eligibility based on configured interface priority.

Router R2 has an OSPF interface priority of 0, which immediately disqualifies it from DR/BDR election eligibility, placing it at the bottom of the election ranking.

An OSPF interface priority of 0 specifies that the router cannot become a DR or BDR on the segment.

Evaluate eligible candidate routers by interface priority to select the DR.

Router R4 has a priority of 20, while Router R1 and Router R3 each have a priority of 10. Router R4 wins the DR election.

OSPF interface priority is the primary criteria for DR/BDR election; the router with the highest numeric priority value wins.

Break the priority tie between remaining eligible routers for BDR selection using Router ID.

Router R3 and Router R1 tie with an interface priority of 10. Router R3 has an active Loopback0 IP of 10.2.2.2, which becomes its Router ID. Router R1 has a Loopback0 IP of 10.1.1.1. Because 10.2.2.2 > 10.1.1.1, Router R3 is elected BDR and Router R1 becomes a DROTHER.

When interface priorities are equal, OSPF uses the highest Router ID (derived automatically from the highest active loopback IP address) as the secondary tie-breaker.

Key Concept

OSPFv2 DR/BDR Election Precedence (Interface Priority > Highest Router ID via Loopback > Priority 0 Ineligibility)

Estimated Time:2m 0s

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