Q1. What is neighbor list?Answer: Each OSPF router uses a neighbor list to track its neighbors discovered, their states, who is DR/BDR it has elected.Q2. How is it being used?Answer: When a router floods a Hello, it wants to tell neighbors about its current list of neighbors by including its neighbor list in the Hello packet. When a router receives a Hello packet, it starts the following update: Add the sending router to its neighbor list. Change the neighbor state of the sender to 2-way if Hello's list of neighbors includes the receiver's name. The router learns the sending router's OSPF priority and Router ID. It re-elects DR and BDR. Q3. When a router receives a Hello, does it add all routers in Hello's list of neighbors as its neighbors?Answer: No. It adds only the Hello sender node as a neighbor. OSPF neighbors are directly connected by a link or a multiaccessing subnet.Q4: What is 2-way?Answer: When a router receives a Hello and sees its name is in the Hello's list of neighbors, then the sender is a 2-way neighbor of the receiver. How ever, the sender may not treats the receiver as a 2-way neighbor.Q5. What are the steps to bring two neighboring nodes 2-way to each other?Answer: Consider a simple topology: R1 is connected to R2.1. R1 sends a Hello to R2 with an empty list. 2. R2 receives Hello from R1. It adds R1 to its neighbor list. R2 treats R1 as a 1-way neighbor. 3. R2 sends a Hello with a neighbor list that contains R1. 4. R1 receives Hello from R2. R1 sees its name in Hello's neighbor list. R1 adds R2 to its list and treats R2 as a 2-way neighbor. 5. R1 sends Hello to R2 again. This time, Hello's list of neighbors contains R2. 6. R2 receives Hello from R1. It sees itself in Hello's list. R2 change R1's state to 2-way.After two rounds of Hello flooding, R1 and R2 become 2-way neighbors. Q6. Why 2-way is so important?Answer: In order to calculate shortest paths correctly, OSPF routers need to gain the same knowledge of network to pology. To achieve this goal, OSPF synchronizes LSDB for all routers. The starting point of this synchronization is the 2-way relation between two neighbors. With a 2-way, two neighbors can start to exchange LSAs to synchronize their LSDB.Q7. In DR election, how many 2-way relationships are created in a subnet of n attached routers?Answer: n*(n-1)/2. For example, a 4 router subnet, there are 6 2-way relationships (4*3/2=6.) In a 10-router subnet, there are 45 2-way relationships (10*9/2=45.)Q8. What are the election rules?Answer: RFC 2328 "9.4. Electing the Designated Router" defines the complete election rules. Here is a simplified version:1. Only priority non-zero routers are eligible for election. 2. Only neighbor routers that are 2-way/full are eligible for election. 3. Group eligible routers into 3 lists o DR candidate list. E.g., Routers who win as DR in Hello. o BDR candidate list. E.g., routers who win as BDR in Hello o Eligible. Routers not shown as DR, nor BDR in H ello.4. Elect BDR first. From the BDR candidate list, elect the highest priority router as BDR. If a tie, elect the highest RID. If BDR candidate list is empty, then elect the highest priority-RID router from eligible list. If eligible list is empty, then BDR cannot be filled. 5. Elect DR. In DR candidate list, elect the highest priority router as DR. If a tie, elect the highest RID. If DR candidate list is empty, then promote BDR just elected as DR and elect a new BDR. 6. If a router is a DR/BDR but lost the title after re-election, this router is put back to eligible list. This article is the FAQ of an OSPF simulation. The simulation animates Hello flooding and neighbor lists changes. It helps readers visualize DR election prior to reading textbooks or RFC.2328 (OSPF 2) in detailsExternal links for OSPF network simulation: 1. OSPF simulation: DR election (This article): /view.php?cid=915&protocol=OSPF&title=1.5%20Neighbor%20State 2. OSPF simulation list (Table of Content): /protocol.overview.php?p=OSPF&protocol=&title=0.%20OSPF%20Overview
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