David Sudjiman

Non Broadcast Multi Access (NBMA) - RFC 2328 Standard

• Full or partial-mesh network topology.
• One IP Subnet.
• Hello Timer 30 seconds.
• Manual adjacency, DR/BDR elected.

From the previous example, we see that we’re using broadcast mode on NBMA network. What if we don’t want to use broadcast mode since it’s a non-broadcast network, anyway?

There is another type of network in OSPF called non-broadcast. Complete command for these three routers are below.

R1#sh run int s1/0
Building configuration...

Current configuration : 219 bytes
!
interface Serial1/0
 ip address 192.168.1.1 255.255.255.0
 encapsulation frame-relay
 ip ospf network non-broadcast
 serial restart-delay 0
 frame-relay map ip 192.168.1.2 102
 frame-relay map ip 192.168.1.3 103
 no frame-relay inverse-arp
end

R1#sh run | s router ospf
router ospf 1
 log-adjacency-changes
 network 1.1.1.1 0.0.0.0 area 0
 network 192.168.1.0 0.0.0.255 area 0
 neighbor 192.168.1.2
 neighbor 192.168.1.3
R1#sh ip ospf nei

Neighbor ID     Pri   State           Dead Time   Address         Interface
2.2.2.2           1   FULL/DROTHER    00:01:39    192.168.1.2     Serial1/0
3.3.3.3           1   FULL/DR         00:01:38    192.168.1.3     Serial1/0

R2#sh run int s1/0
Building configuration...

Current configuration : 219 bytes
!
interface Serial1/0
 ip address 192.168.1.2 255.255.255.0
 encapsulation frame-relay
 ip ospf network non-broadcast
 serial restart-delay 0
 frame-relay map ip 192.168.1.1 201
 frame-relay map ip 192.168.1.3 203
 no frame-relay inverse-arp
end

R2#sh run | s router ospf
router ospf 1
 log-adjacency-changes
 network 2.2.2.2 0.0.0.0 area 0
 network 192.168.1.0 0.0.0.255 area 0
 neighbor 192.168.1.3
 neighbor 192.168.1.1
R2#sh ip ospf nei

Neighbor ID     Pri   State           Dead Time   Address         Interface
3.3.3.3           1   FULL/DR         00:01:40    192.168.1.3     Serial1/0
1.1.1.1           1   FULL/BDR        00:01:40    192.168.1.1     Serial1/0

R3#sh run int s1/0
Building configuration...

Current configuration : 219 bytes
!
interface Serial1/0
 ip address 192.168.1.3 255.255.255.0
 encapsulation frame-relay
 ip ospf network non-broadcast
 serial restart-delay 0
 frame-relay map ip 192.168.1.1 301
 frame-relay map ip 192.168.1.2 302
 no frame-relay inverse-arp
end

R3#sh run | s router ospf
router ospf 1
 log-adjacency-changes
 network 3.3.3.3 0.0.0.0 area 0
 network 192.168.1.0 0.0.0.255 area 0
 neighbor 192.168.1.1
 neighbor 192.168.1.2

R3#sh ip ospf nei

Neighbor ID     Pri   State           Dead Time   Address         Interface
1.1.1.1           1   FULL/BDR        00:01:54    192.168.1.1     Serial1/0
2.2.2.2           1   FULL/DROTHER    00:01:55    192.168.1.2     Serial1/0

R1#sh ip ospf int s1/0
Serial1/0 is up, line protocol is up
  Internet Address 192.168.1.1/24, Area 0
  Process ID 1, Router ID 1.1.1.1, Network Type NON_BROADCAST, Cost: 64
  Transmit Delay is 1 sec, State DR, Priority 1
  Designated Router (ID) 1.1.1.1, Interface address 192.168.1.1
  Backup Designated router (ID) 2.2.2.2, Interface address 192.168.1.2
  Timer intervals configured, Hello 30, Dead 120, Wait 120, Retransmit 5
    oob-resync timeout 120
    Hello due in 00:00:22
  Supports Link-local Signaling (LLS)
  Index 2/2, flood queue length 0
  Next 0×0(0)/0×0(0)
  Last flood scan length is 0, maximum is 1
  Last flood scan time is 0 msec, maximum is 4 msec
  Neighbor Count is 2, Adjacent neighbor count is 2
    Adjacent with neighbor 2.2.2.2  (Backup Designated Router)
    Adjacent with neighbor 3.3.3.3
  Suppress hello for 0 neighbor(s)

You might probably have a question, why should we suse this? can we just use the broadcast network type as previous example? What’s the different?

The different is that each router, in a non-broadcast network, will not sending multicast packets but sending unicast packets.

R1#debug ip ospf hello
OSPF hello events debugging is on
R1#
*Mar  1 01:43:40.059: OSPF: Send hello to 192.168.1.2 area 0 on Serial1/0 from 192.168.1.1
*Mar  1 01:43:40.063: OSPF: Send hello to 192.168.1.3 area 0 on Serial1/0 from 192.168.1.1
*Mar  1 01:43:41.411: OSPF: Rcv hello from 2.2.2.2 area 0 from Serial1/0 192.168.1.2
*Mar  1 01:43:41.415: OSPF: End of hello processing
*Mar  1 01:43:41.635: OSPF: Rcv hello from 3.3.3.3 area 0 from Serial1/0 192.168.1.3
*Mar  1 01:43:41.639: OSPF: End of hello processing

Another difference is that those routers are no longer be able to find each other without manual adjacency (neighbor command). Remember, this is a non-broadcast network.