OSPF DR Election for Beginners (from a companion OSPF simulation)

Q1. What does DR do?

Q1. What does DR do?

Answer: DR (Designated Router) represents router nodes on a broadcast subnet (e.g., Ethernet). It does:

Create a Network LSA to describe nodes that are attached to the subnet (called attached routers.

Synchronize LSDB with other attached routers on the subnet. [a]

Q2. How many LSAs will routers outside the subnet see?

Answer: 5. in this scenario, there are 4 routers attached to the subnet. Each creates a Router LSA. When DR is being elected, it creates Network LSA. After DR and non-DR nodes are synchronized, they all have 5 LSAs.

When the subnet gateway router have synchronized its LSDB with neighboring routers not attached to the subnet, outside nodes will see 5 LSAs from the subnet: four Router LSA, one Network LSA.

Q3. Why do we need DR

Answer: Scalability.Consider an example: If there are 10 OSPF routers (R1~ R10) attached to a subnet. To synchronize LSDBs, OSPF needs to maintain 45 two-way relationships (10*9/2=45). This is a complex task and consumes a lot of network resources. If we elect one as DR and only DR maintains two-way relationships with the other 9 nodes then LSDB synchronization task are greatly reduced.

Q4. How to configure DR?

Answer: Network admin often chooses a big and faster router as DR. He can configure it with a higher OSPF priority. During DR election, the highest priority wins.

Q5: How does RID (Router ID) affect DR election?

Answer: When two routers have the same highest OSPF priority, highest RID wins.

Q6. How to finger out RID's value?

Answer: RID value is assigned as follows:

If RID is set by a command line, then RID uses the assigned value.

If command is not used and there are several loopback addresses, the highest loopback address become RID.

If command is not used and no loopback address is configured, then the highest IP address fromthis node's interfaces becomes RID.

Note [a]: For beginners, watching OSPF DR simulation can be helpful. See the companion simulation listed below.

OSPF DR Election for Beginners (from a companion OSPF simulation)

By: ibc222

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