Q1. What does DR do?
Answer: DR (Designated Router) represents router nodes on a broadcast subnet (eg, 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. (The companion simulation visualizes LSDB synchronization activities.)
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 rout routers not attached to the subnet, outside nodes will see 5 LSAs from the subnet: four Router LSA, one Network LSA. (You can find the topology in the companion topology).
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 from this node's interfaces becomes RID.