OSPF designated router (DR)

The OSPF protocol elects a designated router  that is responsible for keeping
all routers updated on shortest routes. Instead of exchanging route infor-mation with ever other router in the network, routers only communicate
their routes to the designated router. The designated router then updates
everyone on the routes available. Consolidating the exchange of information
about routes to the designated router improves the efficiency of the network
because less bandwidth is consumed by having each router communicate its
routes only to the designated router, instead of communicating with every
other router in their area.
Routers send their LSA packets to a multicast address (224.0.0.6). The DR lis-tens to this multicast IP address. The DR relays routing information back to the routers by sending route updates to the multicast address 224.0.0.5. All
routers running OSPF are listening to the 224.0.0.5 multicast IP address.
Selecting a designated router
The designated router is elected based on the OSPF priority. The OSPF prior-ity is a value between 0 and 255 assigned to a router interface using the  ip
ospf priority value Cisco IOS command. The default OPSF priority is
set to 1.
The router with the highest OPSF priority becomes the designated router.
If two routers have the same OPSF priority, the  router ID (RID)  is used to
decide which one becomes the DR.

It is best practice to configure a core router — or the largest, most highly avail-able, and most powerful router — in your network to be the designated router.
You can control which router is elected DR using either of the following:
 ✦ Loopback interfaces:  Create loopback interfaces on each router in your
network, and set the highest IP address on the one you want to become
the DR.
 ✦ OSPF priority: Set the highest OPSF priority on the router and interface
you want to become the DR.