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2025-01-19 Update From: SLTechnology News&Howtos shulou NAV: SLTechnology News&Howtos > Internet Technology >
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How to understand DR and BDR, I believe that many inexperienced people do not know what to do. Therefore, this paper summarizes the causes and solutions of the problem. Through this article, I hope you can solve this problem.
To reduce OSPF traffic in a multiaccess network, OSPF selects a designated router (DR) and a backup designated router (BDR). When the multiaccess network changes, DR is responsible for updating all other OSPF routers. BDR monitors the status of the DR and takes over its role if the current DR fails. DR and BDR are selected by all routers in the same network segment according to router priority and Router ID through HELLO packet. Only routers with priority greater than 0 are eligible for selection. During the DR/BDR election, each router writes its selected DR into the Hello message and sends it to each router running the OSPF protocol on the network segment. When two routers on the same network segment declare themselves as DR at the same time, the router with the highest priority wins. If the priority is equal, the Router ID winner wins.
The election of DR
1. Record the DR and BDR values in the current network for later comparison.
two。 Calculate the BDR as follows. Only those routers in the list that do not claim to be DR can participate in the BDR election. If more than one router claims to be BDR (for example, they currently list themselves as BDR in their Hello packets, but not DR), the router with the highest router priority is elected BDR. If the router has the same priority, the router with the highest Router ID will be elected; if no router claims to be BDR, the neighbor router with the highest router priority will be elected BDR (also regardless of those routers that claim to be DR), and Router ID will also be used to break the tie.
3. Calculate the new DR for this network as follows. If one or more routers claim to be DR (that is, they currently list themselves as DR in their Hello packets), the router with the highest router priority is declared DR. If there is a draw, the one with the highest Router ID wins; if no router declares itself DR, the newly selected BDR becomes DR.
4. If router X is now selected as the new DR or BDR, or if it is no longer DR or BDR, repeat steps 2 and 3, and then skip to step 5. For example, if router X is now DR, router X will no longer be able to participate in the BDR election when step 2 is repeated. In addition, this ensures that no router will declare itself BDR and DR at the same time.
5. As a result of the election, the router itself may now be a DR or BDR. The interface status of the router will be set accordingly. If the router is now BDR, the new status of the port is backup. Otherwise, the new state of the interface becomes DR Other.
6. If the connected network is a NBMA network and the router itself happens to be elected DR or BDR, it must start sending hello packets to neighbors who do not have the right to be DR. This is achieved by calling each neighbor message "Start" that has a router priority of 0.
7. If the above calculation results in a change in the identity of DR or BDR, the adjacency associated with this port needs to be changed. Some adjacencies need to be established, while others will be broken. To implement this action, the message "AdjOK?" is called for all neighbors with a state of at least "2-Way". This will cause them to re-check the adjacency status.
After reading the above, have you mastered how to understand DR and BDR? If you want to learn more skills or want to know more about it, you are welcome to follow the industry information channel, thank you for reading!
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