First, the essential ideas
- Broadcast network type
- An OSPF mode for a shared multiaccess segment such as this Ethernet VLAN. It elects a DR and BDR to organize topology exchange.
- DR and BDR
- The designated router and backup designated router coordinate adjacency and flooding on one broadcast segment. They are not automatically every packet's next hop.
- DROTHER
- A router that is neither DR nor BDR on the segment. It becomes fully adjacent to those two roles, not necessarily to every other DROTHER.
- Interface priority
- An election value from 0 through 255. Higher eligible priority is preferred during election; zero makes the interface ineligible for DR or BDR.
- Nonpreemptive election
- An established DR is not displaced merely because a new router arrives with a higher priority. Existing roles and startup history matter.
- OSPF cost
- A directional routing metric assigned to an outgoing interface. OSPF selects lower total-cost paths within the applicable route class; cost is not milliseconds.
- Reference bandwidth
- The numerator used with interface bandwidth to derive a default OSPF cost. Consistent reference settings are needed across a domain; an explicit interface cost overrides the calculation.
- Control and forwarding planes
- The control plane learns and calculates routes. The forwarding plane sends data using selected next hops; a DR control role does not require a data detour.
Ask two independent questions: who coordinates OSPF exchange on this segment, and which adjacent router receives this particular data packet? The first is an election question. The second follows the destination route and its metric. Keeping them separate prevents treating the DR as a default gateway for every router.
See the idea
Data forwarding is not the DR role
R3 sends data through SW1 directly to R2. R1's separate DR control role does not make R1 a transit router for that exchange.
Read the data next hop
R3 selects R2 at 192.0.2.2 for target 203.0.113.35; SW1 switches the frame directly to R2.
What this model leaves out: Role comparison, not a serial topology. R1, R2 and R3 share SW1 VLAN 35; the named data path excludes R1. Records are not extra devices. No capture or latency measurement.
Full visual explanation and references
Two unconnected records separate R3-to-R2 data via SW1 from R1's DR control role. The data endpoints are R3 Loopback0 198.51.100.35 and R2 Loopback0 203.0.113.35.
Role comparison, not a serial topology. R1, R2 and R3 share SW1 VLAN 35; the named data path excludes R1. Records are not extra devices. No capture or latency measurement.
- Read the data next hop
R3 selects R2 at 192.0.2.2 for target 203.0.113.35; SW1 switches the frame directly to R2.
www.rfc-editor.org - Separate the DR role
R1 coordinates OSPF exchange as DR. R3's adjacencies with R1 DR and R2 BDR do not require data transit through R1.
www.rfc-editor.org - Cost is not priority
Changing R3's cost from 10 to 100 changes the path metric from 11 to 101, not the DR role or the R2 next hop.
www.cisco.com
One idea at a time
Start with something familiar.
Draw SW1 in the middle with one cable each to R1, R2 and R3. Write DR beside R1, BDR beside R2 and priority 0 beside R3 only after following the controlled startup sequence.
Why learn this?
Changing the wrong OSPF knob can produce a perfectly healthy but unexpected result. Priority cannot make a path cheaper, and a high cost cannot elect a different DR.
An everyday comparison
A meeting chair organizes discussion, but two colleagues can hand a document directly to each other without passing it through the chair.
What happens in a network
The chair resembles the DR's control role. The direct handoff resembles forwarding from R3 to R2 on the same switched segment.
Where the comparison stopsOSPF has specific flooding and adjacency rules beyond a meeting analogy. This comparison does not imply the DR is optional or that every neighbor pair is fully adjacent.
Follow one worked example
- Start R1 before the other routers.
R1 becomes DR on the initially empty segment, then R2 becomes the backup.
Why: Controlled startup makes the expected roles reproducible without disruptive global resets.
- Read R3's destination route.
The next hop for R2's loopback is 192.0.2.2, not R1's 192.0.2.1.
Why: The route points directly at the destination's router on the shared LAN.
- Change only R3's outgoing cost.
The calculated metric rises while the next hop and DR roles remain unchanged.
Why: A metric change is not a measured latency change or necessarily an outage.
Your first small task
With R3's outgoing VLAN cost 10 and R2 Loopback0 cost 1, calculate the expected route metric. Then repeat with outgoing cost 100.
Show a hint
Add the outgoing path costs; do not add election priorities.
Check the expected result
The expected metrics are 11 and 101. R2 remains the next hop in this single-segment topology.
Pause and explain it in your own words
Does R1 being DR require R3's ping to R2 Loopback0 to route through R1?
Show a hint
R2 is directly adjacent on the same LAN and owns the destination.
Show the explanation
No. R3's route should use 192.0.2.2 as next hop. SW1 forwards the frame toward R2; R1's DR role coordinates OSPF exchange rather than imposing a data-path waypoint.
Short answer
What should you understand today?
On a broadcast OSPF segment, DR and BDR roles organize topology exchange. Priority and router ID influence election, while outgoing interface cost influences path calculation. These are different decisions: a data packet can go directly to another router on the shared LAN without passing through the DR. Verify roles, next hop and metric separately.
Before you begin
Learning targets
- Complete point-to-point OSPFv2 neighbors and router-ID basics.
- Use three fresh CML 2.10 IOSv 15.9M routers plus bundled IOSvL2 15.2 in an entitled project.
- Keep this VLAN isolated; do not connect an external bridge or production trunk.
- Explain DR, BDR and DROTHER scope on a broadcast segment.
- Distinguish priority, router ID and cost.
- Predict metric 10 plus destination-loopback cost 1.
- Verify the selected next hop does not require a DR detour.
- Restore one deliberately incorrect interface cost and retest.
Words you will meet
Open a word when you need its meaning. You do not need to memorize the whole list before reading.
Broadcast network type
An OSPF mode for a shared multiaccess segment such as this Ethernet VLAN. It elects a DR and BDR to organize topology exchange.
DR and BDR
The designated router and backup designated router coordinate adjacency and flooding on one broadcast segment. They are not automatically every packet's next hop.
DROTHER
A router that is neither DR nor BDR on the segment. It becomes fully adjacent to those two roles, not necessarily to every other DROTHER.
Interface priority
An election value from 0 through 255. Higher eligible priority is preferred during election; zero makes the interface ineligible for DR or BDR.
Nonpreemptive election
An established DR is not displaced merely because a new router arrives with a higher priority. Existing roles and startup history matter.
OSPF cost
A directional routing metric assigned to an outgoing interface. OSPF selects lower total-cost paths within the applicable route class; cost is not milliseconds.
Reference bandwidth
The numerator used with interface bandwidth to derive a default OSPF cost. Consistent reference settings are needed across a domain; an explicit interface cost overrides the calculation.
Control and forwarding planes
The control plane learns and calculates routes. The forwarding plane sends data using selected next hops; a DR control role does not require a data detour.
A broadcast segment has local elected roles
A broadcast network type can contain several OSPF routers. DR and BDR roles reduce the need for every pair to maintain full database adjacencies. Each DROTHER becomes fully adjacent to the DR and BDR. Two DROTHERs can remain Two-Way normally. Our three-router lab has only one DROTHER, so it cannot demonstrate a DROTHER-to-DROTHER Two-Way pair; that remains an explicitly explained comparison.
- DR and BDR are per-segment roles, not an area-wide king and deputy.
- Do not label every Two-Way observation as a failed adjacency.
Election uses priority and history, not cost
Eligible candidates are compared using interface priority, with router ID breaking ties. Priority 0 cannot win either role. However, a higher-priority newcomer does not simply preempt an established DR. The lab therefore brings R1 online first and confirms its DR role before enabling R2, then confirms the backup before enabling R3. This preserves a clear startup history rather than promising election solely from a final configuration snapshot.
- Higher priority helps an eligible election candidate, not an already completed election.
- A router ID breaks a tie; it does not override priority 0.
Data forwarding follows its selected next hop
The DR participates in topology exchange, but the forwarding path is calculated from the destination and costs. R3 can send directly to R2 across SW1 when the remote loopback belongs to R2. The switch is a Layer 2 transit, not another IP routing hop. Use a destination-route lookup and, if desired, observed packet evidence to establish forwarding; never draw a mandatory detour through the DR just because it organizes OSPF.
- Keep control relationships and packet paths visually distinct.
- The endpoint loopbacks are on R2 and R3; no hidden client devices exist.
Cost is directional and cumulative
OSPF cost represents an outgoing interface contribution. A path's cost adds the relevant contributions toward the destination, not the sum of every interface seen on a diagram. Traditional Cisco default reference bandwidth is 100 Mbps, so faster links can share the minimum cost unless the reference is adjusted consistently. The lab uses explicit costs to avoid hiding arithmetic behind interface bandwidth defaults. The bandwidth command itself is not a physical speed upgrade.
- Cost need not be symmetric in opposite directions.
- Changing cost does not change DR priority or guarantee an application performance improvement.
Repair a metric error with matching evidence
The deliberate fault is a deviation from the intended cost plan, not a fabricated outage. After changing R3 to 100, record the route's higher metric, the unchanged next hop, the stable neighbor roles and the same ping. Restore the exact cost 10 and verify metric 11 returns. Do not clear OSPF, renumber interfaces or change priorities to repair a cost mistake.
- A healthy ping can coexist with an incorrect routing metric.
- Predicted numeric results must be checked on the actual image before being called runtime evidence.
Real-world walkthrough
A priority change did not improve the path
In a hypothetical troubleshooting session, an operator raises DR priority hoping to reduce a route's metric. The value changes but the metric does not, because election preference and path cost are separate controls.
- Record the intended destination and current next hop and metric.
- Identify whether the complaint concerns election role or forwarding choice.
- Compare the outgoing interface cost with the documented cost plan.
- Correct the authorized cost value without changing election priorities.
- Retest route and traffic evidence; do not claim faster performance without measurement.
Guided GNS3 lab
A DR is not a mandatory packet waypoint
Build the complete shared VLAN, establish known roles, verify the direct R3-to-R2 route and restore an intentionally wrong R3 cost. No runtime output is fabricated and no alternate physical path is assumed.
Addressing plan
| Device | Interface | Address | Purpose |
|---|---|---|---|
| R1 | Gi0/0 | 192.0.2.1/24 | Shared LAN, priority 200 |
| R2 | Gi0/0 | 192.0.2.2/24 | Shared LAN, priority 100 |
| R3 | Gi0/0 | 192.0.2.3/24 | Shared LAN, priority 0 |
| R1 | Loopback0 | 198.51.100.135/32 | Advertised router endpoint |
| R2 | Loopback0 | 203.0.113.35/32 | Final target |
| R3 | Loopback0 | 198.51.100.35/32 | Fixed source |
| SW1 | VLAN 35 ports | No IP address | Layer 2-only switching |
Set up the lab
- Use Cisco Modeling Labs 2.10 with its legally entitled bundled IOSv 15.9M reference image. Record the complete show version release; do not export that image to GNS3. No paid API is involved.
- Create fresh nodes and only the listed links. No External Connector, NAT node, bridge, public uplink or production network is permitted. Leave console access independent of the test data links.
- IOSv examples use GigabitEthernet0/0 and GigabitEthernet0/1. Confirm those exact ports in the workbench and show ip interface brief before configuring; if your image differs, document an explicit port substitution first.
- Decline the initial configuration dialog on a new authorized node, press Enter to reach the prompt, then use enable. Do not bypass unknown credentials. All displayed expected results are predictions, not author runtime.
- Without an entitled compatible image, complete the address and route-selection worksheet only. Mark command execution, packet forwarding and recovery unmeasured.
- Add SW1 using the bundled IOSvL2 image. Map the three exact ports above; no uplinks, SVI or external bridge. Read show version on every node before configuration.
- Follow R1, then R2, then R3 startup in order. Wait for the documented interface role before proceeding; a fresh lab has no prior election state.
Build the isolated broadcast VLAN
In the isolated CML workbench, select SW1, open its Console tab and connect before typing. Start at > or #; use end first if a prior task left configuration mode. Confirm the three exact cable mappings and configure only those access ports.
enable
configure terminal
vlan 35
exit
interface range GigabitEthernet0/0 - 2
switchport mode access
switchport access vlan 35
no shutdown
endWhat each command means
enable- Enter authorized privileged EXEC mode; this does not change the network configuration.
configure terminal- Enter global configuration mode on this named router; accepted changes affect its running state.
vlan 35- Create the isolated shared broadcast VLAN.
exit- Return to global switch configuration.
interface range GigabitEthernet0/0 - 2- Select exactly the three declared router-facing ports.
switchport mode access- Make each selected port an untagged access port.
switchport access vlan 35- Put all three ports into the same isolated VLAN.
no shutdown- Enable the three mapped ports.
end- Return to privileged EXEC; this neither saves nor reverses accepted configuration changes.
- What you should see
- The three router-facing ports share VLAN 35 without any routed SVI.
- Why this step matters
- The shared Layer 2 segment is real and completely specified, not an assumed invisible switch.
Start the first election candidate
In the isolated CML workbench, select R1, open its Console tab and connect before typing. Start at > or #; use end first if a prior task left configuration mode. Configure only R1 now; leave R2 and R3 unconfigured until R1's DR state is checked.
enable
configure terminal
router ospf 1
router-id 1.1.1.1
exit
interface GigabitEthernet0/0
ip address 192.0.2.1 255.255.255.0
ip ospf network broadcast
ip ospf priority 200
ip ospf 1 area 0
no shutdown
endWhat each command means
enable- Enter authorized privileged EXEC mode; this does not change the network configuration.
configure terminal- Enter global configuration mode on this named router; accepted changes affect its running state.
router ospf 1- Create this router's fresh local OSPF process.
router-id 1.1.1.1- Set a unique router identity before enabling OSPF.
exit- Return to global configuration.
interface GigabitEthernet0/0- Select the cable to SW1.
ip address 192.0.2.1 255.255.255.0- Assign this router's address in the shared /24.
ip ospf network broadcast- Explicitly select broadcast OSPF behavior.
ip ospf priority 200- Set DR/BDR election eligibility and preference, not path cost.
ip ospf 1 area 0- Enable the interface in area 0.
no shutdown- Enable the shared-LAN connection.
end- Return to privileged EXEC; this neither saves nor reverses accepted configuration changes.
- What you should see
- R1 should elect itself DR after the wait interval on the empty segment.
- Why this step matters
- Startup order makes the role prediction reproducible without preempting a live DR.
Set R1 costs and confirm DR
In the isolated CML workbench, select R1, open its Console tab and connect before typing. Start at > or #; use end first if a prior task left configuration mode. Add its endpoint, then wait for the interface display to show DR before starting R2.
enable
configure terminal
interface Loopback0
ip address 198.51.100.135 255.255.255.255
ip ospf 1 area 0
ip ospf cost 1
exit
interface GigabitEthernet0/0
ip ospf cost 10
end
show ip ospf interface GigabitEthernet0/0What each command means
enable- Enter authorized privileged EXEC mode; this does not change the network configuration.
configure terminal- Enter global configuration mode on this named router; accepted changes affect its running state.
interface Loopback0- Create this router's traffic endpoint.
ip address 198.51.100.135 255.255.255.255- Assign its distinct /32 loopback address.
ip ospf 1 area 0- Advertise the loopback in area 0.
ip ospf cost 1- Make the loopback's advertised cost explicit.
exit- Return to global configuration.
interface GigabitEthernet0/0- Select the outgoing shared-LAN interface.
ip ospf cost 10- Set an explicit baseline outgoing OSPF cost.
end- Return to privileged EXEC; this neither saves nor reverses accepted configuration changes.
show ip ospf interface GigabitEthernet0/0- Confirm broadcast type, priority 200 and DR state.
- What you should see
- Expect R1 DR with LAN cost 10 and loopback cost 1.
- Why this step matters
- The next stage depends on observed role completion, not a guessed delay.
Start the backup candidate
In the isolated CML workbench, select R2, open its Console tab and connect before typing. Start at > or #; use end first if a prior task left configuration mode. Join the established segment with a different router ID and eligible priority 100.
enable
configure terminal
router ospf 1
router-id 2.2.2.2
exit
interface GigabitEthernet0/0
ip address 192.0.2.2 255.255.255.0
ip ospf network broadcast
ip ospf priority 100
ip ospf 1 area 0
no shutdown
endWhat each command means
enable- Enter authorized privileged EXEC mode; this does not change the network configuration.
configure terminal- Enter global configuration mode on this named router; accepted changes affect its running state.
router ospf 1- Create this router's fresh local OSPF process.
router-id 2.2.2.2- Set a unique router identity before enabling OSPF.
exit- Return to global configuration.
interface GigabitEthernet0/0- Select the cable to SW1.
ip address 192.0.2.2 255.255.255.0- Assign this router's address in the shared /24.
ip ospf network broadcast- Explicitly select broadcast OSPF behavior.
ip ospf priority 100- Set DR/BDR election eligibility and preference, not path cost.
ip ospf 1 area 0- Enable the interface in area 0.
no shutdown- Enable the shared-LAN connection.
end- Return to privileged EXEC; this neither saves nor reverses accepted configuration changes.
- What you should see
- R2 should become BDR without displacing the established R1 DR.
- Why this step matters
- OSPF's existing-role behavior differs from a simplistic always-highest-wins claim.
Set R2 endpoint and confirm BDR
In the isolated CML workbench, select R2, open its Console tab and connect before typing. Start at > or #; use end first if a prior task left configuration mode. Create the final target and confirm its BDR state before enabling R3.
enable
configure terminal
interface Loopback0
ip address 203.0.113.35 255.255.255.255
ip ospf 1 area 0
ip ospf cost 1
exit
interface GigabitEthernet0/0
ip ospf cost 10
end
show ip ospf interface GigabitEthernet0/0What each command means
enable- Enter authorized privileged EXEC mode; this does not change the network configuration.
configure terminal- Enter global configuration mode on this named router; accepted changes affect its running state.
interface Loopback0- Create this router's traffic endpoint.
ip address 203.0.113.35 255.255.255.255- Assign its distinct /32 loopback address.
ip ospf 1 area 0- Advertise the loopback in area 0.
ip ospf cost 1- Make the loopback's advertised cost explicit.
exit- Return to global configuration.
interface GigabitEthernet0/0- Select the outgoing shared-LAN interface.
ip ospf cost 10- Set an explicit baseline outgoing OSPF cost.
end- Return to privileged EXEC; this neither saves nor reverses accepted configuration changes.
show ip ospf interface GigabitEthernet0/0- Confirm R1 DR, R2 BDR and local cost 10.
- What you should see
- Expect R2 BDR and its advertised /32 at cost 1.
- Why this step matters
- The target address and cost are explicit inputs to the later calculation.
Join an ineligible election participant
In the isolated CML workbench, select R3, open its Console tab and connect before typing. Start at > or #; use end first if a prior task left configuration mode. Use priority 0 while still enabling normal OSPF route learning.
enable
configure terminal
router ospf 1
router-id 3.3.3.3
exit
interface GigabitEthernet0/0
ip address 192.0.2.3 255.255.255.0
ip ospf network broadcast
ip ospf priority 0
ip ospf 1 area 0
no shutdown
endWhat each command means
enable- Enter authorized privileged EXEC mode; this does not change the network configuration.
configure terminal- Enter global configuration mode on this named router; accepted changes affect its running state.
router ospf 1- Create this router's fresh local OSPF process.
router-id 3.3.3.3- Set a unique router identity before enabling OSPF.
exit- Return to global configuration.
interface GigabitEthernet0/0- Select the cable to SW1.
ip address 192.0.2.3 255.255.255.0- Assign this router's address in the shared /24.
ip ospf network broadcast- Explicitly select broadcast OSPF behavior.
ip ospf priority 0- Set DR/BDR election eligibility and preference, not path cost.
ip ospf 1 area 0- Enable the interface in area 0.
no shutdown- Enable the shared-LAN connection.
end- Return to privileged EXEC; this neither saves nor reverses accepted configuration changes.
- What you should see
- R3 should remain DROTHER and form full adjacencies to R1 and R2.
- Why this step matters
- Election ineligibility does not prevent routing or adjacency to the elected routers.
Set R3 source and baseline cost
In the isolated CML workbench, select R3, open its Console tab and connect before typing. Start at > or #; use end first if a prior task left configuration mode. Create the actual test source and explicitly set outgoing LAN cost 10.
enable
configure terminal
interface Loopback0
ip address 198.51.100.35 255.255.255.255
ip ospf 1 area 0
ip ospf cost 1
exit
interface GigabitEthernet0/0
ip ospf cost 10
endWhat each command means
enable- Enter authorized privileged EXEC mode; this does not change the network configuration.
configure terminal- Enter global configuration mode on this named router; accepted changes affect its running state.
interface Loopback0- Create this router's traffic endpoint.
ip address 198.51.100.35 255.255.255.255- Assign its distinct /32 loopback address.
ip ospf 1 area 0- Advertise the loopback in area 0.
ip ospf cost 1- Make the loopback's advertised cost explicit.
exit- Return to global configuration.
interface GigabitEthernet0/0- Select the outgoing shared-LAN interface.
ip ospf cost 10- Set an explicit baseline outgoing OSPF cost.
end- Return to privileged EXEC; this neither saves nor reverses accepted configuration changes.
- What you should see
- R3's source /32 is advertised with cost 1.
- Why this step matters
- Fixed source and explicit costs make the reverse path and metric independently checkable.
Record roles and the direct next hop
In the isolated CML workbench, select R3, open its Console tab and connect before typing. Start at > or #; use end first if a prior task left configuration mode. Wait for synchronization; record both neighbors and the remote endpoint route.
enable
show ip ospf neighbor
show ip route 203.0.113.35
ping 203.0.113.35 source 198.51.100.35What each command means
enable- Enter authorized privileged EXEC mode to inspect this named router.
show ip ospf neighbor- Expect R1 as DR and R2 as BDR, both Full.
show ip route 203.0.113.35- Expect next hop 192.0.2.2 and OSPF metric 11.
ping 203.0.113.35 source 198.51.100.35- Verify the complete source-to-target exchange.
- What you should see
- Expect direct R2 next hop, not a routed detour through R1.
- Why this step matters
- The data-plane destination choice is distinct from the DR's control role.
Introduce a cost-plan error
In the isolated CML workbench, select R3, open its Console tab and connect before typing. Start at > or #; use end first if a prior task left configuration mode. Change only the outgoing cost from 10 to 100; do not alter priority or addressing.
enable
configure terminal
interface GigabitEthernet0/0
ip ospf cost 100
endWhat each command means
enable- Enter authorized privileged EXEC mode; this does not change the network configuration.
configure terminal- Enter global configuration mode on this named router; accepted changes affect its running state.
interface GigabitEthernet0/0- Select R3's outgoing shared-LAN interface.
ip ospf cost 100- Deliberately raise the metric contribution by 90.
end- Return to privileged EXEC; this neither saves nor reverses accepted configuration changes.
- What you should see
- The metric to R2's loopback should rise to 101.
- Why this step matters
- This is a safe isolated metric fault, not a claim that connectivity must fail.
Compare metric, role and traffic
In the isolated CML workbench, select R3, open its Console tab and connect before typing. Start at > or #; use end first if a prior task left configuration mode. After recalculation, compare all three observations with baseline.
enable
show ip route 203.0.113.35
show ip ospf neighbor
ping 203.0.113.35 source 198.51.100.35What each command means
enable- Enter authorized privileged EXEC mode to inspect this named router.
show ip route 203.0.113.35- Check metric 101 with the same R2 next hop.
show ip ospf neighbor- Verify the existing DR/BDR relationships remain.
ping 203.0.113.35 source 198.51.100.35- Check that the single available path still carries the exchange.
- What you should see
- Expect a higher metric but unchanged role and viable path.
- Why this step matters
- With no alternate route, increased cost alone does not create a new next hop or guaranteed outage.
Restore the intended cost
In the isolated CML workbench, select R3, open its Console tab and connect before typing. Start at > or #; use end first if a prior task left configuration mode. Restore the documented explicit value instead of resetting unrelated OSPF state.
enable
configure terminal
interface GigabitEthernet0/0
ip ospf cost 10
endWhat each command means
enable- Enter authorized privileged EXEC mode; this does not change the network configuration.
configure terminal- Enter global configuration mode on this named router; accepted changes affect its running state.
interface GigabitEthernet0/0- Select the same outgoing interface.
ip ospf cost 10- Restore the original cost-plan value.
end- Return to privileged EXEC; this neither saves nor reverses accepted configuration changes.
- What you should see
- The route should return to its baseline metric after recalculation.
- Why this step matters
- Using an explicit restoration avoids relying on an image-dependent default bandwidth value.
Repeat the original checks
In the isolated CML workbench, select R3, open its Console tab and connect before typing. Start at > or #; use end first if a prior task left configuration mode. Compare the exact destination and source with the first baseline.
enable
show ip route 203.0.113.35
show ip ospf neighbor
ping 203.0.113.35 source 198.51.100.35What each command means
enable- Enter authorized privileged EXEC mode to inspect this named router.
show ip route 203.0.113.35- Verify metric 11 and next hop 192.0.2.2.
show ip ospf neighbor- Confirm the original roles and Full adjacencies.
ping 203.0.113.35 source 198.51.100.35- Repeat the initial endpoint exchange.
- What you should see
- Expect the original metric and working exchange restored.
- Why this step matters
- This closes the fault with both control and forwarding evidence.
Verify
- Confirm switch ports share VLAN 35 and SW1 has no routing role.
- Retain startup evidence for R1 DR and R2 BDR before R3 joins.
- Show R3's remote route points at R2, not at the DR merely because of its role.
- Compare metrics 11 and 101 without describing them as milliseconds.
Troubleshoot
- If roles differ, inspect previous election state and startup order; do not assume a priority change preempts an existing DR.
- If neighbors are missing, compare VLAN membership, broadcast type, area and compatible Hello parameters.
- If metric arithmetic differs, inspect actual outgoing and destination-loopback costs before assuming the ping path changed.
GNS3 does not provide Cisco images. Use software only when the applicable license or entitlement permits that environment. Do not share or redistribute image files. Cisco Modeling Labs is the official alternative; its reference images are licensed for use within CML unless separately licensed. A paper exercise needs no image, but does not establish hands-on competence. Virtual routers do not reproduce physical radios, PoE delivery, hardware forwarding rates or failover timing.
- Restore R3 Gi0/0 cost 10 and leave the original election priorities unchanged.
- Stop the isolated four-node project after retaining actual evidence; no role-reset or device erase is needed.
Practice set
Answer before you reveal
01Does the DR have to forward every data packet?
In this shared-LAN example R3 reaches R2 directly through SW1. R1's DR role is a control-plane relationship.
02Can priority 0 win DR or BDR?
It may still discover neighbors and route normally. A high router ID does not override election ineligibility.
03Does a higher-priority newcomer preempt a stable DR?
Startup history matters. This lab explicitly establishes R1 first so the predicted roles do not rely on simultaneous boot timing.
04What is the expected baseline metric to R2's loopback?
The switch is not an IP routing hop contributing another OSPF cost, and DR priority is not part of the calculation.
05Why does cost 100 not break this lab's ping?
A higher cost can alter selection when alternatives exist, but this topology intentionally supplies no alternative routed path.
06Can this three-router lab show two DROTHERs in Two-Way?
The normal DROTHER-to-DROTHER behavior is taught as a comparison, not fabricated as an observation from this node count.
Knowledge check
Quiz: prove the reasoning
Keep these
Technical takeaways
- DR and BDR organize topology exchange on a segment.
- Priority 0 prevents election, not normal routing.
- Existing DR roles are not simply preempted by newcomers.
- Forwarding does not require a DR detour.
- Path cost is directional arithmetic, not latency.
- A cost-plan fault can exist while packets still arrive.