In Hub & Spoke topology, the Hub MUST be the DR. Spoke not always have connectivity with other routers.
On the Spokes, configure ip ospf priority 0.
In this mode, you have to configure neighbors on the HUB router. There is no need to configure the neighbor on the Spokes: spokes will reply to unicast hellos from the Hub.
On r1:
!
interface Loopback0
ip address 150.1.1.1 255.255.255.0
!
interface FastEthernet0/0
ip address 150.10.0.11 255.255.255.0
ip ospf network point-to-point
duplex auto
speed auto
!
interface Serial0/0
ip address 150.0.0.1 255.255.255.0
encapsulation frame-relay
frame-relay map ip 150.0.0.4 104 broadcast
frame-relay map ip 150.0.0.3 103 broadcast
frame-relay map ip 150.0.0.2 102 broadcast
no frame-relay inverse-arp
!
router ospf 1
router-id 150.1.1.1
log-adjacency-changes
passive-interface Loopback0
network 150.0.0.1 0.0.0.0 area 0
network 150.1.1.1 0.0.0.0 area 0
network 150.10.0.11 0.0.0.0 area 0
neighbor 150.0.0.2
neighbor 150.0.0.3
neighbor 150.0.0.4
!
R1 S0/0 is NON_BROADCAST and R1 is the DR (default priority is 1).
Three neighbors are configured.
r1#show ip ospf interface S0/0
Serial0/0 is up, line protocol is up
Internet Address 150.0.0.1/24, Area 0
Process ID 1, Router ID 150.1.1.1, Network Type NON_BROADCAST, Cost: 64
Transmit Delay is 1 sec, State DR, Priority 1
Designated Router (ID) 150.1.1.1, Interface address 150.0.0.1
No backup designated router on this network
Timer intervals configured, Hello 30, Dead 120, Wait 120, Retransmit 5
oob-resync timeout 120
Hello due in 00:00:24
Supports Link-local Signaling (LLS)
Cisco NSF helper support enabled
IETF NSF helper support enabled
Index 1/1, flood queue length 0
Next 0x0(0)/0x0(0)
Last flood scan length is 1, maximum is 1
Last flood scan time is 4 msec, maximum is 4 msec
Neighbor Count is 3, Adjacent neighbor count is 3
Adjacent with neighbor 150.2.2.2
Adjacent with neighbor 150.3.3.3
Adjacent with neighbor 150.4.4.4
Suppress hello for 0 neighbor(s)
The neighbors are in FULL/DROTHER state, they are no DR, nor BDR.
r1#show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
55.0.0.5 0 FULL/ - 00:00:35 150.10.0.15 FastEthernet0/0
150.2.2.2 0 FULL/DROTHER 00:01:59 150.0.0.2 Serial0/0
150.3.3.3 0 FULL/DROTHER 00:01:41 150.0.0.3 Serial0/0
150.4.4.4 0 FULL/DROTHER 00:01:36 150.0.0.4 Serial0/0
r1#
On r2:
!
interface Loopback0
ip address 150.2.2.2 255.255.255.0
!
interface Serial0/0
ip address 150.0.0.2 255.255.255.0
encapsulation frame-relay
ip ospf priority 0
frame-relay map ip 150.0.0.4 201
frame-relay map ip 150.0.0.3 201
frame-relay map ip 150.0.0.1 201 broadcast
no frame-relay inverse-arp
!
router ospf 1
router-id 150.2.2.2
log-adjacency-changes
passive-interface Loopback0
network 150.0.0.2 0.0.0.0 area 0
network 150.2.2.2 0.0.0.0 area 0
!
r2#show ip ospf interface Serial 0/0
Serial0/0 is up, line protocol is up
Internet Address 150.0.0.2/24, Area 0
Process ID 1, Router ID 150.2.2.2, Network Type NON_BROADCAST, Cost: 64
Transmit Delay is 1 sec, State DROTHER, Priority 0
Designated Router (ID) 150.1.1.1, Interface address 150.0.0.1
No backup designated router on this network
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)
Cisco NSF helper support enabled
IETF NSF helper support enabled
Index 1/1, flood queue length 0
Next 0x0(0)/0x0(0)
Last flood scan length is 1, maximum is 1
Last flood scan time is 0 msec, maximum is 4 msec
Neighbor Count is 1, Adjacent neighbor count is 1
Adjacent with neighbor 150.1.1.1 (Designated Router)
Suppress hello for 0 neighbor(s)
r2#
r2#show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
150.1.1.1 1 FULL/DR 00:01:57 150.0.0.1 Serial0/0
r2#
And so on for r3 and r4.
The routing table on r2 is the following:
r2#show ip route ospf
55.0.0.0/32 is subnetted, 1 subnets
O 55.0.0.5 [110/66] via 150.0.0.1, 00:03:58, Serial0/0
150.1.0.0/32 is subnetted, 1 subnets
O 150.1.1.1 [110/65] via 150.0.0.1, 01:11:34, Serial0/0
150.3.0.0/32 is subnetted, 1 subnets
O 150.3.3.3 [110/65] via 150.0.0.3, 00:15:45, Serial0/0
150.4.0.0/32 is subnetted, 1 subnets
O 150.4.4.4 [110/65] via 150.0.0.4, 01:10:26, Serial0/0
150.10.0.0/24 is subnetted, 1 subnets
O 150.10.0.0 [110/65] via 150.0.0.1, 00:08:48, Serial0/0
We can see that the next-hop is handled as on BROADCAST networks. Even if the packets have to go via the Hub, NEXT-HOP is the spoke router that announces the route.
This behaviour implies that frame-relay mapping is present between each spokes.
This behaviour implies that frame-relay mapping is present between each spokes.
r2#show ip route 150.4.4.4
Routing entry for 150.4.4.4/32
Known via "ospf 1", distance 110, metric 65, type intra area
Last update from 150.0.0.4 on Serial0/0, 00:06:46 ago
Routing Descriptor Blocks:
* 150.0.0.4, from 150.4.4.4, 00:06:46 ago, via Serial0/0
Route metric is 65, traffic share count is 1
r2#show ip route 55.0.0.5
Routing entry for 55.0.0.5/32
Known via "ospf 1", distance 110, metric 66, type intra area
Last update from 150.0.0.1 on Serial0/0, 00:10:36 ago
Routing Descriptor Blocks:
* 150.0.0.1, from 55.0.0.5, 00:10:36 ago, via Serial0/0
Route metric is 66, traffic share count is 1
r1 is seen as a transit network, so it has the hability to change the next-hop:
r2#show ip ospf database router 150.4.4.4
OSPF Router with ID (150.2.2.2) (Process ID 1)
Router Link States (Area 0)
LS age: 222
Options: (No TOS-capability, DC)
LS Type: Router Links
Link State ID: 150.4.4.4
Advertising Router: 150.4.4.4
LS Seq Number: 8000000B
Checksum: 0xB608
Length: 48
Number of Links: 2
Link connected to: a Stub Network
(Link ID) Network/subnet number: 150.4.4.4
(Link Data) Network Mask: 255.255.255.255
Number of TOS metrics: 0
TOS 0 Metrics: 1
Link connected to: a Transit Network
(Link ID) Designated Router address: 150.0.0.1
(Link Data) Router Interface address: 150.0.0.4
Number of TOS metrics: 0
TOS 0 Metrics: 64
Notes/Tips/Traps:
Using ospf network NON_BROADCAST type in Hub & Spoke topology requires that a mapping exists on each spokes to reach each other spoke because the spoke's IP address is the next-hop for the prefix it advertises in the NBMA network.
So, if static mapping is not allowed, we are facing an issue with ospf network NON_BROADCAST type.
The broadcast statement in the mapping is not mandatory as the hellos are sent unicast.
