Editing Open Shortest Path First (section)

===Operation modes===
The OSPF can have different operation modes on the following setups on an interface or network:
* [[Point-to-point (network topology)|Point-to-point]]. Each router advertises itself by periodically multicasting hello packets. No designated router is elected. The interface can be [[IPv4#Unnumbered interface|IP unnumbered]] (without a unique IP address assigned to it).
* [[Broadcasting (networking)|Broadcast]] (default), each router advertises itself by periodically multicasting hello packets.
* [[Non-broadcast multiple-access network|Non-broadcast multi-access]], with the use of designated routers. May need static configuration. Packets are sent as [[unicast]]. 
* [[Point-to-multipoint]], where OSPF treats neighbours as a collection of point-to-point links. No designated router is elected. Separate hello packets are sent to each neighbor.
* Point to Multipoint Non Broadcast (P2MP-NB), No designated router is elected. Separate hello packets are sent to each neighbor, Packets are sent as [[unicast]].
* Passive, Only advertised to other neighbours. No adjacency is advertised on network.

====Indirect connections====

Virtual link over Virtual links, [[Tunneling protocol|tunneling]] and sham links, are a form of connections that goes over the routing engine, and is not a direct connection to the remote host.

* Virtual links: The packets are sent as [[unicast]]. Can only be configured on a non-backbone area (but not stub-area). Endpoints need to be ABR, the virtual links behave as unnumbered point-to-point connections. The cost of an intra-area path between the two routers is added to the link.
* Virtual link over [[Tunneling protocol|tunneling]] (like [[Generic Routing Encapsulation|GRE]] and [[WireGuard]]): Since OSPF does not support virtual links for areas other than the backbone, a workaround is the use of tunneling.<ref>{{Cite web|title=[Junos] GRE Configuration Example - Juniper Networks|url=https://kb.juniper.net/InfoCenter/index?page=content&id=KB19371|access-date=2021-11-28|website=kb.juniper.net|archive-date=November 28, 2021|archive-url=https://web.archive.org/web/20211128172846/https://kb.juniper.net/InfoCenter/index?page=content&id=KB19371|url-status=live}}</ref> If the same [[IPv4|IP]] or [[IPv4#Unnumbered interface|router ID]] is used, the link creates two equal-cost routes to the destination.<ref>{{Cite web|title=Generic Routing Encapsulation (GRE) {{!}} Interfaces User Guide for Switches {{!}} Juniper Networks TechLibrary|url=https://www.juniper.net/documentation/us/en/software/junos/interfaces-ethernet-switches/topics/topic-map/switches-interface-gre.html|access-date=2021-11-28|website=www.juniper.net|archive-date=November 28, 2021|archive-url=https://web.archive.org/web/20211128172845/https://www.juniper.net/documentation/us/en/software/junos/interfaces-ethernet-switches/topics/topic-map/switches-interface-gre.html|url-status=live}}</ref>
* Sham link{{Ref RFC|4577}}:<ref name="Juniper-sham-link">{{cite web |title=Understanding OSPF Sham Links - Technical Documentation - Support - Juniper Networks |url=https://www.juniper.net/documentation/en_US/junose15.1/topics/concept/ospf-sham-links-overview.html |website=www.juniper.net |access-date=14 November 2021 |archive-date=November 14, 2021 |archive-url=https://web.archive.org/web/20211114144842/https://www.juniper.net/documentation/en_US/junose15.1/topics/concept/ospf-sham-links-overview.html |url-status=live }}</ref><ref name="Cico-sham-link">{{cite web |title=IP Routing: OSPF Configuration Guide, Cisco IOS Release 15SY - OSPF Sham-Link Support for MPLS VPN [Cisco IOS 15.1SY] |url=https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_ospf/configuration/15-sy/iro-15-sy-book/iro-sham-link.html |website=Cisco |access-date=14 November 2021 |language=en |archive-date=November 1, 2021 |archive-url=https://web.archive.org/web/20211101213057/https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_ospf/configuration/15-sy/iro-15-sy-book/iro-sham-link.html |url-status=live }}</ref> An intra-area link that connects two sites via the MPLS VPN backbone that is preferred to an internal intra-area "OSPF backdoor link" between the same two sites. A sham link is only needed if the MPLS VPN backbone is preferred over the OSPF backdoor link.