1. © 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—3-1 Implementing a Scalable Multiarea Network OSPF- Based Solution Improving Routing Performance in a Complex Enterprise Network

2. © 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—3-2 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. © 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—3-3 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. © 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—3-4 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. © 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—3-5 OSPF Adjacency Over Metro Ethernet and EoMPLS  EoMPLS and Metro Ethernet 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. © 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—3-6 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. © 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—3-7 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. © 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—3-8 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. ip ospf priority 3 DR(config-if)#

9. © 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—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. © 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—3-10 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. © 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—3-11 Frame Relay Topologies

12. © 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—3-12 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. © 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—3-13 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. © 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—3-14 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. © 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—3-15 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. neighbor 192.168.1.2 priority 0 R1(config-router)# ip ospf network non-broadcast R1(config-if)#

16. © 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—3-16 NBMA Configuration Example

17. © 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—3-17 The show ip ospf neighbor Command

18. © 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—3-18 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. © 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—3-19 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. interface serial 0/0/0.1 point-to-point R1(config)#

20. © 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—3-20 Point-to-Point Subinterface Example

21. © 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—3-21 Multipoint Subinterfaces  Multiple PVCs are on a single subinterface.  Each subinterface requires a subnet.  OSPF nonbroadcast mode is the default. –The DR is used. –Neighbors need to be statically configured. interface serial 0/0/0.1 multipoint R1(config)#  This shows how to configure a multipoint subinterface.

22. © 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—3-22 Multipoint Subinterface Example

23. © 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—3-23 Point-to-Point Mode  Leased-line emulation  Automatic configuration of adjacency  DR is not used  Only a single subnet is used

24. © 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—3-24 Point-to-Point Configuration Example

25. © 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—3-25 Point-to-Point Verification Example

26. © 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—3-26 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. © 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—3-27 Point-to-Multipoint Configuration Example

28. © 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—3-28 Point-to-Multipoint Verification Example

29. © 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—3-29 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 ip ospf network point-to-multipoint non-broadcast R1(config-if)#

30. © 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—3-30 OSPF Mode NBMA Preferred Topology Subnet Address Hello Timer Adjacency RFC or Cisco Broadcast Full- or partial- mesh Same10 sec Automatic, DR/BDR elected Cisco Nonbroadcast (NBMA) Full- or partial- mesh Same30 sec Manual configuration, DR/BDR elected RFC Point-to- multipoint Partial-mesh or star Same30 sec Automatic, no DR/BDR RFC Point-to- multipoint nonbroadcast Partial-mesh or star Same30 sec Manual configuration, no DR or BDR Cisco Point-to-point Partial-mesh or star, using subinterface Different for each subinterface 10 sec Automatic, no DR or BDR Cisco OSPF over NBMA Topology Summary

31. © 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—3-31 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. © 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—3-32 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.  With point-to-point mode, leased line is emulated, the adjacency is automatically configured, 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.

33. © 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—3-33