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CCNA lab 21 of 46

OSPF network types and the DR/BDR election

Four routers share one Ethernet segment and elected a designated router by accident. Read the FULL and 2-WAY states, control the election with priorities, compare the broadcast and point-to-point network types, and break an adjacency with mismatched timers.

Run this lab

It needs Lite: its 5 nodes are set to 5 vCPU and 5,120 MB in total. That fits the free demo too.

In your workspace: Shared, then CCNA 200-301 guided labs, then 21 - OSPF network types and the DR-BDR election

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Level
Advanced
Time
About 50 minutes
Nodes need
5 vCPU, 5,120 MB
Smallest plan
Lite

The scenario

Severn Freight's four core routers all plug into one switch (S1) on the shared segment 10.21.0.0/24, and R1 and R2 also have a direct link. OSPF works, but the designated router is whichever router happened to boot at the right moment. The design says R1 must be the DR, R2 the BDR, and R4 (an old router with little memory) must never take either role.

RouterGi0/0 (shared, via S1)OtherRouter ID
R110.21.0.1/24Gi0/1 10.0.12.1/30 to R21.1.1.1
R210.21.0.2/24Gi0/1 10.0.12.2/30 to R12.2.2.2
R310.21.0.3/24–3.3.3.3
R410.21.0.4/24–4.4.4.4

Every router has Loopback0 10.255.255.n/32 in OSPF area 0.

What you’ll practise

  • Explain why OSPF elects a DR and BDR on multi-access networks
  • Interpret FULL/DR, FULL/BDR and 2WAY/DROTHER neighbour states
  • Control the election with interface priority and explain why it isn't pre-emptive
  • Compare broadcast and point-to-point network types
  • Diagnose a hello and dead timer mismatch

The tasks

  1. Read the neighbour states
  2. Learn the election rules
  3. Set the priorities and force a new election
  4. Change the direct link to point-to-point
  5. Break an adjacency with timers

The Lab Tasks pane has the scenario, these tasks, the checks to run, hints and the full solution.

Topology and devices

The network as it opens in EVE-NG, drawn from the lab’s own canvas, and the image each node runs.

Topology of OSPF network types and the DR-BDR election5 nodes, 5 links. Every link is listed as text below the diagram.R1R2R3R4S1Gi0/0Gi0/0Gi0/0Gi0/1Gi0/0Gi0/2Gi0/0Gi0/3Gi0/1Gi0/1
5 nodes, with the interface names each link uses. On a narrow screen, scroll the diagram sideways.
Every link, as text
  • R1 Gi0/0 to S1 Gi0/0
  • R2 Gi0/0 to S1 Gi0/1
  • R3 Gi0/0 to S1 Gi0/2
  • R4 Gi0/0 to S1 Gi0/3
  • R1 Gi0/1 to R2 Gi0/1
Nodes in OSPF network types and the DR/BDR election, with the image each runs and the vCPU and RAM it’s set to
NodeDeviceImage in Add NodeSet to
R1Cisco IOSvvios-adventerprisek9-m.SPA.159-3.M61 vCPU, 1,024 MB
R2Cisco IOSvvios-adventerprisek9-m.SPA.159-3.M61 vCPU, 1,024 MB
R3Cisco IOSvvios-adventerprisek9-m.SPA.159-3.M61 vCPU, 1,024 MB
R4Cisco IOSvvios-adventerprisek9-m.SPA.159-3.M61 vCPU, 1,024 MB
S1Cisco IOSvL2viosl2-adventerprisek9-m.ssa.high_iron_202009291 vCPU, 1,024 MB

How to open this lab

  1. Sign in to your workspace at lab.cloudlabbox.com.

  2. In File Manager, open Shared, then the folder CCNA 200-301 guided labs.

  3. Open 21 - OSPF network types and the DR-BDR election and start its nodes, then open Lab Tasks for the scenario and the steps.

The lab is read-only. You can start the nodes and configure the devices, but changes to the topology can’t be saved. To change it, rebuild it in your own folder.

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CloudLabBox is independent of Cisco. The lab follows exam topics and isn’t vendor training material; vendor names identify the exams and the images in the workspace.