OSPF Network Type – Point-to-Multipoint

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Points to remember about Point-to-Multipoint OSPF network type.

1. No Designated Router (DR) and Back Designated Router(BDR) election.
2. Uses multicast to send OSPF hello.
3. Changes next-hop IP address.
4. Advertises /32 host route instead of a network route.
5. All routers in the segment must agree on the network type.

         L0:10.0.0.1/24         L0:20.0.0.2/24
            Spoke                   Spoke
           .-----.                 .-----.
           | R01 |---.        .----| R02 |
           '-----'   |        |    '-----'
                     |        |
                S0/0 |        | S0/0
                .1   v        v .2  
                 .------------------.
                / Frame Relay Switch \
               (     10.0.0.0/24      )
                \                    /
                 '------------------'
                           ^
                           | .3   
                           | S0/0 
                        .-----.
                        | R03 |
                        '-----'
                          Hub
		    L0:30.0.0.3/24

R01’s routing table indicates that R02’s loopback0 network 20.0.0.2/24 is advertised by 10.0.0.3 (R03’s serial 0/0). This means that point-to-multipoint OSPF network type changes the next-hop address before sending routes to its neighbors.

R1(config-if)#do sh ip route    
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area 
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2
       i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
       ia - IS-IS inter area, * - candidate default, U - per-user static route
       o - ODR, P - periodic downloaded static route

Gateway of last resort is not set

     20.0.0.0/32 is subnetted, 1 subnets
O       20.0.0.2 [110/129] via 10.0.0.3, 00:04:02, Serial0/0
     10.0.0.0/8 is variably subnetted, 3 subnets, 2 masks
O       10.0.0.2/32 [110/128] via 10.0.0.3, 00:04:02, Serial0/0
O       10.0.0.3/32 [110/64] via 10.0.0.3, 00:04:02, Serial0/0
C       10.0.0.0/24 is directly connected, Serial0/0
     11.0.0.0/24 is subnetted, 1 subnets
C       11.0.0.0 is directly connected, Loopback0
     30.0.0.0/32 is subnetted, 1 subnets
O       30.0.0.3 [110/65] via 10.0.0.3, 00:04:02, Serial0/0

Spokes must only have one “frame-relay map ip” statement pointing to the hub with the broadcast keyword. The Broadcast keyword allows broadcast and multicast processing.

R1(config-if)#do sh run int s0/0
!
interface Serial0/0
 ip address 10.0.0.1 255.255.255.0
 encapsulation frame-relay
 ip ospf network point-to-multipoint
 clock rate 2000000
 frame-relay map ip 10.0.0.3 103 broadcast

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