Ospf what is dr

DR and BDR elections are not-preemptive, this means that once a DR and BDR is established, they keep their statuses despit new routers becomig active on the same broadcast area that may even have beeter priorities. In other words, there are no new elections held for every OSPF router that comes late. OSPF Broadcast Network Separated I created this topology with two switched specifically for this possibility of having the routers separated in pairs by loosing connection between the switches.

A router's Hello Packet contains its Router Priority, which is configurable on a per-interface basis. In general, when a router's interface to a network first becomes functional, it checks to see whether there is currently a Designated Router for the network. If there is, it accepts that Designated Router, regardless of its Router Priority. This makes it harder to predict the identity of the Designated Router, but ensures that the Designated Router changes less often.

See below. Otherwise, the router itself becomes Designated Router if it has the highest Router Priority on the network.

A more detailed and more accurate description of Designated Router election is presented in Section 9. Only those routers on the list that have not declared themselves to be Designated Router are eligible to become Backup Designated Router. In In a point-to-point network, the DR is not elected, but R1 and R5 are the neighbors i. Facebook twitter Hatena Pocket Copy. The figure is for clarity only and does not show the actual LSA transmission behavior itself.

Related article. There are two types of relationships between OSPF routers: neighbor and adjacency. How show ip ospf neighbor looks on Ethernet. We will configure Router3 with a priority of 0. The default priority is 1. There are several reasons for rigging the DR election process, as we have done in this recipe.

The most common reason is simply to ensure that the router closest to the network core is responsible for distributing routing information. This is actually a somewhat aesthetic requirement, because all of the routers in an area see all of the LSAs for that area, so nobody's routing table is more accurate than anybody else's.

But it can result in faster convergence in some network configurations. But there are two times when it is critical to force a particular router to become the DR. In this case, all of the routers are members of the same subnet, but only the central hub router can talk directly to the branch devices.

A branch router should never act as DR because it can't talk directly to any of the other branches. The central router is the only device that can be the DR, or the routing updates will not work. See Recipe 8. It is important to note that setting a higher priority can help to rig the DR election process, but it doesn't guarantee that another lower priority router device won't become DR if it happens to be there first.

If the higher priority router isn't available when a lower priority router joins the segment, then the lower priority router will become DR. Once a router is DR, it will remain DR until you either manually reset the neighbor relationships or until there is a network failure on the segment that forces the change.