What's OSPF DR/BDR Election Process and Function Description

Created Dec 29, 2018 11:20:53Latest reply Dec 30, 2018 14:31:58 185 2 1 0
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I am learning Huawei's solution, and I am learning the huawei IP by myself, please I need to know the OSPF DR/BDR Election Process and Function Descripiton

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yWX511501  Novice   Created Dec 29, 2018 11:22:54 Helpful(1) Helpful(1)

Designated Router (DR)

Backup Designated Router (BDR): Backs up a specified router.

DROther(Designated Router Other), member router

Now, what is the role of the two roles?
1. In a multi-access network environment, routers establish adjacencies with each other. If there are x routers, x(x-1)/2 adjacencies are established. Each router advertises x-1 LSAs to neighboring routers that have adjacencies with each other, add a network LSA. The calculation result shows that n2 LSAs are generated on the network. (The large part of the LSA is from books and I cannot calculate it.)
2. The flooding of the multiple access network is very chaotic. A router sends a notification to the adjacent router. Similarly, the routers send the copy of the advertisement to the adjacent router. In this way, a large number of duplicate advertisement copies are generated on the network.
The LSA sent by 3.DR describes other routers connected to the multi-access network.
4. Manage the flooding of LSAs on the multi-access network.

Each multi-access network environment has a DR. All routers on the network only establish adjacencies with the DR/BDR. This is irrelevant to the area. As long as multiple accesses are available, a DR exists, that is, if a router has more than two interfaces, the A interface may be the DR of a network, and the network where the B interface resides may be the DRother. In short, the role of the DR/BDR is based on the interface, the router is not the entire router.

An important problem about the DR is that if only one DR exists on the network, if the DR fails, all the adjacencies become invalid. In addition, all routers need to re-elect a DR, in addition, the database is synchronized with the DR again, and then the neighbor relationship is re-established. When this occurs, the network breaks down and data packets cannot be transmitted.

To solve this problem, in addition to the DR on a network, a router needs to be elected as the BDR and DR/BDR. All routers on the network establish adjacencies with the DR/BDR. In this way, when the DR fails, the BDR is promoted to the DR. Because the DRother maintains adjacencies with the BDR, the network impact can be minimized.

The DR/BDR election is triggered by an interface state machine. To implement the entire process, the following conditions must be met:
1. Each access interface has a priority. It is a 8-bit unsigned integer. The range is 0-255. The interface with the highest priority becomes the DR. When the interface priority is 0, the interface does not participate in the DR/BDR election. That is, the interface cannot be the DR/BDR. Although the DR/BDR cannot be preempted, when the priority of the DR or BDR interface is set to 0, the interface immediately discards the DRother.
The 2.Hello package lists the priority of the originating interface and may be the IP address of the DR/BDR.
3. When an interface takes effect on multiple access networks, the DR/BDR field in the Hello packet sent by the interface is set to 0.0.0.0 and the wait time is reset to DealInterval. The function of this time is as follows: If the router does not receive Hello packets from other router interfaces during this period, it considers itself as a DR.
4. In a multiple-access network, an interface adds an existing DR/BDR to an interface data structure table.

The following describes the DB/BDR election process.
1. The router must be 2-way, that is, bidirectional communication has been set up. Then, the router sets the DR/BDR field in the Hello message to the IP address of the originating interface. In this way, the router declares itself as the DR/BDR, then, the router checks all received Hello packets and lists the interfaces that are eligible for election. (The priority is not 0, and the neighbor status is 2-way.)
2. In the list of selected routers, create a set of all routes that are not advertised as DRs.
3. If one or more neighbor routers exist in the set and they set their own interface IP addresses in the BDR field of hello packets, they compare them and become BDRs, when the priorities are the same and the Router-id is high, the BDR becomes the BDR. After the BDR is elected, the BDR automatically becomes the DR. The BDR automatically becomes when there is no DR on the network, this action does not generate any packets to notify other neighbors. The DR field is the IP address of the DR interface.

In summary, when an OSPF router is valid and discovers its neighbor, it checks the existing. If the existing exists, it accepts the existing DR/BDR. If the existing DR/BDR does not exist, an election process is performed, the BDR is elected first. If no DR exists on the current network, the BDR is directly promoted to the DR, and then an election process is performed to elect the BDs.

After the DR/BDR router is elected successfully, other routers (DRother) maintain the adjacency relationship with the DR and BDR, and all routers send Hello packets in multicast 224.0.0.5 mode. Therefore, they can trace their neighbor routes, however, the DRother only sends the update packet to the 224.0.0.6, and only the DR is available. The BDR listens to the address. Otherwise, the update packet address sent by the DR/BDR is 224.0.0.5. In this way, all the DRother can receive the update packet.

In a multi-access network environment, if only one router is qualified to participate in the selection, the router becomes the DR. All other routers only maintain adjacencies with the DR. If no router is qualified, in this network, no router becomes the DR. All routers stay in the 2-way state.
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Mysterious.color  Novice   Created Dec 30, 2018 14:31:58 Helpful(0) Helpful(0)

it's just a mechanism to reduce the load on network interfaces
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find what you love and let it kill you.

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