About this interactive
On a multi-access segment such as Ethernet, n routers that all paired up would need n(n - 1) / 2 neighborships. OSPF elects a **designated router (DR)** and a **backup (BDR)** instead: the others (DROthers) send updates to the DR on 224.0.0.6, and the DR sends them on to everyone on 224.0.0.5. Each DROther keeps full adjacencies with the DR and the BDR.
The election, run when the segment comes up:
1. Highest **interface priority** (0 to 255, default 1; 0 is never elected).
2. Then highest **router ID**.
The result is not re-run when a router joins. If the DR fails, the BDR takes over and a new BDR is elected.
A Cisco router's ID is the one configured under OSPF; else the **highest** loopback IP address; else the highest IP address of an active interface. The video says the lowest-numbered loopback and interface.
What you'll learn
- Work out the DR and BDR on a multi-access segment: highest interface priority, then highest router ID, priority 0 never elected, and no re-election when a router joins
- Work out a router's ID (configured, else the highest loopback address, else the highest active interface address) and how many neighborships a segment needs with and without a DR
Aligned to
Cisco CCNA
3.4
Configure and verify single area OSPFv2
Key terms
- Backup Designated Router
BDR
- An OSPF router elected on a multi-access network segment as the backup to the Designated Router, ready to take over DR functions if the DR fails. The BDR maintains adjacencies with all other routers on the segment to minimize reconvergence time.
- Router ID
- A 32-bit value formatted like an IP address that uniquely identifies an OSPF router and is used as a tiebreaker in DR/BDR elections when interface priorities are equal.
- OSPF Priority
- A configurable interface value (0–255) used during DR and BDR elections; a higher value increases the likelihood of being elected DR, and a value of 0 prevents election.
- OSPF Interface Priority
- A configurable value (0–255) assigned to a router interface that determines its preference in the DR/BDR election; a value of 0 disqualifies the interface from election.
Topics
Ospf
Dr Bdr
Router Id
Ospf Priority
Multi Access
Interactive
Calculation
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