OSPF network types
There are three types of networks that are known to OSPF:
- point to point
- non broadcast multiaccess NBMA
For each one of these network types, OSPF has an operational mode. The default operational modes are not always suitable for a specific network. So that’s why sometimes we need to change it.
To change the operational mode of OSPF on a particular network type:
Adjacency behaviour on point-to-point network types
- This network type encompasses the following technologies
- physical serial interfaces
- serial point-to-point sub-interfaces
- No DR or BDR elections
- Neighbors dynamically become adjacent
- Hello timer = 10 sec
- Dead Interval = 40 sec (4 times the Hello timer)
Adjacency behaviour on broadcast network types
- On each broadcast network a DR and a BDR are elected.
- routers establish full adjacency with the DR only
- routers communicate with the DR through the multicast address 126.96.36.199
- DR communicate with routers through the multicast address 188.8.131.52
- all routers have the same synchronized LSDB
Adjacency behavior on non-broadcast network types
In this type of networks, OSPF provides 5 operational modes. Two of them are RFCs and the rest is Cisco-proprietary:
OSPF RFC-based operational modes:
- Non Broadcast
- Point to Multipoint
Cisco-based operational modes:
- Point to Multipoint non Broadcast
- Point to multipoint
In NBMA networks, a DR and a BDR are elected.
If there is a tie, then the router with highest Router ID wins.