OSPF - Broadcast Networks

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