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Is it possible, to have a DR as a Slave Router?

So here is my knowledge:

The DR and the BDR are elected based on priority and then highest RID in an area, so every router can communicate with the DR instead of every router with every router, to minimize network traffic. Although it is possible, that a router with lower RID is elected, if it started faster.

The Master and the Slave router is elected only during LSA exchange. The master is elected from a higher RID and the Slave with the lower RID. After the exchange, the master and slave "descriptions" are dropped.

Example for when it could happen:

  • Router 1 (ID: 1.1.1.1, Prio: 5)
  • Router 2 (ID: 2.2.2.2, Prio: 1)
  • Router 3 (ID: 3.3.3.3, Prio: 3)

From my knowledge, R1 will be selected as the DR, R3 as the BDR and R2 as normal router. During an exchange between R1 and R2, R1 will be the Slave and R2 will be the Master.

Here are some links from my research:

All my collegues, including my boss says that the DR and the Master router is the exact same thing, so please correct me if I am wrong or give me some sources to prove them wrong.

Thanks :)

1 Answer 1

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Foreword

Okay I have read the entire RFC 2328 (Official OSPF Version 2 Standart Documentation) and here is what I have found out:

Answer

Those are two completely different things. I have even tested the example and yes, the Router1 (Designated Router) became indeed the Slave Router. Here the explanation and sources:


Abbreviations used

  • DR: Designated Router
  • RID: Router Identifier
  • OSPF: Open Shortest Path First Protocol

Explanation

Designated Router:

  • If a OSPF Network has more than 2 routers, it will elect a DR based on priority and then highest RID
  • The only purpose is to reduce network traffic by reducing the communication needed

Master Router:

  • During the Database Exchange Process the two routers talking form a master/slave relationship based on highest RID
  • Master and Slave dictate how the routers should behave during that process

Proof

Definition:

1.2 Definitions of commonly used terms - Designated Router (Page 10):

Each broadcast and NBMA network that has at least two
attached routers has a Designated Router. The Designated
Router generates an LSA for the network and has other
special responsibilities in the running of the protocol.
The Designated Router is elected by the Hello Protocol.
The Designated Router concept enables a reduction in the
number of adjacencies required on a broadcast or NBMA
network.  This in turn reduces the amount of routing

7.2. The Synchronization of Databases (Page 53):

This sending and receiving of Database Description packets is
called the "Database Exchange Process".  During this process,
the two routers form a master/slave relationship

Election

9.4. Electing the Designated Router (Page 75):

the one having highest Router Priority is declared
to be Designated Router.  In case of a tie, the one having
the highest Router ID is chosen

10.10 An Example (Page 105):

When it sees that RT2 is indeed the master (because of RT2's higher Router ID)
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  • 1
    You only have a DR on a broadcast medium. Something like a point-to-point network will not have a DR, but it will still require a master for the exchange.
    – Ron Maupin
    Commented May 6 at 21:04
  • Ohh yes of course, that is right ^^
    – Bog
    Commented May 7 at 7:52

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