DR/BDR Election and VRP OSPF Configuration
Key Takeaways
- On multi-access networks OSPF elects a DR and BDR; higher priority wins, then higher Router ID; priority 0 makes a router ineligible to become DR or BDR.
- DR/BDR election is not preemptive in the classic sense: a new higher-priority router joining later may not immediately take over until restart/clear of existing DR roles—design priorities before bringing up the segment.
- Huawei basic config: ospf process with router-id, area, and network commands (wildcard mask) or interface ospf enable; verify with display ospf peer and display ospf lsdb.
- Interface cost, timers, priority, and passive interfaces tune behavior; area and network-type mismatches are top adjacency failures.
- Operational checks: peer Full/2-Way patterns, LSDB types present, and display ospf routing for installed OSPF routes.
DR/BDR Election and VRP OSPF Configuration
Quick Answer: On multi-access segments, OSPF elects a DR and BDR (highest priority, then highest Router ID; priority 0 = never DR/BDR). Configure Huawei with
ospf <process> router-id,area, andnetworkwildcard masks or interfaceospf enable. Verify usingdisplay ospf peeranddisplay ospf lsdb. Match area, timers, and network type or adjacency fails.
This section operationalizes the theory: who becomes Designated Router, how to write VRP configuration, and which show commands prove success. HCIA scenarios often combine a small topology, a config snippet, and a question about DR identity or missing neighbors.
Why DR and BDR exist
On a multi-access network with n routers, a full mesh of adjacencies would mean each router exchanges DBDs with every other router—O(n²) adjacencies. Instead:
- Elect one DR and one BDR.
- All routers form Full adjacencies with DR and BDR.
- DROther–DROther pairs stay at 2-Way.
- Topology updates for the segment center on the DR (Type 2 LSA).
If the DR fails, the BDR promotes to DR and a new BDR is elected—faster recovery than electing from scratch among all DROthers with no backup.
Election rules
| Step | Rule |
|---|---|
| 1 | Routers with priority 0 cannot be DR or BDR (DROther only). |
| 2 | Among eligible routers, highest interface priority wins DR. |
| 3 | If priority ties, highest Router ID wins. |
| 4 | BDR is the next-best eligible router by the same rules. |
| 5 | Election participates from 2-Way visibility on the segment. |
Default priority is commonly 1. Setting a core switch/router VLANIF to priority 100 and access devices to 0 is a standard design pattern so flaky edge devices never become DR.
Non-preemptive behavior (exam nuance)
Classic OSPF DR election is not fully preemptive: if a DR already exists and a new router with higher priority appears, the existing DR often remains DR until OSPF restarts on the DR, the DR is disqualified, or the segment’s DR state is lost. Therefore set priorities before stable production adjacency, or plan a controlled restart when redesigning DR placement. Exam questions may ask whether “adding priority 100 later instantly steals DR”—often no until re-election conditions occur.
Point-to-point note
On true point-to-point OSPF network types, DR/BDR election is not used the same way; the two peers adjacency goes Full without multi-access DR roles. Mis-setting network type (broadcast vs P2P) on serial-like or tunnel links is a frequent lab error.
Router ID selection and stability
Always configure:
[Router] ospf 1 router-id 10.0.0.1
Stable Router IDs (often matching a loopback plan: 10.0.0.1, 10.0.0.2, …) make display ospf peer readable and avoid RID changes when interfaces flap. Changing Router ID typically requires the OSPF process to restart to take effect—plan maintenance windows.
Huawei VRP basic configuration patterns
Pattern A — network command under area (classic teaching)
[Router] ospf 1 router-id 1.1.1.1
[Router-ospf-1] area 0.0.0.0
[Router-ospf-1-area-0.0.0.0] network 10.1.1.0 0.0.0.255
[Router-ospf-1-area-0.0.0.0] network 192.168.1.1 0.0.0.0
Wildcard mask (inverse mask) selects which interface addresses enable OSPF and which connected prefixes enter the area. 0.0.0.0 matches a single host address (often a loopback). This is not the same as a subnet mask; 0.0.0.255 matches a /24 worth of host bits.
Pattern B — interface enable (common on modern VRP styles)
[Router] ospf 1 router-id 1.1.1.1
[Router] interface GigabitEthernet0/0/0
[Router-GigabitEthernet0/0/0] ospf enable 1 area 0.0.0.0
Exact keywords can vary slightly by software train; the exam concept is: process + area + enable on the interface or network statement under area.
Multi-area ABR sketch
[ABR] ospf 1 router-id 2.2.2.2
[ABR-ospf-1] area 0.0.0.0
[ABR-ospf-1-area-0.0.0.0] network 10.0.0.0 0.0.0.255
[ABR-ospf-1] area 0.0.0.10
[ABR-ospf-1-area-0.0.0.10] network 10.10.0.0 0.0.255.255
Interfaces in both areas make the device an ABR. Forgetting to put the core-facing link in Area 0 while putting access in Area 10 breaks hierarchical design.
Interface tuning examples
[Router-GigabitEthernet0/0/0] ospf cost 100
[Router-GigabitEthernet0/0/0] ospf dr-priority 100
[Router-GigabitEthernet0/0/0] ospf timer hello 10
[Router-GigabitEthernet0/0/0] ospf timer dead 40
[Router-ospf-1] silent-interface GigabitEthernet0/0/10
- cost influences SPF path selection (lower better).
- dr-priority (priority) influences DR election; 0 refuses DR/BDR.
- silent-interface / passive-style configuration prevents Hellos on edge interfaces while still advertising the network when included in OSPF.
- Keep Hello/Dead consistent on both ends.
Verification command pack
| Command | What you confirm |
|---|---|
display ospf peer / peer brief | Neighbor RID, state (Full/2-Way), interface, Dead remaining |
display ospf interface | Area, network type, cost, priority, DR/BDR on segment, Hello timers |
display ospf lsdb | LSA types present; area-scoped consistency |
display ospf routing | OSPF-learned prefixes and path types |
display ospf brief / process overview | Router ID, areas, statistics (wording varies) |
display ip routing-table | Whether OSPF routes installed in global RIB vs static/preference winners |
Interpreting DR fields
On display ospf interface for a multi-access interface, note:
- Local priority
- DR Router ID / IP
- BDR Router ID / IP
- Neighbor count and adjacency states
If your router shows priority 0, it must not appear as DR. If two routers both believe they are DR, you have a split (often two-way communication failure or mismatched area/segment).
Configuration and adjacency failure matrix
| Problem | Symptom | Fix direction |
|---|---|---|
| Area mismatch | No neighbor / stuck Down | Align area IDs on the link |
| Timer mismatch | No stable neighbor | Match Hello/Dead both sides |
| Auth mismatch | No neighbor | Match authentication |
| Network type mismatch | Stuck states / no DR agreement | Align broadcast vs P2P |
| Wrong wildcard / network | Interface not running OSPF | Fix network or ospf enable |
| Passive/silent on peer link | No Hellos | Remove silent on inter-router links |
| ACL blocks multicast | Down/Init issues | Allow OSPF (IP protocol 89) / multicast |
| Duplicate Router ID | Unstable, weird LSDB | Unique RIDs |
| Priority 0 everywhere | No DR elected properly | Ensure at least one eligible router |
End-to-end lab checklist (HCIA style)
- Address interfaces; ping connected neighbors.
- Set router-id on each device uniquely.
- Enable OSPF; place links in correct areas (Area 0 for backbone links).
- Set dr-priority on multi-access segments intentionally.
display ospf peer→ expect Full (or 2-Way only where DROther–DROther).display ospf lsdb→ Type 1 present; Type 2 if multi-access DR exists; Type 3/5 if multi-area/external.display ospf routing/display ip routing-table→ prefixes learned; traceroute validates path.- Fail a DR candidate in the lab (shutdown) and observe BDR promotion (learning outcome).
Cost and path selection reminder
Even with perfect adjacency, traffic follows lowest cost SPF paths. A secondary high-cost link may stay unused until the primary fails. Preference versus other protocols still applies at RIB installation time (OSPF versus static/direct). Floating statics can back up OSPF if administrative preference is set correctly—as covered in routing fundamentals chapters.
Exam checklist
- Elect DR/BDR: highest priority, then highest RID; priority 0 cannot win.
- Know non-preemption: late higher priority may not instantly take DR.
- Write VRP:
ospf,router-id,area,networkwith wildcard, or interface enable. - Verify:
display ospf peer,display ospf lsdb, interface/priority/DR fields. - Troubleshoot mismatches: area, timers, auth, network type, silent interface, RID.
- Connect back to theory: DR originates Type 2; Full vs 2-Way pattern; Area 0 multi-area rule.
Mastering this chapter means you can design a small multi-area campus OSPF domain, predict neighbor states and LSA types, elect DRs deliberately, and prove operation on Huawei VRP—core skills for HCIA-Datacom IP routing topics.
A router’s OSPF interface priority is set to 0 on a multi-access LAN. What is the effect?
Two eligible OSPF routers on the same Ethernet segment have equal interface priority. Which becomes DR?
On Huawei VRP, which command pair is most appropriate to confirm OSPF neighbors reached Full and to inspect LSA contents?
In the VRP network command network 10.1.1.0 0.0.0.255 under area 0, what does 0.0.0.255 represent?