Implementing a Scalable Multiarea Network OSPF-Based Solution

1. Improving Routing Performance in a Complex Enterprise Network Implementing a Scalable Multiarea Network OSPF-Based Solution

2. OSPF Network Types • Point-to-point: A network that joins a single pair of routers. • Broadcast: A multiaccess broadcast network, such as Ethernet. • Nonbroadcast multiaccess (NBMA): A network that interconnects more than two routers but that has no broadcast capability. • Examples: Frame Relay, ATM, and X.25 • Five modes of OSPF operation are available for NBMA networks

3. Point-to-Point Links • Usually a serial interface running either PPP or HDLC • May also be a point-to-point subinterface running Frame Relay or ATM • Does not require DR or BDR election • Is automatically detected by OSPF • Sends OSPF packets using multicast 224.0.0.5

4. Multiaccess Broadcast Network • This generally applies to LAN technologies like Ethernet. • DR and BDR selection are required. • All neighbor routers form full adjacencies with the DR and BDR only. • Packets to the DR and the BDR use 224.0.0.6. • Packets from DR to all other routers use 224.0.0.5.

5. OSPF Adjacency Over Metro Ethernet and EoMPLS • EoMPLS and MetroEthernet service does not participate in STP,nor does it learn MAC addresses • Customer routers R1 and R2 exchange Ethernet frames via an interface or VLAN subinterfaces • OSPF behaves the same as on Ethernet • OSPF network type = Multiaccess Broadcast Network • DR and BDR are elected • Routers form full adjacencies with the DR and BDR only

6. OSPF Adjacency Over MPLS VPN • Customer routers run OSPF and exchange routing updates with the PE routers • PE routers appear as another router in the customer’s network • Service provider’s P routers are hidden from the customer • Customer routers are unaware of MPLS VPN • Customer and service provider must agree on OSPF parameters • Customer Routers to PE connection can be of any type • OSPF behaves per the connection type (point-to-point, broadcast, NBMA)

7. Electing the DR and BDR • Hello packets are exchanged via IP multicast • DR: The router with the highest OSPF priority • BDR: The router with the second-highest priority value • The OSPF router ID is used as the tiebreaker • The DR election is nonpreemptive

8. Setting the Priority for DR Election • This interface configuration command assigns the OSPF priority to an interface. • Different interfaces on a router may be assigned different values. • The default priority is 1. The range is from 0 to 255. • “0” means the router cannot be the DR or BDR. • A router that is not the DR or BDR is DROTHER. DR(config-if)# ip ospf priority 3

9. NBMA Topology • A single interface interconnects multiple sites • NBMA topologies support multiple routers, but without broadcasting capabilities • Five modes of OSPF operation are available

10. DR Election in NBMA Topology • OSPF considers NBMA to be like other broadcast media. • The DR and BDR need to have fully meshed connectivity with all other routers, but NBMA networks are not always fully meshed. • The DR and BDR each need a list of neighbors. • OSPF neighbors are not automatically discovered by the router.

11. Frame Relay Topologies

12. OSPF over NBMA Topology Modes of Operation • There are five modes of OSPF operation. • RFC 2328-compliant modes are as follows: • Nonbroadcast (NBMA) • Point-to-multipoint • Additional modes from Cisco are as follows: • Point-to-multipoint nonbroadcast • Broadcast • Point-to-point

13. Nonbroadcast Mode (NBMA Mode) • Treated as a broadcast network by OSPF (like a LAN) • All serial ports are part of the same IP subnet • Frame Relay, X.25, and ATM networks default to nonbroadcast mode • Duplicates LSA updates

14. Steps to Configure NBMA Mode • Enable the OSPF routing process • Define the interfaces that OSPF will run on • NBMA-specific configuration: • Statically define a neighbor relationship • Define the OSPF network type

15. Nonbroadcast Mode Operation • Neighbors must be statically configured • The OSPF network type must be defined • Use this command to statically define neighbor relationships in an NBMA network. • This command defines the OSPF non-broadcast network type. R1(config-router)# neighbor 192.168.1.2 priority 0 R1(config-if)# ip ospf network non-broadcast

16. NBMA Configuration Example

17. The show ip ospf neighbor Command

18. Using Subinterfaces • Several logical subinterfaces can be created over all multiaccess WAN networks: • point-to-point • multipoint • Each subinterface requires an IP subnet. • Logical interfaces behave in exactly the same way as physical interfaces for routing purposes • Statistics and traffic shaping behavior differs between interfaces and subinterfaces

19. Point-to-Point Subinterfaces • Each PVC gets its own subinterface. • PVCs are treated like point-to-point links. • Each subinterface requires a subnet. • OSPF point-to-point mode is the default. • DR and BDR are not used. • You do not need to configure neighbors. • This shows how to configure a point-to-point subinterface. R1(config)# interface serial 0/0/0.1 point-to-point

20. Point-to-Point Subinterface Example

21. Multipoint Subinterfaces • Multiple PVCsare on a single subinterface. • Each subinterface requires a subnet. • OSPF nonbroadcast mode is the default. • The DR isused. • Neighbors need to be statically configured. R1(config)# interface serial 0/0/0.1 multipoint • This shows how to configure a multipointsubinterface.

22. Multipoint Subinterface Example

23. Point-to-Point Mode • Leased-line emulation • Automatic configuration of adjacency • DR is not used • Only a single subnet is used

24. Point-to-Point Configuration Example

25. Point-to-PointVerification Example

26. Point-to-Multipoint Mode • Fixes partial-mesh and star topologies • Automatic configuration of adjacency • DR is not used • Only a single subnet is used

27. Point-to-Multipoint Configuration Example

28. Point-to-Multipoint Verification Example

29. Point-to-Multipoint Nonbroadcast • Cisco extension to the RFC-compliant point-to-multipoint mode • Must manually define neighbors—as with NBMA mode • DR, BDR not used—as with point-to-multipoint mode • Used in special cases where neighbors cannot be automatically discovered • Example: Virtual circuits without multicast and broadcast enabled • Defines the OSPF network type R1(config-if)# ip ospf network point-to-multipoint non-broadcast

30. OSPF over NBMA Topology Summary

31. Summary • OSPF defines three types of networks: point-to-point, broadcast, and NBMA. • On point-to-point links, the adjacency is dynamic, uses multicast addresses, and has no DR or BDR. • On broadcast links, the adjacency is dynamic and includes election of a DR and BDR. All updates are sent to the DR, which forwards the updates to all routers. • OSPF over Metro Ethernet and EoMPLS requires no changes to the OSPF configuration from the customer perspective. • OSPF over MPLS VPN requires the customer routers to run OSPF and exchange routing updates with the PE routers. • The router with the highest OSPF priority is selected as the DR. The router with the second-highest priority value is selected as the BDR.

32. Summary (Cont.) • The OSPF mode of operation on Frame Relay depends on the underlying Frame Relay network. OSPF mode options include nonbroadcast, broadcast, point-to-multipoint, point-to-multipoint nonbroadcast, and point-to-point. • By default on NBMA links, adjacency requires the manual definition of neighbors for the DR and BDR, because OSPF will consider the network similar to broadcast media. • A physical interface can be split into multiple logical interfaces called subinterfaces. Each subinterface requires an IP subnet. • Withpoint-to-point mode, leased line is emulated, the adjacency is automaticallyconfigured, and no DR is required. • In point-to-multipoint mode, no DR or BDR is needed and neighbors are automatically discovered. In point-to-multipoint nonbroadcast mode, no DR or BDR is needed, but neighbors must be statically configured.