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OSPF over Frame Relay Configuration Options

Types of Frame Relay Topologies:

• Star Topology
  • aka hub-and-spoke configuration.
  • Remote sites connect to a central site.
  • The central router provides a multipoint connection because it typically uses a single interface to interconnect multiple PVCs.
  • Least expensive type and thus most commonly used topology.
• Full-mesh Topology
  • All routers have direct connections (VCs) to all other routers.
  • Its the most expensive topology. As more routers are added the more costly it becomes.
  • The formula to determine the number of VCs needed: n(n-1)/2, where n is the number of nodes in the network.
• Partial-mesh Topology
  • Only some routers have direct access to central site.
  • Cheaper to implement than a full-mesh.

OSPF over NBMA Topology Modes of Operation

To configure OSPF mode, the following interface configuration command is used:

ip ospf network {broadcast | non-broadcast | point-to-multipoint [non-broadcast] | point-to-point}

The following describes the type and parameters used in the ip ospf network command:

Two official modes in NBMA topologies, as described in RFC 2328:

• Nonbroadcast
  • Simulates the operation of OSPF in broadcast networks
  • Same IP subnet.
  • Neighbors must be configured manually.
  • DR and BDR election is required.
  • DR and BDR need to have full connectivity with all other routers
  • Configuration typically for fully-meshed networks (but can be partial-meshed)
  • Advantage is that it has less overhead traffic as compared to point-to-multipoint.
• Point to Multipoint
  • Treats the nonbroadcast network as a collection of point-to-point links
  • Routers automatically identify their neighboring routers. Uses a multicast hello packet to automatically discover the neighbors.
  • Do not elect DR and BDR. The router sends additional LSAs with more information about neighboring routers.
  • Configuration typically for partial-meshed, but also used for star topologies.
  • Advantage is that it requires less manual configuration

Cisco Modes of Operation for NBMA Network:

• Point-to-Multipoint Nonbroadcast
  • Neighbors must be configured manually
  • Does not require a DR or BDR
  • This mode should be used (instead of the RFC-compliant point-to-multipoint mode) if multicast and broadcast are not enabled on the VC.
    • That is because the router cannot dynamically discover its neighboring routers using the multicast hello packets.
• Broadcast
  • Uses one IP subnet
  • Makes the WAN interface appear to be a LAN
  • Uses a multicast OSPF hello packet to automatically discover neighbors.
  • DR and BDR are elected
  • Full or partial-mesh topology.
• Point-to-point
  • Each point-to-point connection has a different IP subnet
  • No DR or BDR election required
  • Only used between two routers that need to form an adjacency on a pair of interfaces.
  • Interfaces can either be LAN or WAN.

Defaul OSPF Modes

• On point-to-point Frame Relay subinterface - point-to-point mode
• On Frame Relay multipoint subinterface - nonbroadcast mode
• On a main Frame Relay interface - nonbroadcast mode.

OSPF Broadcast Mode Configuration

Sample configuration:

R1(config)#interface serial 1/0
R1(config-if)#encapsulation frame-relay
R1(config-if)#ip ospf network broadcast

• Neighbors must be manually configured on a nonbroadcast mode. Broadcast mode is a workaround for statically listing all existing neighbour routers.
• The interface is set to broadcast and behaves as though the router connects to a LAN.
• Because a DR and BDR election is required, make sure to use either a full-mesh topology or a static configuration of the DR based on the interface priority.

OSPF Nonbroadcast Mode Configuration

• Emulates operation over a broadcast network.
• All routers should be on the same IP subnet
• A DR and BDR are elected for the NBMA network
  • DR originates LSAs for the network.
• Best if the topology is fully-meshed
  • If not fully-meshed, select the DR and BDR manually. The goal is that the selecte DR and BDR have full connectivity to all other neighbors.
• The LSU packets must be replicated for each PVC. They are sent to each of the interface’s neighboring routers, as defined in the neighbor table.
• The command to statically define the adjacent relationships in NBMA networks using nonbroadcast mode:

R1(config-router)#neighbor ip-address [priority number] [poll-interval number] [cost number] [database-filter all]

• The parameters are described as follows:
  • ip-address
    • The IP address of the neighboring router
  • priority number
    • Optional parameter that sets the priority of the neighbor
    • 0 is the default, which means that the neighboring router does not participate in DR/BDR election
  • poll-interval number
    • Optional parameter that sets the length of time (in seconds) that an NBMA interface waits before sending hellos to the neighbors even if the neighbor is inactive.
  • cost number
    • Optional parameter that assigns a cost to the neighbor using any value from 1 to 65535.
    • If now specific cost is configured for a neighbor, the neighbor assumes the cost of the interface based on the ip ospf cost command.
    • For point-to-multipoint interfaces, the cost number keyword/argument parameters are the only options that are applicable
    • This keyword does not apply to nonbroadcast mode.
  • database-filter all
    • Optional parameter that filters outgoing LSAs to an OSPF neighbor.

Using the neighbor command in Nonbroadcast Mode

Router1 Configuration

interface Serial2
ip address 1.1.1.2 255.255.255.0
encapsulation frame-relay
ip ospf priority 2
no keepalive
frame-relay map ip 1.1.1.1 16
!
router ospf 1
network 1.1.1.0 0.0.0.255 area 0
neighbor 1.1.1.1

Router2 Configuration

interface Serial1/0
ip address 1.1.1.1 255.255.255.0
encapsulation frame-relay
no keepalive
clockrate 2000000
frame-relay map ip 1.1.1.2 16
!
router ospf 1
network 1.1.1.0 0.0.0.255 area 0
neighbor 1.1.1.2

• The ip opsf priority 2 on Router1 sets it as a DR because it has a higher priority value. The only other router (Router2) in this scenario has a default value of, which makes Router2 a BDR
  • To remove Router2 from becoming a BDR, configure an ip ospf priority 0 on Router2’s s1/0 interface.
  • In fact, with multiple routers and no full-mesh topology, set the spoke routers’ priority to 0 to ensure that only the hub becomes the DR - because the hub is the only one that has connectivity to all other routers.
• Though it is sufficient in this example to configure the neighbor command on one end to form adjacency, it is good practice to configure it on both routers, as shown in the scenario.
• Additionally, the frame-relay map commands did not need the broadcast parameter because the OSPF packets are unicasted with the neighbor statement.

• In nonbroadcast mode, neighbor statements are required only on DR and BDR.
• In a hub-and-spoke topology, neighbor statements must be placed on the hub.
  • The hub must be configured to become DR by assigning a higher priority.
• It is not mandatory to configure neighbor statements on spoke routers.
• In a full-mesh NBMA topology, it might be necessary to configure neighbor statements on all routers unless the DR/BDR are statically configured using the ip ospf priority command.
• The following is what the show ip ospf neighbor would display if ran on Router1.

OSPF Configuration in Point-to-Multipoint Mode (RFC-compliant)

• RFC-compliant point-to-multipoint mode is designed for partial-mesh or star topology.
  • OSPF treats router-to-router connections as if they are point-to-point links.
  • Multicast packets discover neighboring routers dynmically
• DRs are not used
• Type 2 Network LSAs are not flooded.
• Works by exchanging LSUs that are designed to automatically discover neighboring routers and add them to the neighbor table.
• Properties of point-to-multipoint mode:
  • Full-mesh network not necessary
    • Two routers can exchange routes without being directly connected. They are, however, connected to a router that has VCs to each of the two routers.
  • No static neighbor configuration
    • Point-to-multipoint mode treats the network as a collection of point-to-point links.
    • Hellos, updates and acknowledgments were sent using multicast. In particular, multicast hellos discovered all neighbors dynamically.
  • One subnet
    • With nonbroadcast mode, point-to-multipoint mode has all routers on the same subnet.
  • Duplicates LSA packets
    • Also similar to nonbroadcast mode, the router replicates the LSU packets and sent out to each of the interfaces neighboring routers.

OSPF Point-to-Multipoint Configuration

Router R1 Configuration

interface serial 1/0
ip address 10.0.0.1 255.0.0.0
ip ospf network point-to-multipoint
encapsulation frame-relay
frame-relay map ip 10.0.0.2 102 broadcast
frame-relay map ip 10.0.0.3 103 broadcast
frame-relay map ip 10.0.0.4 104 broadcast
!
router ospf 1
network 10.0.0.0 0.0.0.255 area 0

Router R2 Configuration

interface serial 1/0
ip address 10.0.0.2 255.0.0.0
ip ospf network point-to-multipoint
encapsulation frame-relay
frame-relay map ip 10.0.0.1 201 broadcast
frame-relay map ip 10.0.0.3 203 broadcast

!
router ospf 1
network 10.0.0.0 0.0.0.255 area 0

Cisco Point-to-Multipoint Nonbroadcast mode

• This is a Cisco extension to the RFC-compliant mode
• With this mode, neighbors are statically configured, just like nonbroadcast modes.
  • DRs and BDRs are not elected.
• Modify the neighbor link cost to reflect the different bandwidth of each link.
• Used for VCs that cannot use multicasts or broadcasts
  • RFC point-to-multipoint mode was developed to support underlying point-to-multipoint VCs that support multicast and broadcast

Using Subinterfaces in OSPF over Frame Relay Configuration

• Subinterfaces are accomplished by splitting a physical interface into multiple logical interfaces.
  • Each interface can be defined as a point-to-point or a multipoint interface.
  • They were originally created to handle problems with split horizon over NBMA using distance-vector protocols.
  • Each subinterface is a different subnet
  • A point-to-point subinterface is similar to a physical point-to-point link.
  • To define the subinterface use use the global command:

interface serial number.subinterface-number {multipoint | point-to-point}

• The choice of multipoint or point-to-point affects OSPF operation

Point-to-Point Subinterfaces

• On a point-to-point subinterface, each VC has its own subinterface.
• Because it operates just like a physical point-to-point, there is no DR/BDR.
  • Neighbor discovery is automatic
  • Neighbors don’t need to be configured
• A point-to-point subinterface is usually used with a point-to-point mode, where only two nodes exist on the NBMA network.
• Each point-to-point connection is a separate subnet.

Multipoint Subinterfaces

• With this configuration, a single interface has multiple VCs
• Multipoint Frame Relay subinterfaces default to OSPF nonbroadcast mode.
  • This implies that neighbors need to be statically configured.
  • A DR and BDR are also required.

Resources:

1. OSPF Design Guide
2. Initial Configurations for OSPF over Non-Broadcast Links
3. Adjacencies on Non-Broadcast Multi-Access (NBMA) Networks
4. Configuring OSPF
   • OSPF Configuration Examples

This entry is not an authoritative guide. These are merely notes and rehash of the primary text materials and resources that I use. For a thorough guide of the BSCI course, consider purchasing Building Scalable Cisco Internetworks (BSCI) (Authorized Self-Study Guide) (3rd Edition) by Diane Teare and Catherine Paquet, as well as following the links on the resources section of this entry.