OSPF uses broadcast and DR/BDR elections which means some issues with underlying L2 protocols. So let's consider all L2 options and how OSPF handles them.
Point-to-point.
It is the easiest one:
interface Serial1/0
ip address 10.0.0.1 255.255.255.0
ip ospf 1 area 0
end
R1#show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
2.2.2.2 0 FULL/ - 00:00:31 10.0.0.2 Serial1/0
R1#show ip ospf interface serial 1/0
Serial1/0 is up, line protocol is up
Internet Address 10.0.0.1/24, Area 0
Process ID 1, Router ID 1.1.1.1, Network Type POINT_TO_POINT, Cost: 64
Enabled by interface config, including secondary ip addresses
Transmit Delay is 1 sec, State POINT_TO_POINT
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
It is default network type for serial links. This type of media supports broadcast (and, therefore, OSPF multicast) so neighboring routers are automatically discovered.
The main reason for DR/BDR election is reducing of full-mesh LSA flooding between adjacent routers on the media. But serial links by default aren't multiaccess media, so DR/BDR election is not necessary.
Broadcast.
The second easiest one:
interface FastEthernet0/0
ip address 10.0.0.1 255.255.255.0
ip ospf 1 area 0
end
R1#show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
2.2.2.2 1 2WAY/DROTHER 00:00:39 10.0.0.2 FastEthernet0/0
3.3.3.3 1 FULL/BDR 00:00:37 10.0.0.3 FastEthernet0/0
4.4.4.4 1 FULL/DR 00:00:36 10.0.0.4 FastEthernet0/0
R1#show ip ospf interface FastEthernet 0/0
FastEthernet0/0 is up, line protocol is up
Internet Address 10.0.0.1/24, Area 0
Process ID 1, Router ID 1.1.1.1, Network Type BROADCAST, Cost: 10
Enabled by interface config, including secondary ip addresses
Transmit Delay is 1 sec, State DROTHER, Priority 1
Designated Router (ID) 4.4.4.4, Interface address 10.0.0.4
Backup Designated router (ID) 3.3.3.3, Interface address 10.0.0.3
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
Non-Broadcast Multiaccess.
The rest of this post is related to obscure Frame Relay technology which I have never seen in real production networks. Configuration examples can be found here.
With Frame Relay you have several interface configuration options, namely - physical interface, point-to-point subinterfaces, point-to-multipoint subinterfaces. Let's consider all of them.
Connection using physical interfaces.
R1#show running-config interface s1/0
interface Serial1/0
ip address 10.0.0.1 255.255.255.0
encapsulation frame-relay
ip ospf 1 area 0
no keepalive
frame-relay interface-dlci 102
frame-relay interface-dlci 103
frame-relay lmi-type ansi
end
Because only R1 has DLCIs to all other routers, it should become DR, on other routers the ospf priority is set to 0:
R2#show running-config interface s1/0
interface Serial1/0
ip address 10.0.0.2 255.255.255.0
encapsulation frame-relay
ip ospf priority 0
ip ospf 1 area 0
no keepalive
frame-relay map ip 10.0.0.3 201
frame-relay interface-dlci 201
frame-relay lmi-type ansi
end
R1#show ip ospf interface serial 1/0
Serial1/0 is up, line protocol is up
Internet Address 10.0.0.1/24, Area 0
Process ID 1, Router ID 1.1.1.1, Network Type NON_BROADCAST, Cost: 64
Enabled by interface config, including secondary ip addresses
Transmit Delay is 1 sec, State DR, Priority 1
Designated Router (ID) 1.1.1.1, Interface address 10.0.0.1
No backup designated router on this network
Timer intervals configured, Hello 30, Dead 120, Wait 120, Retransmit 5
The network type is non-broadcast which means that no OSPF multicast messages are allowed. So you need to manually specify neighbors, forcing OSPF to use unicast messages:
R1#show running-config | section router ospf
router ospf 1
neighbor 10.0.0.2
neighbor 10.0.0.3
R1#show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
2.2.2.2 0 FULL/DROTHER 00:01:41 10.0.0.2 Serial1/0
3.3.3.3 0 FULL/DROTHER 00:01:55 10.0.0.3 Serial1/0
Notice, that with Frame Relay you actually can specify ospf network type as broadcast. But this will require frame-relay mappings with "broadcast" keyword:
R1#show running-config interface serial 1/0
interface Serial1/0
ip address 10.0.0.1 255.255.255.0
encapsulation frame-relay
ip ospf network broadcast
ip ospf 1 area 0
no keepalive
frame-relay map ip 10.0.0.3 103 broadcast
frame-relay map ip 10.0.0.2 102 broadcast
frame-relay interface-dlci 102
frame-relay interface-dlci 103
frame-relay lmi-type ansi
end
R1#show ip ospf interface serial 1/0
Serial1/0 is up, line protocol is up
Internet Address 10.0.0.1/24, Area 0
Process ID 1, Router ID 1.1.1.1, Network Type BROADCAST, Cost: 64
Enabled by interface config, including secondary ip addresses
Transmit Delay is 1 sec, State DR, Priority 1
Designated Router (ID) 1.1.1.1, Interface address 10.0.0.1
No backup designated router on this network
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
R1#show running-config | section router ospf 1
router ospf 1
log-adjacency-changes
Router will send multicast packets as unicast according to configured Frame Relay mappings, which will lead essentially to dynamic neighbor discovery.
R1#show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
2.2.2.2 0 FULL/DROTHER 00:00:36 10.0.0.2 Serial1/0
3.3.3.3 0 FULL/DROTHER 00:00:30 10.0.0.3 Serial1/0
Connection using point-to-point subinterfaces.
The topology is the same:
interface Serial1/0
no ip address
encapsulation frame-relay
no keepalive
end
R1#show running-config interface serial 1/0.102
interface Serial1/0.102 point-to-point
ip address 10.0.0.1 255.255.255.128
ip ospf 1 area 0
frame-relay interface-dlci 102
end
R1#show running-config interface serial 1/0.103
interface Serial1/0.103 point-to-point
ip address 10.0.0.129 255.255.255.128
ip ospf 1 area 0
frame-relay interface-dlci 103
On other routers the configuration is almost the same.
R1#show ip ospf interface serial 1/0.102
Serial1/0.102 is up, line protocol is up
Internet Address 10.0.0.1/25, Area 0
Process ID 1, Router ID 1.1.1.1, Network Type POINT_TO_POINT, Cost: 64
Enabled by interface config, including secondary ip addresses
Transmit Delay is 1 sec, State POINT_TO_POINT
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
As we can see IOS uses point-to-point network type, which is pretty logical.
R1#show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
2.2.2.2 0 FULL/ - 00:00:34 10.0.0.2 Serial1/0.102
3.3.3.3 0 FULL/ - 00:00:38 10.0.0.130 Serial1/0.103
And as long these interfaces are point-to-point we don't need DR/BDR on them.
Connection using point-to-multipoint subinterfaces.
interface Serial1/0
no ip address
encapsulation frame-relay
no keepalive
end
R1#show running-config interface serial 1/0.1
interface Serial1/0.1 multipoint
ip address 10.0.0.1 255.255.255.0
ip ospf network point-to-multipoint
ip ospf 1 area 0
snmp trap link-status
frame-relay map ip 10.0.0.3 103 broadcast
frame-relay map ip 10.0.0.2 102 broadcast
end
R1#show ip ospf interface serial 1/0.1
Serial1/0.1 is up, line protocol is up
Internet Address 10.0.0.1/24, Area 0
Process ID 1, Router ID 1.1.1.1, Network Type POINT_TO_MULTIPOINT, Cost: 64
Enabled by interface config, including secondary ip addresses
Transmit Delay is 1 sec, State POINT_TO_MULTIPOINT
Timer intervals configured, Hello 30, Dead 120, Wait 120, Retransmit 5
By default OSPF treats these multipoint-subinterfaces as regular non-broadcast interfaces, preventing dynamic neighbor discovery. Therefore "ip ospf network point-to-multipoint" is required. Frame relay mappings with "broadcast" keyword required as well. After this configuration OSPF treats each pair of routers (that have common DLCI) as point-to-point neighbors and DR/BDR therefore aren't needed:
R1#show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
3.3.3.3 0 FULL/ - 00:01:43 10.0.0.3 Serial1/0.1
2.2.2.2 0 FULL/ - 00:01:59 10.0.0.2 Serial1/0.1
Alternatively you can use "ip ospf network point-to-multipoint non-broadcast". As with regular non-broadcast interfaces you have to specify neighbors manually, but no DR/BDR elections will occur, because each pair of adjacencies will be treated as point-to-point:
R1#show running-config interface serial 1/0.1
interface Serial1/0.1 multipoint
ip address 10.0.0.1 255.255.255.0
ip ospf network point-to-multipoint non-broadcast
ip ospf 1 area 0
frame-relay interface-dlci 102 #Notice, that there is no more frame-relay mappings due to lack of multicast requirement.
frame-relay interface-dlci 103
end
R1#show running-config | section router ospf
router ospf 1
neighbor 10.0.0.3
neighbor 10.0.0.2
R1#show ip ospf interface serial 1/0.1
Serial1/0.1 is up, line protocol is up
Internet Address 10.0.0.1/24, Area 0
Process ID 1, Router ID 1.1.1.1, Network Type POINT_TO_MULTIPOINT, Cost: 64
Enabled by interface config, including secondary ip addresses
Transmit Delay is 1 sec, State POINT_TO_MULTIPOINT
Timer intervals configured, Hello 30, Dead 120, Wait 120, Retransmit 5
To sum up the information above:
| Type | DR/BDR | Dynamic Neighbors | Hello/Dead | Labels |
|---|---|---|---|---|
| Broadcast | Y | Y | 10/40 | Default for Ethernet links, requires broadcast mappings (for Frame Relay) |
| Non-Broadcast | Y | N | 30/120 | Default for Frame Relay physical and multipoint interfaces |
| Point-to-Point | N | Y | 10/40 | Default for Serial links and Frame Relay point-to-point subinterfaces |
| Point-to-Multipoint | N | Y | 30/120 | Requires static neighbors and broadcast mappings (for Frame Relay) |
| Point-to-Multipoint Non-Broadcast | N | N | 30/120 | Requires static neighbors |
And the last important thing - you can mix network types on different ends of the link as long as they can interoprate with each other, in other words when these two types of networks both (not) require DR/BDR, dynamic neighbor discovery and have identical timers values:
OSPF over Frame-Relay – Part 6: Troubleshooting
Understanding OSPF Network Types
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