In the world of networking, OSPF (Open Shortest Path First) is one of the most widely used dynamic routing protocols. But did you know that behind the scenes, two special routers —the **Designated Router (DR)** and **Backup Designated Router (BDR)** — work tirelessly to keep your network running smoothly?
In this blog post, we’ll uncover:
- 1. What do DR and BDR do for our
network?
- 2. How do they get elected, and why is
this election important?
- 3. What are the Common mistakes in OSPF
configuration?
- 4. What are the best practices for optimizing
DR/BDR selection?
Let's dive
in-à
·
Why
do we need DR and BDR?
Ø Imagine a conference room where
everyone is shouting updates at the same time--- chaos, right? That’s what happens
in an OSPF multi-access network (Ethernet) without a DR and BDR.
·
The
problem: toooo many LSAs*
Ø Every OSPF router floods Link-State
Advertisements (LSAs) to announce network changes. In a large network, this
creates excessive traffic and inefficiency.
·
The
Solution: DR and BDR to the Rescue**
>Designated Router DR -> becomes the central hub for LSA
distribution.
> BDR Backup DR-> standby
to take over if the DR fails.
> All the other routers (***DROTHERs***) only talk to the DR and BDR,
reducing unnecessary chatter.
Ø Result: A cleaner, more efficient
OSPF network**
·
How
does the DR and BDR election work?
Ø Not every router gets to be the DR-
there’s a strict election process.
Ø Only routers in ** multi-access
networks*** (Ethernet, Frame Relay) elect a DR/BDR.
Ø Point-to-point links (like PPP) don’t
need DR/BDR.
·
What
are the Election Rules?
Ø OSPF Priority (0-255)
Ø Default - ***1***
Ø If OSPF priority is 0, it means the router
is ineligible to be elected
Ø If the priorities are equal, the
router with the highest RID wins
Ø Router ID is chosen from—manually configured
RID, the highest loopback IP, highest active physical IP.
·
The
surprising Truth: Elections are non-preemptive.
Ø Once elected, the DR/BDR OSPF
routers stay in power unless they fail.
Ø A new router with a higher priority won't
take over the current DR BDR. Unless the current DR/BDR fails.
·
Common
DR/BDR configuration mistakes.
Ø Ignoring priority setting > if
all routers have default priority 1, the highest router ID wins, which may not
be optimal. FIX – manually set priorities on critical routers.
Ø Adding a new high-priority router
won’t change the DR unless the current one fails. FIX restart OSPF on the
interface if needed.
Ø If router IDs are unstable, for
example, if they're based on a physical interface, DR election can be unpredictable.
FIX always set a STATIC router ID in OSPF configuration.
·
Best
practice for DR/BDR optimization.
Ø Set priority 100 on the routers you want
as DR/BDR, and set priority 0 on routers that should never become DR.
Ø Avoid DR on low power routers- the
DR does extra work, do not make a weak router the DR.
Ø Use the loopback interface for router-ID.
Let's see the configuration=>https://www.youtube.com/@internetworkss
Topology:-
- Configure the topology as per the diagram
- Configure the IP addresses as per the topology
- Configure OSPF 1 and advertise directly connected interfaces
- Configure router 1 DR, router 2, and router 3 Drothers
- Configure Router 4 as backup DR
- VERIFY THE CONFIGURATIONS.
