OSPF–Point To Multipoint Networks

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using the same setup we used in the last 2 posts for Broadcast and Non-Broadcast Networks, but we will create multipoint interface on each router

R1

interface Serial0/0
 no ip address
 encapsulation frame-relay
 no frame-relay inverse-arp
!
interface Serial0/0.123 multipoint
 ip address 10.1.123.1 255.255.255.0
 frame-relay map ip 10.1.123.2 102 broadcast
 frame-relay map ip 10.1.123.3 103 broadcast

R2

interface Serial0/0
 no ip address
 encapsulation frame-relay
 no frame-relay inverse-arp
!
interface Serial0/0.123 multipoint
 ip address 10.1.123.2 255.255.255.0
 frame-relay map ip 10.1.123.1 201 broadcast

R3

interface Serial1/0
 no ip address
 encapsulation frame-relay
 no frame-relay inverse-arp
!
interface Serial0/0.123 multipoint
 ip address 10.1.123.3 255.255.255.0
 frame-relay map ip 10.1.123.1 301 broadcast
we will enable OSPF under all sub interfaces
interf s0/0.123
ip ospf 1 area 0

R2
interf s0/0.123
ip ospf 1 area 0

R3
interf s1/0.123
ip ospf 1 area 0
if we would check the default OSPF network for multipoint subinterface it should be Non-Broadcast similar to the physical interfaces
R3(config)#do sh ip os inter 
Serial1/0.123 is up, line protocol is up
  Internet Address 10.1.123.3/24, Area 0
  Process ID 1, Router ID 3.3.3.3, Network Type NON_BROADCAST, Cost: 781
  Enabled by interface config, including secondary ip addresses
we will change the OSPF network type to point-to-multipoint under all subinterfaces
We will change R1 s0/0.123 and see what happens
interf s0/0.123
ip ospf network point-to-multipoint
Once you do that you will notice that OSPF adjacencies are coming up between the R1 – R2 and R1 – R3
R1
R1(config-router)#do sh ip os inter
Serial0/0.123 is up, line protocol is up
  Internet Address 10.1.123.1/24, 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

R1(config-router)#do sh ip ospf ne
Neighbor ID     Pri   State           Dead Time   Address         Interface
2.2.2.2           0   FULL/  -        00:01:37    10.1.123.2      Serial0/0.123
3.3.3.3           0   FULL/  -        00:01:46    10.1.123.3      Serial0/0.123
R1(config-router)#do sh ip route ospf
R1(config-router)#
R2
R2(config)#do sh ip os ne

Neighbor ID     Pri   State           Dead Time   Address         Interface
1.1.1.1           1   FULL/BDR        00:01:35    10.1.123.1      Serial0/0.123


R2(config)#do sh ip route ospf
R2(config)#
R3
R3(config)#do sh ip os ne

Neighbor ID     Pri   State           Dead Time   Address         Interface
1.1.1.1           1   FULL/BDR        00:01:50    10.1.123.1      Serial1/0.123
We can clearly see from the output above that even we have OSPF adjacencies between the 3 routers, the OSPF database are not exchanged correctly between them, simply every router has a different view of that area.
Things should be sorted out after changing the OSPF network types on R2 and R3
R1(config)#do sh ip os ne

Neighbor ID     Pri   State           Dead Time   Address         Interface
2.2.2.2           0   FULL/  -        00:01:55    10.1.123.2      Serial0/0.123
3.3.3.3           0   FULL/  -        00:01:58    10.1.123.3      Serial0/0.123
R1(config)#do sh ip route


R1(config)#do sh ip route os
     2.0.0.0/32 is subnetted, 1 subnets
O       2.2.2.2 [110/65] via 10.1.123.2, 00:00:30, Serial0/0.123
     3.0.0.0/32 is subnetted, 1 subnets
O       3.3.3.3 [110/65] via 10.1.123.3, 00:00:30, Serial0/0.123
     10.0.0.0/8 is variably subnetted, 3 subnets, 2 masks
O       10.1.123.2/32 [110/64] via 10.1.123.2, 00:00:30, Serial0/0.123
O       10.1.123.3/32 [110/64] via 10.1.123.3, 00:00:30, Serial0/0.123
R2
R2(config)#do sh ip os ne

Neighbor ID     Pri   State           Dead Time   Address         Interface
1.1.1.1           0   FULL/  -        00:01:47    10.1.123.1      Serial0/0.123

R2(config)#do sh ip route ospf
     1.0.0.0/32 is subnetted, 1 subnets
O       1.1.1.1 [110/65] via 10.1.123.1, 00:04:40, Serial0/0.123
     3.0.0.0/32 is subnetted, 1 subnets
O       3.3.3.3 [110/129] via 10.1.123.3, 00:04:40, Serial0/0.123
     10.0.0.0/8 is variably subnetted, 3 subnets, 2 masks
O       10.1.123.1/32 [110/64] via 10.1.123.1, 00:04:40, Serial0/0.123
O       10.1.123.3/32 [110/128] via 10.1.123.1, 00:04:40, Serial0/0.123
R3
R3(config)#do sh ip os ne

Neighbor ID     Pri   State           Dead Time   Address         Interface
1.1.1.1           0   FULL/  -        00:01:43    10.1.123.1      Serial1/0.123
R3(config)#do sh ip route ospf
     1.0.0.0/32 is subnetted, 1 subnets
O       1.1.1.1 [110/782] via 10.1.123.1, 00:24:25, Serial1/0.123
     2.0.0.0/32 is subnetted, 1 subnets
O       2.2.2.2 [110/846] via 10.1.123.2, 00:24:25, Serial1/0.123
     10.0.0.0/8 is variably subnetted, 3 subnets, 2 masks
O       10.1.123.1/32 [110/781] via 10.1.123.1, 00:24:25, Serial1/0.123
O       10.1.123.2/32 [110/845] via 10.1.123.1, 00:24:25, Serial1/0.123
few notes on the outputs above
  • We can see that point-to-multipoint OSPF networks don’t require DR/BDR election
  • We can also see the next hop for R2/R3 loopback interfaces on R3/R2 respectively are pointing to R1 as the Hub that's why we don't need the extra frame relay mapping on the spokes unlike broadcast  and non broadcast networks 
  • Finally we can see the host routes on the three routers, while the major subnets are suppressed and not being advertised by the Hub

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