OSPF - Broadcast Networks

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We are building this lab on the previous lab OSPF – Non Broadcast Networks

We will remove the neighbor statements from R1 OSPF process and also changing the OSPF network type under the serial interfaces to Broadcast

R1

interface Serial0/0
 ip address 10.1.123.1 255.255.255.0
 encapsulation frame-relay
 ip ospf network broadcast
 ip ospf 1 area 0
 frame-relay map ip 10.1.123.2 102
 frame-relay map ip 10.1.123.3 103
 no frame-relay inverse-arp

R2
interface Serial0/0
 ip address 10.1.123.2 255.255.255.0
 encapsulation frame-relay
 ip ospf network broadcast
 ip ospf priority 0
 ip ospf 1 area 0
 frame-relay map ip 10.1.123.1 201
 frame-relay map ip 10.1.123.3 201
 no frame-relay inverse-arp

R3
interface Serial1/0
 ip address 10.1.123.3 255.255.255.0
 encapsulation frame-relay
 ip ospf network broadcast
 ip ospf priority 0
 ip ospf 1 area 0
 frame-relay map ip 10.1.123.1 301
 frame-relay map ip 10.1.123.2 301
 no frame-relay inverse-arp

Remember the OSPF network is broadcast so we still need a DR and BDR election

also we need to change the frame-relay mappings under all the serial interfaces to include the broadcast keyword so hello packets can be sent to the Multicast addresses

R1(config-if)#frame-relay map ip 10.1.123.2 102 b
R1(config-if)#frame-relay map ip 10.1.123.3 103 b

R2(config-if)#frame-relay map ip 10.1.123.1 201 b

R3(config-if)#frame-relay map ip 10.1.123.1 301 b

Note: we don’t need to include the broadcast key-word for the spoke-spoke mapping since they only reach other through the hub anyway

Check the OSPF adjacencies

R1(config)#do sh ip os ne

Neighbor ID     Pri   State           Dead Time   Address         Interface
2.2.2.2           0   FULL/DROTHER    00:00:38    10.1.123.2      Serial0/0
3.3.3.3           0   FULL/DROTHER    00:00:32    10.1.123.3      Serial0/0


R2(config-if)#do sh ip os ne

Neighbor ID     Pri   State           Dead Time   Address         Interface
1.1.1.1           1   FULL/DR         00:00:30    10.1.123.1      Serial0/0

R3(config-if)#do sh ip os ne

Neighbor ID     Pri   State           Dead Time   Address         Interface
1.1.1.1           1   FULL/DR         00:00:37    10.1.123.1      Serial1/0

Routing between spokes are still required mapping between different spokes similar to Non-Broadcast example we saw earlier

R3(config-if)#do sh ip route 2.2.2.2
Routing entry for 2.2.2.2/32
  Known via "ospf 1", distance 110, metric 782, type intra area
  Last update from 10.1.123.2 on Serial1/0, 00:08:09 ago
  Routing Descriptor Blocks:
  * 10.1.123.2, from 2.2.2.2, 00:08:09 ago, via Serial1/0
      Route metric is 782, traffic share count is 1

Although many similarities between the Broadcast and Non-Broadcast OSPF networks, timers are different 

in Non-Broadcast OSPF network hello are sent every 30 seconds while dead interval is 120 sec

In Broadcast OSPF networks hellos are sent every 10 seconds and dead interval is 40 seconds

R3(config-if)#do sh ip os interf s1/0
Serial1/0 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 BROADCAST, Cost: 781
  Enabled by interface config, including secondary ip addresses
  Transmit Delay is 1 sec, State DROTHER, Priority 0
  Designated Router (ID) 1.1.1.1, Interface address 10.1.123.1
  No backup designated router on this network
  Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5

Because of the many similarities between the two Network types, technically we can have them working together if we change the timers to each other

so in our case we can change R2 s0/0 to non-broadcast and change the timers to match R1 10 and 40 seconds and neighbor will come up

R2(config-if)#ip os network non-broadcast
R2(config-if)#ip ospf hello-interval 10

R2(config-if)#do sh ip os  nei  

Neighbor ID     Pri   State           Dead Time   Address         Interface
1.1.1.1           1   FULL/DR         00:00:36    10.1.123.1      Serial0/0

Serial0/0 is up, line protocol is up
  Internet Address 10.1.123.2/24, Area 0
  Process ID 1, Router ID 2.2.2.2, Network Type NON_BROADCAST, Cost: 64
  Enabled by interface config, including secondary ip addresses
  Transmit Delay is 1 sec, State DROTHER, Priority 0
  Designated Router (ID) 1.1.1.1, Interface address 10.1.123.1
  No backup designated router on this network
  Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
    oob-resync timeout 40
    Hello due in 00:00:03

while R1

R1(config)#do sh ip os interf s0/0
Serial0/0 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 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 10.1.123.1
  No backup designated router on this network
  Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5

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