Ospf dr/BDR selection rules

OSPF is an Open Shortest Path priority protocol. It is one of the gateway protocols used within the autonomous region and is developed based on the link status.

Ospf has five network types:

1. Point-to-point type, proposed by Cisco, automatically discovers neighbors and does not elect DR/BDR. The hello Message release time is 10 s.

2. Broadcast type: the network type proposed by Cisco, which automatically discovers neighbors and selects DR/BDR. The release time of the hello message is 10 s.

3. Non-broadcast network. The network type proposed by RFC is manually configured with neighbors. The release time of DR/BDR and hello packets is 30 s.

4. Point-to-point networks, which are proposed by RFC and automatically discover neighbors. No DR/BDR is elected, and the hello message is published for 30 s.

5. Point-to-point non-broadcast, the network type proposed by cisco, manual configuration of neighbors, no DR/BDR election, hello Message release time 30 s.

The point-to-point network type is better understood, so this article focuses on the broadcast network type.

Before the emergence of DR and BDR in broadcast ospf, because any vro and its neighbors need to establish an adjacent relationship, there will be many adjacent relationships between vrouters, in this way, too many LSA will be sent, and the LSA sent to the neighbor will be re-sent from the neighbor in the broadcast type, resulting in a lot of duplicate information in the network. Therefore, DR | BDR is generated.

DR | the BDR task is:

1. Describe the multi-access network and other related routers on the network.

2. Manage the flooding process on the multi-access network.

3. For redundancy, a bdr is also selected for Dual Backup.

DR | selection rules of BDR

1. each router's route port has a priority. The priority ranges from 0 to 255. The default priority of the port is 1, when the priority is changed to 0, it cannot be elected as DR | BDR. The code for setting the port priority is ip ospf priority.

2. The hello message contains the port priority, which may also be the IP address of the DR | BDR port.

3. When the port is started for the first time in multiple routes, it sets DR | BDR to 0.0.0.0, and sets the wait timer value to equal to the router's invalid interval.

Router route ID definition:

1. You can set the route ID in ospf.

2. If no route ID is defined and the vro defines loopback, select the loopback port with the largest IP address as the route ID.

3. If none of them exist, use the IP address of the physical port with the largest IP address as the route ID.

DR | BDR selection process:

1. After the router establishes two-way communication with the neighbor, check the primary, DR, and BDR fields in the neighbor hello message to list all the neighbors that may participate in the election of DR | BDR (except for the primary value 0)

2. if multiple routers declare that they are BDR (that is, they are expressed as BDR in the hello Message), the higher priority will be elected as BDR. If the priority is the same, then, select the vro with a large ID as the BDR. If no vro declares that you are a BDR instance, select a vro with a higher priority as the BDR instance (and exclude the vro that declares that you are a DR instance). If the priorities are the same, the vro ID is used to determine the BDR instance.

3. if one or more routers declare that they are DR, select the high-priority router as DR. If the priority is the same, select the high-priority router as DR; if no vro declares that you are a DR, use the new BDR in the previous step as the DR.

4. If the vro is new to DR or BDR, or the vro is new to exit DR or BDR, repeat steps 2 and 3 and end the election to prevent a vro from declaring DR and BDR at the same time.

5. There is no preemption in ospf, that is, after ospf selects a new DR and BDR, it does not re-elect even if there is a router with a higher priority.

6. after the DR/BDR election is complete, DRother only forms an adjacent relationship with DR/BDR. all routers route Hello packets to the AllSPFRouters address 224.0.0.5 so that they can track the information of other neighbors. That is, DR sends the flood update packet to 224.0.0.5; DRother only broadcasts update packet to AllDRouter address 224.0.0.6, only DR | BDR can listen to this address