Network types

Understand

Defaults are platform-dependent. NBMA assumes a transit-network model even when the underlying WAN lacks a full mesh; DR placement and spoke reachability matter. Point-to-multipoint can better model hub-and-spoke reachability and commonly adds host routes for neighbor endpoints. Network type affects the LSDB topology and next-hop calculation, not just timers.

Lab setup

Use a fresh lab for this lesson; changes from other lessons are not cumulative. All outputs are illustrative excerpts, not recorded router tests. Transit cost is explicitly 10 and Loopback0 cost is 1. Interface names may vary by image.

Loopback0 is 10.255.0.<router number>/32; area assignments: R1: area 0, R2: area 0.

configure terminal
hostname R1
router ospf 1
 router-id 10.255.0.1
 passive-interface Loopback0
 exit
interface Loopback0
 ip address 10.255.0.1 255.255.255.255
 ip ospf 1 area 0
 ip ospf cost 1
 exit
interface GigabitEthernet1
 ip address 192.0.2.1 255.255.255.252
 ip ospf network point-to-point
 ip ospf cost 10
 ip ospf 1 area 0
 no shutdown
 exit
end

configure terminal
hostname R2
router ospf 1
 router-id 10.255.0.2
 passive-interface Loopback0
 exit
interface Loopback0
 ip address 10.255.0.2 255.255.255.255
 ip ospf 1 area 0
 ip ospf cost 1
 exit
interface GigabitEthernet1
 ip address 192.0.2.2 255.255.255.252
 ip ospf network point-to-point
 ip ospf cost 10
 ip ospf 1 area 0
 no shutdown
 exit
end

Apply

The pair baseline explicitly uses point-to-point. Change both ends to broadcast on this isolated Ethernet link; retain the same /30 addressing and cost. Two routers on broadcast Ethernet still elect a DR and BDR.

R1 and R2 · independently

configure terminal
interface GigabitEthernet1
 ip ospf network broadcast
end

Wait for election and adjacency. The sample below assumes R2 became DR. Election history can produce a different valid role assignment.

Verify and interpret

R1# show ip ospf interface GigabitEthernet1
  Network Type BROADCAST, Cost: 10
  State BDR, Priority 1
R1# show ip ospf neighbor
Neighbor ID     Pri State    Dead Time Address     Interface
10.255.0.2        1 FULL/DR  00:00:35  192.0.2.2  GigabitEthernet1

Broadcast creates a network-LSA when a DR has a Full neighbor. FULL/DR refers to the neighbor’s role.

R1 and R2 · restore

configure terminal
interface GigabitEthernet1
 ip ospf network point-to-point
end

R1# show ip ospf interface GigabitEthernet1
  Network Type POINT_TO_POINT, Cost: 10
  State POINT_TO_POINT
R1# show ip ospf neighbor
Neighbor ID     Pri State   Dead Time Address     Interface
10.255.0.2        0 FULL/-  00:00:38  192.0.2.2  GigabitEthernet1

No DR or BDR is needed. Never force a multi-router LAN into point-to-point merely to avoid an election.

Technical references

OSPFv2 standard · Router configuration guide