Editing Open Shortest Path First (section)

==Non-point-to-point network==
[[File:OSPF-type 2 Network-LSA figur.drawio.png|OSPF-type 2 Network-LSA figur.drawio|frameless|right]]
On networks (same subnet) with networks type of:
* Broadcast 
* Non-Broadcast Multi-Access (NBMA) 

A system of ''designated router'' (DR) and ''backup designated router'' (BDR), is used to reducing network traffic by providing a source for routing updates.
This is done using multicast addresses:

* [[Multicast address#Notable IPv4 multicast addresses|{{IPaddr|224.0.0.5}}]], all routers in the topology will listen on that multicast address.
* [[Multicast address#Notable IPv4 multicast addresses|{{IPaddr|224.0.0.6}}]], DR and BDR will listen on that multicast address.

The DR and BDR maintains a complete topology table of the network and sends the updates to the other routers via multicast. All routers in a multi-access network segment will form a leader/follower relationship with the DR and BDR. They will form adjacencies with the DR and BDR only. Every time a router sends an update, it sends it to the DR and BDR on the multicast address {{IPaddr|224.0.0.6}}. The DR will then send the update out to all other routers in the area, to the multicast address {{IPaddr|224.0.0.5}}. This way all the routers do not have to constantly update each other, and can rather get all their updates from a single source. The use of multicasting further reduces the network load. DRs and BDRs are always setup/elected on OSPF broadcast networks. DR's can also be elected on NBMA (Non-Broadcast Multi-Access) networks such as Frame Relay or ATM. DRs or BDRs are not elected on point-to-point links (such as a point-to-point WAN connection) because the two routers on either side of the link must become fully adjacent and the bandwidth between them cannot be further optimized. DR and non-DR routers evolve from 2-way to full adjacency relationships by exchanging DD, Request, and Update.

===Designated router===
A ''designated router'' (DR) is the router interface elected among all routers on a particular multiaccess network segment, generally assumed to be broadcast multiaccess. Special techniques, often vendor-dependent, may be needed to support the DR function on [[non-broadcast multiaccess]] (NBMA) media. It is usually wise to configure the individual virtual circuits of an NBMA subnet as individual point-to-point lines; the techniques used are implementation-dependent.

===Backup designated router===
A ''backup designated router'' (BDR) is a router that becomes the designated router if the current designated router has a problem or fails. The BDR is the OSPF router with the second-highest priority at the time of the last election.

A given router can have some interfaces that are designated (DR) and others that are backup designated (BDR), and others that are non-designated. If no router is a DR or a BDR on a given subnet, the BDR is first elected, and then a second election is held for the DR.<ref name="rfc2328" />{{rp|75}}

===DR Other===
A router that has not been selected to be ''designated router'' (DR) or ''backup designated router'' (BDR). The router forms an adjacency to both the ''designated router'' (DR) and the ''backup designated router'' (BDR).

For other non (B)DR, the adjacency stops at ''2-ways'' State.

===Designated router election===
The DR is elected based on the following default criteria:
*  If the priority setting on an OSPF router is set to 0, that means it can NEVER become a DR or BDR.
*  If no DR exists on the network, routes will wait until ''Wait Timer'' runs out.
*  When a DR fails and the BDR takes over, there is another election to see who becomes the replacement BDR.
*  The router sending the Hello packets with the highest priority wins the election.
*  If two or more routers tie with the highest priority setting, the router sending the Hello with the highest RID (Router ID) wins. NOTE: a RID is the highest logical (loopback) IP address configured on a router, if no logical/loopback IP address is set then the router uses the highest IP address configured on its active interfaces (e.g. {{IPaddr|192.168.0.1}} would be higher than {{IPaddr|10.1.1.2}}).
*  Usually the router with the second-highest priority number becomes the BDR.
*  The priority values range between 0 – 255,<ref>{{cite web |url=http://www.cisco.com/en/US/docs/ios/iproute_ospf/command/reference/iro_cr_book.pdf |title=Cisco IOS IP Routing: OSPF Command Reference |date=April 2011 |url-status=dead |archive-url=https://web.archive.org/web/20120425041808/http://www.cisco.com/en/US/docs/ios/iproute_ospf/command/reference/iro_cr_book.pdf |archive-date=April 25, 2012 |df=mdy-all |publisher=[[Cisco Systems]]}}</ref> with a higher value increasing its chances of becoming DR or BDR.
*  If a higher priority OSPF router comes online after the election has taken place, it will not become DR or BDR until (at least) the DR and BDR fail.
*  If the current DR 'goes down' the current BDR becomes the new DR and a new election takes place to find another BDR. If the new DR then 'goes down' and the original DR is now available, still previously chosen BDR will become DR.

===Routing update flow===

==== When {{font color|white|lightpink|DR}} has Routing update ====

# {{font color|white|lightpink|DR}} sends LSU to {{IPaddr|224.0.0.5}}
# {{font color|white|lightgreen|BDR}} sends LSUAck to {{IPaddr|224.0.0.5}}
# {{font color|white|lightBlue|DR Other}} sends LSUAck to {{IPaddr|224.0.0.6}}

==== When {{font color|white|lightgreen|BDR}} has Routing update ====

#  {{font color|white|lightgreen|BDR}} sends LSU to {{IPaddr|224.0.0.5}}
#  {{font color|white|lightpink|DR}} sends LSUAck to {{IPaddr|224.0.0.5}}
#  {{font color|white|lightBlue|DR Other}} sends LSUAck to {{IPaddr|224.0.0.6}}

==== When {{font color|white|lightBlue|DR Other}} has Routing update ====

#  {{font color|white|lightBlue|DR Other}} sends LSU to {{IPaddr|224.0.0.6}}
#  {{font color|white|lightpink|DR}} sends LSA to {{IPaddr|224.0.0.5}}
#  {{font color|white|lightgreen|BDR}} sends LSUAck to {{IPaddr|224.0.0.5}}
#  Non-source routers, {{font color|white|lightBlue|DR Other}} sends LSUAck to {{IPaddr|224.0.0.6}}