David Sudjiman

This tutorial will cover three basic different configurations for Single Area OSPF on Frame Relay Full Mesh Network using nonbroadcast, broadcast, and point-to-point OSPF network mode.

Full Mesh Network using NBMA (nonbroadcast)

For Full mesh network we can use command ip ospf network nonbroadcast to define that the network is a nonbroadcast network. In a nonbroadcast (NBMA) network, routers can not send broadcast or multicast network. However, OSPF uses multicast address set to AllSPFRouters (224.0.0.5) to send Hello packet periodically. Therefore, to be able to make OSPF routing sent its Hello packet to its adjacent router, we need to define its neighbor. Thus, using this neighbor command, router will be able to sent Hello packet and elect DR/BDR. After DR/BDR elected, LSA packets are sent using unicast addresses.

Characteristics

• Full mesh network topology in one subnet.
• For Serial interface with Frame Relay encapsulation, the default network type is nonbroadcast. Therefore, command ip ospf network nonbroadcast is actually not necessary.
• Hello packet is sent every 30 seconds using Unicast addresses and link will be considered dead if not receiving Hello packet within 4×30 seconds (120 seconds).
• The will be DR/BDR election.

Router 1

interface Serial0
 ip address 200.1.1.1 255.255.255.0
 encapsulation frame-relay
 ip ospf network non-broadcast
!
router ospf 1
 network 200.1.1.0 0.0.0.255 area 0
 neighbor 200.1.1.2 priority 1
 neighbor 200.1.1.3 priority 1

Router 2

interface Serial0
 ip address 200.1.1.2 255.255.255.0
 encapsulation frame-relay
 ip ospf network non-broadcast
!
router ospf 1
 network 200.1.1.0 0.0.0.255 area 0
 neighbor 200.1.1.3 priority 1
 neighbor 200.1.1.2 priority 1

Router 3

interface Serial0
 ip address 200.1.1.3 255.255.255.0
 encapsulation frame-relay
 ip ospf network non-broadcast
!
router ospf 1
 network 200.1.1.0 0.0.0.255 area 0
 neighbor 200.1.1.2 priority 1
 neighbor 200.1.1.1 priority 1

Full Mesh Network using NBMA (broadcast)

Using the NBMA nonbroadcast mode is basically compliant to RFC-2328. Cisco provides NBMA broadcast which actually force the network to use broadcast mode thus avoiding to use neighbor command

Characteristics

• Full mesh network topology or star in one subnet. Usually found in LAN network such as Ethernet, Token Ring, and FDDI.
• For Serial interface with Frame Relay encapsulation, we have to define command ip ospf network broadcast otherwise nonbroadcast will be used.
• Hello packet is sent every 10 seconds using Multicast addresses and link will be considered dead if not receiving Hello packet within 4×10 seconds (40 seconds)
• The will be DR/BDR election.

Router 1

interface Serial0
 ip address 200.1.1.1 255.255.255.0
 encapsulation frame-relay
 ip ospf network broadcast
!
router ospf 1
 network 200.1.1.0 0.0.0.255 area 0

Router 2

nterface Serial0
 ip address 200.1.1.2 255.255.255.0
 encapsulation frame-relay
 ip ospf network broadcast
!
router ospf 1
 network 200.1.1.0 0.0.0.255 area 0

Router 3

interface Serial0
 ip address 200.1.1.3 255.255.255.0
 encapsulation frame-relay
 ip ospf network broadcast
!
router ospf 1
 network 200.1.1.0 0.0.0.255 area 0

Full Mesh Network using Point-to-point

Still using the same topology as shown on Figure 1. Instead using one subnet to cover all of the nodes, each PVC will use one subnet and create point-to-point network. There will be no DR/BDR election for this point-to-point network as each router will directly connected to its adjacent.

To create a point-to-point network on topology shown on Figure 1, each router will have to have two link available to connect to two other routers and it means to use two physical serial interfaces. While this is not a very feasible suggestion, we can use subinterfaces on each physical interface to create two PVCs. Subinterfaces basically a way to create another interfaces under one physical interface.

For this purpose, we will create a point-to-point subinterfaces and luckily, Cisco will automatically configure your OSPF mode to point-to-point.

Characteristics

• Point-to-point network.
• For Serial interface with Frame Relay encapsulation, it is recommended to use subinterfaces with point-to-point mode and ip ospf network broadcast.
• Hello packet is sent every 10 seconds using Multicast addresses and link will be considered dead if not receiving Hello packet within 4×10 seconds (40 seconds).
• No DR/BDR election as the network will only recognize two adjacent routers.

Router 1

hostname R1
!
!
interface Serial0
 no ip address
 encapsulation frame-relay
!
interface Serial0.1 point-to-point
 ip address 10.1.1.1 255.255.255.252
 frame-relay interface-dlci 222
!
interface Serial0.2 point-to-point
 ip address 10.1.1.10 255.255.255.252
 frame-relay interface-dlci 333
!
router ospf 1
 log-adjacency-changes
 network 10.1.1.0 0.0.0.15 area 0

Router 2

hostname R2
!
!
interface Serial0
 no ip address
 encapsulation frame-relay
!
interface Serial0.1 point-to-point
 ip address 10.1.1.2 255.255.255.252
 frame-relay interface-dlci 111
!
interface Serial0.2 point-to-point
 ip address 10.1.1.5 255.255.255.252
 frame-relay interface-dlci 333
!
router ospf 1
 log-adjacency-changes
 network 10.1.1.0 0.0.0.15 area 0

Router 3

hostname R3
!
interface Serial0
 no ip address
 encapsulation frame-relay
!
interface Serial0.1 point-to-point
 ip address 10.1.1.6 255.255.255.252
 frame-relay interface-dlci 222
!
interface Serial0.2 point-to-point
 ip address 10.1.1.9 255.255.255.252
 frame-relay interface-dlci 111
!
router ospf 1
 log-adjacency-changes
 network 10.1.1.0 0.0.0.15 area 0

Sources.

• Cisco System 2006, Configuring OSPF, viewed 15 May 2006, http://www.cisco.com/…[cut]…chapter09186a00800b3f2e.html
  
• Gough, C 2005, CCNP BSCI Exam Certification Guide 3rd Edn., Cisco Press, Indianapolis. (ISBN: 1587200856)