OSPF - point-to-multipoint network type
When configuring OSPF, the configured network type affects how OSPF will operate. The Point-to-Multipoint network type has the following characteristics:
Characteristics
- OSPF treats Point-to-Multipoint networks as a collective of point-to-point links, thus no DR/BDR elections
- Since no elections take place, Point-to-Multipoint networks do not maintain a DR/BDR relationship.
- Point-to-Multipoint networks will have an automatic neighbor discovery, so no need to manually configure them.
- The Point-to-Multipoint network type has a 30 second hello and 120 second dead timer.
- OSPF advertises its own tunnel interface IP address as a /32 route to other routers, even if those interfaces are configured with other subnet masks. Thus, OSPF routers will learn about those subnets as /32 from OSPF, not as a directly connected routes, and will have an OSPF-learned entry to a /32 destination.
Use of multicast
OSPF views each connection to a neighbor as a point-to-point link, even though there might be multiple neighbors over a single physical (or logical) interface. As a result, OSPF Hello packets and updates are unicasted directly to each neighboring router's address rather than using multicast. This is simply done by design.
Keep in mind that in topologies that require an IOSPF point-to-point network type, multicast is typically supported. As a result, OSPF does use multicast to initially automatically form neighbor adjacencies. Multicast is also used for LSAs and other OSPF packets. It's just the Hellos and LSUs that still use multicast using the 224.0.0.5 and 224.0.0.6 addresses.
Topologies that use this network type
Point-to-multipoint network type is used in topologies including:
- Frame-Relay with the broadcast keyword used for Frame-Relay maps
- DMVPN topologies using Phase 1 and 3
Links:
https://forum.networklessons.com/t/dmvpn-phase-1-ospf-routing/1304/31?u=lagapides