DROTHER

[Raha_1990]

Hi!i'm a newbie here.i can't understand the answer given in the question.how could you explain it?

best  regards,Abdurahman

[Abzal · Accepted solution]

Hi,

Because on point to point link there is no DR/BDR election. It happens only on broadcast network like fasterhernet, when multiple routers share one subnet.
This is the simplest network type for OSPF. Two routers connected via point-to-point link. For this particular network, there is no need to have DR and BDR. Why, because each router will sending updates to each other and having DR and BDR will not change the situation.
Link
http://www.davidsudjiman.info/2008/04/25/ospf-network-types-point-to-point-cisco-standard/

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[Rolf Fischer]

This question combines a basic knowledge about the different networktypes in OSPF and some experience in the output of the "show ip ospf neighbor" command.

One design goal when developing routing protocols is to keep the protocol traffic as low as possible.

So, depending of the capabilities of a link (which is the connection between two or more routers), there are different ways to exchange routing data.

The most important capabilities are:

- can a router have more than 1 neighbor on a link?

- can the link transmit broadcasts?

(*)

If a link can have more than 1 neighbor, a Designated Router (DR) and (for redundancy) a Backup DR will be elected. The DR/BDR are responsible to send routing updates to all the routers on the segment. This is much more efficient in terms of bandwidth, at least when you have many routers on a subnet.

I'd like to encourage you to play a little bit in a lab environment or a simulator:

R1(config)#int f0/0

! the default network type in Ethernet is "Broadcast"

R1(config-if)#do show ip ospf interface f0/0 | i Network Type

Process ID 1, Router ID 0.0.0.1, Network Type BROADCAST, Cost: 1

! the neighbor has been elected as Designated Router

R1(config-if)#do show ip ospf neighbor

Neighbor ID     Pri   State           Dead Time   Address         Interface

0.0.0.2           1   FULL/DR         00:00:30    172.16.12.2     FastEthernet0/0

! let's change the network type to Point-to-Point:

R1(config-if)#ip ospf network point-to-point

R1(config-if)#do show ip ospf interface f0/0 | i Network Type

Process ID 1, Router ID 0.0.0.1, Network Type POINT_TO_POINT, Cost: 1

! a P-2-P link doesn't need to elect DR/BDR

R1(config-if)#do show ip ospf neighbor

Neighbor ID     Pri   State           Dead Time   Address         Interface

0.0.0.2           0   FULL/  -        00:00:39    172.16.12.2     FastEthernet0/0

HTH

Rolf

(*): There is also an important capability called "transitive connectivity",  but let's keep it simple.