OSPF–Point To Multipoint Non-Broadcast

Using same setup that we have been using in the last 3 posts, we will change the network type to point-to-multipoint Non-Broadcast

we will also add a Ethernet link between R2 and R3 and enable OSPF on it

Our final configuration should be something like this

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  ip ospf network point-to-multipoint non-broadcast  ip ospf 1 area 0  frame-relay map ip 10.1.123.2 102  frame-relay map ip 10.1.123.3 103 !         router ospf 1  router-id 1.1.1.1  log-adjacency-changes  neighbor 10.1.123.2  neighbor 10.1.123.3

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  ip ospf network point-to-multipoint non-broadcast  ip ospf 1 area 0  frame-relay map ip 10.1.123.1 201

R3

interface Serial0/0  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  ip ospf network point-to-multipoint non-broadcast  ip ospf 1 area 0  frame-relay map ip 10.1.123.1 301

We notice that since the OSPF network is PTM Non-Broadcast I don’t need the broadcast key-word with the frame relay  mapping however we do need the OSPF neighbors statements under the OSPF process of R1

If we check the OSPF routes on R1

R1(config)#do sh ip route ospf 1      2.0.0.0/32 is subnetted, 1 subnets O       2.2.2.2 [110/65] via 10.1.123.2, 00:10:26, Serial0/0.123      3.0.0.0/32 is subnetted, 1 subnets O       3.3.3.3 [110/65] via 10.1.123.3, 00:10:26, Serial0/0.123      10.0.0.0/8 is variably subnetted, 4 subnets, 2 masks O       10.1.23.0/24 [110/65] via 10.1.123.3, 00:10:26, Serial0/0.123                      [110/65] via 10.1.123.2, 00:10:26, Serial0/0.123 O       10.1.123.2/32 [110/64] via 10.1.123.2, 00:10:26, Serial0/0.123 O       10.1.123.3/32 [110/64] via 10.1.123.3, 00:10:26, Serial0/0.123

We can see that the output is identical to the point-to-multipoint OSPF networks

I will shut down f0/0 on R3 so we can see the routes through the FR

R3(config-if)#do sh ip route os      1.0.0.0/32 is subnetted, 1 subnets O       1.1.1.1 [110/782] via 10.1.123.1, 00:00:23, Serial1/0.123      2.0.0.0/32 is subnetted, 1 subnets O       2.2.2.2 [110/846] via 10.1.123.1, 00:00:23, Serial1/0.123      10.0.0.0/8 is variably subnetted, 4 subnets, 2 masks O       10.1.123.1/32 [110/781] via 10.1.123.1, 00:00:23, Serial1/0.123 O       10.1.123.2/32 [110/845] via 10.1.123.1, 00:00:23, Serial1/0.123

When we unshut F0//0 R3 and R2 should be routing through the Ethernet link between each other

From R1 perspective 10.1.23.0/24 is received  from R2 and R3 and the cost to reach these two routers is the same so R1 is load balancing between them but what if one of the PVC has less than BW than the other and we want to use it over the second PVC

With Broadcast and Non Broadcast OSPF networks this wouldn’t be possible, however with PTM and PTM Non-Broadcast  we can configure cost with the neighbor statements so we can choose to route over a certain neighbor if we wish , So may PVC 103 is 512 Kbps while PVC 102 is 512 and we want to reach 10.1.23.0/24 through R3

Using cost command on R1 we can achieve that

R1(config)#router ospf 1 R1(config-router)#neighbor 10.1.123.2 cost 100 R1(config-router)#neighbor 10.1.123.2 cost 50

Now let’s check the routes on R1

R1(config-router)#do sh ip route os      2.0.0.0/32 is subnetted, 1 subnets O       2.2.2.2 [110/101] via 10.1.123.2, 00:01:56, Serial0/0.123      3.0.0.0/32 is subnetted, 1 subnets O       3.3.3.3 [110/51] via 10.1.123.3, 00:01:56, Serial0/0.123      10.0.0.0/8 is variably subnetted, 4 subnets, 2 masks O       10.1.23.0/24 [110/51] via 10.1.123.3, 00:01:56, Serial0/0.123 O       10.1.123.2/32 [110/100] via 10.1.123.2, 00:01:56, Serial0/0.123 O       10.1.123.3/32 [110/50] via 10.1.123.3, 00:01:56, Serial0/0.123