Network Types
The OSPF 'network type' refers to the type of connection between OSPF neighbors (Ethernet, etc).
There are three main OSPF network types:
Broadcast
enabled by default on Ethernet and FDDI (Fiber Distributed Data Interfaces) interfaces.
Point-to-point
enabled by default on PPP (Point-to-Point Protocol) and HDLC (High-Level Data Link Control) interfaces.
Non-broadcast
enabled by default on Frame Relay and X.25 interfaces.
Broadcast
Enabled on Ethernet and FDDI interfaces by default.
Routers dynamically discover neighbors by sending/listening for OSPF Hello messages using multicast address 224.0.0.5
A DR (designated router) and BDR (backup designated router) must be elected on each subnet (only DR if there are no OSPF neighbors, ie. R1's G1/0 interface).
Routers that aren't the DR or BDR become a DROther.
The DR/BDR election order of priority:
Highest OSPF interface priority.
Highest OSPF Router ID.
'First place' becomes the DR for the subnet, 'second place' becomes the BDR.
The default OSPF interface priority is 1 on all interfaces.
If you set the OSPF interface priority to 0, the router cannot be the DR/BDR for the subnet.
The DR/BDR election is 'non-preemptive'. Once the DR/BDR is selected they will keep their role until the OSPF is reset, the interface fails/is shut down, etc. That is why the state has not changed.
R4 became the DR, not R2. R2 became the BDR.
When the DR goes down, the BDR becomes the new DR. Then an election is held for the next DBR.
R3 is a DROther, and is stable in the 2-way state.
DROther (R3 and R5 in this subnet) will only move to the FULL state with the DR and BDR. The neighbor state with other DROther will be 2-way.
In the broadcast network type, routers will only form a full OSPF adjacency with the DR and BDR of the segment.
Therefore, routers only exchange LSAs with the DR and BDR. DROthers will not echange LSAs with each orther.
All routers will still have the same LSDB, but this reduces the amount of LSAs flooding the network.
Messages to the DR/BDR are multicast using address 224.0.0.6
Point-to-Point
Enabled on serial interfaces using PPP or HDLC encapsulation by default.
Routers dynamically discover neighbors by sending/listening for OSPF Hello messages using multicast address 224.0.0.5.
A DR and BDR are not elected.
Ther encapsulation are used for 'point-to-point' connections.
Therefore there is no point in electing a DR anb BDR
The two routers will form a Full adjacency with each other.
Serial interfaces
One side of a serial connection functions as DCE (Data Communications Equipment).
The other side function as DTE (Data Terminal Equipment).
The DCE side needs to specify the clock rate (speed) of the connection.
Ethernet interfaces use the speed command to configure the interface's operating speed. Serial interfaces use the clock rate command.
The default encapsulation on a serial interface is HDLC. (actually cHDLC, Cisco HDLC).
If you change the encapsulation, it must match on both ends of the interface will go down.
Manually configure OSPF Network type
You can configure the OSPF network type on an interface with ip ospf network type command.
For example, if two routers are directly connected with an Ethernet link, there is no need for a DR/BDR. You can configure the point-to-point network type in this case.
Not all network types work on all link types (for example, a serial link cannot use the broadcast network type).
Broadcast vs Point-to-Point
| Broadcast | Point-to-Point |
|---|---|
Default on Ethernet, FDDI interfaces | Default on HDLC, PPP (serial) interfaces |
DR/BDR elected | No DR/BDR |
Neighbors dynamically discovered | Neighbors dynamically discovered |
Default timers: Hello 10, Dead 40 | Default timers: Hello 10, Dead 40 |
Non-broadcast network type default timers = Hello 30, Dead 120
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