Study Notes; OSPF over NBMA

OSPF characterizes FR networks as non-broadcast multiple access (NBMA)
because they can support more than two attached routers, but do not have a broadcast capability.

In an NBMA network topology, which is inherently nonbroadcast, neighbors are not discovered automatically.
OSPF tries to elect a DR and a BDR due to the multi-access nature of the network, but the election fails since neighbors are not discovered.
Neighbors must be configured manually to overcome these problems.

OSPF can run into 2 modes.
NBMA and Point-to-multipoint

Configurations for NBMA (Using Network Type Broadcast)
Router1
interface Loopback0
ip address 3.3.3.3 255.255.255.255
!
!
interface Serial2
ip address 1.1.1.2 255.255.255.0
encapsulation frame-relay
ip ospf network broadcast
no keepalive
frame-relay map ip 1.1.1.1 16 broadcast
!
!
router ospf 1
network 1.1.1.0 0.0.0.255 area 0

Router2
interface Loopback0
ip address 2.2.2.2 255.255.255.255
!
interface Serial1/0
ip address 1.1.1.1 255.255.255.0
encapsulation frame-relay
ip ospf network broadcast
no keepalive
clockrate 2000000
frame-relay map ip 1.1.1.2 16 broadcast
!
router ospf 1
network 1.1.1.0 0.0.0.255 area 0

Router1# show ip ospf neighbor

Neighbor ID Pri State Dead Time Address Interface
2.2.2.2 1 FULL/BDR 00:00:37 1.1.1.1 Serial2

Router1# show ip ospf interface s2
Serial2 is up, line protocol is up
Internet Address 1.1.1.2/24, Area 0
Process ID 1, Router ID 3.3.3.3, Network Type BROADCAST, Cost: 64
Transmit Delay is 1 sec, State DR, Priority 1

Configurations for NBMA (Using Neighbor Statements)
Router1
interface Loopback0
ip address 3.3.3.3 255.255.255.255
!
interface Serial2
ip address 1.1.1.2 255.255.255.0
encapsulation frame-relay
ip ospf priority 2
no keepalive
frame-relay map ip 1.1.1.1 16
!
router ospf 1
network 1.1.1.0 0.0.0.255 area 0
neighbor 1.1.1.1

Router2
interface Loopback0
ip address 2.2.2.2 255.255.255.255
!
interface Serial1/0
ip address 1.1.1.1 255.255.255.0
encapsulation frame-relay
no keepalive
clockrate 2000000
frame-relay map ip 1.1.1.2 16
!
router ospf 1
network 1.1.1.0 0.0.0.255 area 0
neighbor 1.1.1.2
!
frame-relay map commands do not need to have the broadcast parameter because the OSPF packets are unicasted with the neighbor statement.

Router2# show ip ospf neighbor

Neighbor ID Pri State Dead Time Address Interface
3.3.3.3 1 FULL/DR 00:01:49 1.1.1.2 Serial1/0

Router2# show ip ospf interface s1/0
Serial1/0 is up, line protocol is up
Internet Address 1.1.1.1/24, Area 0
Process ID 1, Router ID 2.2.2.2, Network Type NON_BROADCAST, Cost: 64
Transmit Delay is 1 sec, State BDR, Priority 1

Configurations for Point-to-Multipoint
Router1
interface Loopback0
ip address 3.3.3.3 255.255.255.255
!
interface Serial2
ip address 1.1.1.2 255.255.255.0
encapsulation frame-relay
ip ospf network point-to-multipoint
no keepalive
frame-relay map ip 1.1.1.1 16 broadcast
!
router ospf 1
network 1.1.1.0 0.0.0.255 area 0
!

Router2

interface Loopback0
ip address 2.2.2.2 255.255.255.255
!
interface Serial1/0
ip address 1.1.1.1 255.255.255.0
encapsulation frame-relay
ip ospf network point-to-multipoint
no keepalive
clockrate 2000000
frame-relay map ip 1.1.1.2 16 broadcast
!
router ospf 1
network 1.1.1.0 0.0.0.255 area 0

Router1# show ip ospf neighbors

Neighbor ID Pri State Dead Time Address Interface
2.2.2.2 1 FULL/ - 00:01:53 1.1.1.1 Serial2
Router1# show ip ospf interface s2
Serial2 is up, line protocol is up
Internet Address 1.1.1.2/24, Area 0
Process ID 1, Router ID 3.3.3.3, Network Type POINT_TO_MULTIPOINT, Cost: 64
Transmit Delay is 1 sec, State POINT_TO_MULTIPOINT,

There are no DR and BDR elected when the NBMA network is configured as Point-to-Multipoint, since it is treated as a collection of point-to-point links.