OSPF Area Types - Broadcast
In this post and the following posts we will try to explore OSPF network types, OSPF is a core technology in the CCIE lab so you need to be able to configure it without referring to the DoC-CD
In Cisco world, OSPF comes in 6 Network types
1- Broadcast
2- Non-Broadcast
3- point-to-multipoint
4- point-to-multipoint Non-Broadcast (Cisco proprietary)
5- Loopback
We will examine each of these network types over NBMA (Non Broadcast Multiple Access) Media like frame relay network since this is where the confusion coming from.
We will start with the Non-Broadcast networks since it is the default OSPF network when you configure OSPF under frame relay physical interfaces
Initial configurations
R1
| ! interface Serial0/0 ip address 10.1.123.1 255.255.255.0 encapsulation frame-relay frame-relay map ip 10.1.123.2 102 broadcast frame-relay map ip 10.1.123.3 103 broadcast no frame-relay inverse-arp interf lo0 ip add 1.1.1.1 255.255.255.255 |
R2
| ! interface Serial0/0 ip address 10.1.123.2 255.255.255.0 encapsulation frame-relay frame-relay map ip 10.1.123.1 201 no frame-relay inverse-arp interf lo0 ip add 2.2.2.2 255.255.255.255 |
R3
| ! interface Serial1/0 ip address 10.1.123.3 255.255.255.0 encapsulation frame-relay frame-relay map ip 10.1.123.1 301 no frame-relay inverse-arp interf lo0 ip add 2.2.2.2 255.255.255.255 |
Few notes on the cconfigs, note that we haven’t configured any mapping between R2-R3 and we will see how this will affect our connectivity between the spokes
Now let’s configure OSPF
under all interfaces issue this command
ip ospf 1 area 0
You can wait as much as you can but no OSPF neighborship will ever come up!
Simply the OSPF network is Non-Broadcast so OSPF will not be trying to multicast Hello packets, it needs to unicast the Hello packets to a neighbor that you define manually
if we configure the following debug on any of these routers we should should prove that
| ! R1#sh access-list Extended IP access list 101 10 permit ospf any any R1#debug ip ospf packet R1#debug ip packet detail 101 IP packet debugging is on (detailed) for access list 101 |
Under the Hub R1 we need to define the OSPF neighbors
| R1(config)#router ospf 1 R1(config-router)#neighbor 10.1.123.2 R1(config-router)#neighbor 10.1.123.3 |
Now let’s check the OSPF adjacencies
R1(config)#do sh ip os ne Neighbor ID Pri State Dead Time Address Interface 2.2.2.2 1 FULL/DR 00:01:59 10.1.123.2 Serial0/0 3.3.3.3 0 FULL/DROTHER 00:01:59 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/BDR 00:01:46 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 00:01:57 10.1.123.1 Serial1/0 |
As you can tell from the output above, the OSPF adjacencies are totally mixed up!
R1 thinks that R2 is the DR and no BDR
R2 thinks that it’s the DR while R1 is the BD and R3 is DROTHER
R3 thinks that R1 is the DR and have no idea about R2
This is because the DR role is distribute the routing updates on a broadcast networks, so updates get sent to the DR from different routers on the segment on 224.0.0.6 and then the DR replicates the updates on 224.0.0.6 to every other router on that segment, So by definition the DR needs to have connectivity to all other routers on the broadcast domain, In our case R1 should be elected as the DR
Since DR election is non preemptive in OSPF, lowering the priority of non-DR routers is not enough because they will still participate in DR election, the non-DR routers’ priority needs to be set to zero
| R2(config-if)#interf s0/0 R2(config-if)#ip os priority 0 R3(config-if)#interf s1/0 R3(config-if)#ip os priority 0 |
Clear the IP OSPF process on both R2 and R3
| R1(config)#do sh ip os ne Neighbor ID Pri State Dead Time Address Interface 2.2.2.2 0 FULL/DROTHER 00:01:41 10.1.123.2 Serial0/0 3.3.3.3 0 FULL/DROTHER 00:01:58 10.1.123.3 Serial0/0 R2(config)#do sh ip os ne Neighbor ID Pri State Dead Time Address Interface 1.1.1.1 1 FULL/DR 00:01:36 10.1.123.1 Serial0/0 Neighbor ID Pri State Dead Time Address Interface 1.1.1.1 1 FULL/DR 00:01:51 10.1.123.1 Serial1/0 |
As we can see things are more consistent now, however when we check the routing on either R2 or R3 we will notice that next hob value for their loopback interfaces are their serial interfaces
| R2(config)#do sh ip route ospf 1.0.0.0/32 is subnetted, 1 subnets O 1.1.11.1 [110/65] via 10.1.123.1, 00:11:19, Serial0/0 3.0.0.0/32 is subnetted, 1 subnets O 3.3.3.3 [110/65] via 10.1.123.3, 00:11:19, Serial0/0 R2(config)#do p 3.3.3.3 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 3.3.3.3, timeout is 2 seconds: . Success rate is 0 percent (0/1) |
Since 3.3.3.3 recuses to 10.1.123.3 and R2 doesn’t have a frame relay map to 10.1.123.3 the encapsulation will fail and R2 will not be able to reach R3 loopback and vice versa
To work out this issue without changing the OSPF network, we need to add frame relay map statements on both R2 and R3
| R2 interface Serial0/0 frame-relay map ip 10.1.123.3 201 R3 interface Serial1/0 frame-relay map ip 10.1.123.2 301 |
When we test ping now we should have connectivity between R2 and R3
| R2(config)#do p 3.3.3.3 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 3.3.3.3, timeout is 2 seconds: !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 112/119/132 ms |
Which we do
Last thing to mention on Non-Broadcast OSPF networks is the Hello/Dead Intervals which are set to 30 and 120 sec by default
| R2(config)#do sh ip os interf 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 30, Dead 120, Wait 120, Retransmit 5 |
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