© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Configuring OSPF OSPF Network Types

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v OSPF Network Types The three types of networks defined by OSPF are: Point-to-point: A network that joins a single pair of routers. Broadcast: A multiaccess broadcast network, such as Ethernet. Nonbroadcast multiaccess (also called NBMA): A network that interconnects more than two routers but that has no broadcast capability. Frame Relay, ATM, and X.25 are examples of NBMA networks. –Five modes of OSPF operation are available for NBMA networks.

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v 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. No DR or BDR election required. OSPF autodetects this interface type. OSPF packets are sent using multicast

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Multiaccess Broadcast Network Generally these are, LAN technologies like Ethernet and Token Ring. 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 Packets from DR to all other routers use

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Electing the DR and BDR Hello packets are exchanged via IP multicast. The router with the highest OSPF priority is selected as the DR. The router with the second-highest priority value is the BDR. Use the OSPF router ID as the tiebreaker. The DR election is nonpreemptive.

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Setting 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 means the router cannot be the DR or BDR. A router that is not the DR or BDR is DROTHER. ip ospf priority number Router(config-if)#

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v NBMA Topology A single interface interconnects multiple sites. NBMA topologies support multiple routers, but without broadcasting capabilities.

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v 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 need a list of neighbors. OSPF neighbors are not automatically discovered by the router.

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Frame Relay Topologies

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v OSPF over NBMA Topology Modes of 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

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Selecting the OSPF Network Type for NBMA Networks ip ospf network [{broadcast | non-broadcast | point-to- multipoint [non-broadcast] | point-to-point}] Defines OSPF network type Router(config-if)# Router(config)#interface serial 0/0/0 Router(config-if)#encapsulation frame-relay Router(config-if)#ip ospf network broadcast Example: Broadcast Mode

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Nonbroadcast Mode (NBMA Mode) Treated as a broadcast network by OSPF (acts like a LAN). All serial ports are part of the same IP subnet. Frame Relay, X.25, and ATM networks default to nonbroadcast mode. Neighbors must be statically configured. Duplicates LSA updates. Complies with RFC 2328.

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Using the neighbor Command Used to statically define neighbor relationships in an NBMA network neighbor ip-address [priority number] [poll-interval number] [cost number] [database-filter all] Router(config-router)#

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v neighbor Command Example RouterA(config)# router ospf 100 RouterA(config-router)# network area 0 RouterA(config-router)# neighbor priority 0 RouterA(config-router)# neighbor priority 0 RouterA(config-router)# network area 0

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v RouterA# show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface FULL/DROTHER 00:01: Serial0/0/ FULL/DROTHER 00:01: Serial0/0/ FULL/BDR 00:00: FastEthernet0/0 The show ip ospf neighbor Command

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Point-to-Multipoint Mode The point-to-multipoint mode allows for NBMA networking. The point-to-multipoint mode fixes partial-mesh and star topologies. No DR is required and only a single subnet is used. A 30-second hello is used. This mode is RFC 2328-compliant.

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Point-to-Multipoint Configuration interface Serial0/0/0 ip address encapsulation frame-relay ip ospf network point-to-multipoint router ospf 100 log-adjacency-changes network area 0 network area 0 interface Serial0/0/0 ip address encapsulation frame-relay ip ospf network point-to-multipoint ip ospf priority 0 Router A Router C

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Point-to-Multipoint Example RouterA#sh ip ospf int s0/0/0 Serial0/0/0 is up, line protocol is up Internet Address /24, Area 0 Process ID 100, Router ID , Network Type POINT_TO_MULTIPOINT, Cost: 781 Transmit Delay is 1 sec, State POINT_TO_MULTIPOINT Timer intervals configured, Hello 30, Dead 120, Wait 120, Retransmit 5 oob-resync timeout 120 Hello due in 00:00:26 Supports Link-local Signaling (LLS) Index 2/2, flood queue length 0 Next 0x0(0)/0x0(0) Last flood scan length is 1, maximum is 1 Last flood scan time is 0 msec, maximum is 4 msec Neighbor Count is 2, Adjacent neighbor count is 2 Adjacent with neighbor Adjacent with neighbor Suppress hello for 0 neighbor(s) RouterA#

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Point-to-Multipoint Nonbroadcast Cisco extension to RFC-compliant point-to-multipoint mode Must statically define neighbors, like nonbroadcast mode Like point-to-multipoint mode, DR and BDR not elected Used in special cases where neighbors cannot be automatically discovered

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Using Subinterfaces The physical serial port becomes multiple logical ports. Each subinterface requires an IP subnet. interface serial number.subinterface-number {multipoint | point-to-point} Router(config)#

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Point-to-Point Subinterfaces Each PVC and SVC gets its own subinterface. OSPF point-to-point mode is the default on point-to-point Frame Relay subinterfaces. –No DR/BDR –Do not need to configure neighbors interface serial number.subinterface-number point-to-point Router(config)#

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Point-to-Point Subinterface Example PVCs are treated like point-to-point links. Each subinterface requires a subnet.

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Multipoint Subinterfaces Multiple PVCs and SVCs are on a single subinterface. OSPF nonbroadcast mode is the default. –DR and BDR are required. –Neighbors need to be statically configured. interface serial number.subinterface-number multipoint Router(config)#

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Multipoint Subinterface Example Single interface serial 0/0/0 has been logically separated into two subinterfaces: one point-to-point (S0/0/0.1) and one point-to-multipoint (S0/0/0.2). Each subinterface requires a subnet. OSPF defaults to point-to-point mode on point-to-point subinterfaces. OSPF defaults to nonbroadcast mode on point-to-multipoint subinterfaces.

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v 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/BDR Cisco Point-to-point Partial-mesh or star, using subinterface Different for Each Subinterface 10 sec Automatic, no DR/BDR Cisco OSPF over NBMA Topology Summary

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Creation of Adjacencies for Point-to-Point Mode RouterA# debug ip ospf adj OSPF: Interface Serial0/0/0.1 going Up OSPF: Build router LSA for area 0, router ID , seq 0x OSPF: Rcv DBD from on Serial0/0/0.1 seq 0xCF0 opt 0x52 flag 0x7 len 32 mtu 1500 state INIT OSPF: 2 Way Communication to on Serial0/0/0.1, state 2WAY OSPF: Send DBD to on Serial0/0/0.1 seq 0xF4D opt 0x52 flag 0x7 len 32 OSPF: NBR Negotiation Done. We are the SLAVE OSPF: Send DBD to on Serial0/0/0.1 seq 0xCF0 opt 0x52 flag 0x2 len 132 OSPF: Rcv DBD from on Serial0/0/0.1 seq 0xCF1 opt 0x52 flag 0x3 len 132 mtu 1500 state EXCHANGE OSPF: Send DBD to on Serial0/0/0.1 seq 0xCF1 opt 0x52 flag 0x0 len 32 OSPF: Database request to OSPF: sent LS REQ packet to , length 12 OSPF: Rcv DBD from on Serial0/0/0.1 seq 0xCF2 opt 0x52 flag 0x1 len 32 mtu 1500 state EXCHANGE OSPF: Exchange Done with on Serial0/0/0.1 OSPF: Send DBD to on Serial0/0/0.1 seq 0xCF2 opt 0x52 flag 0x0 len 32 OSPF: Synchronized with on Serial0/0/0.1, state FULL %OSPF-5-ADJCHG: Process 100, Nbr on Serial0/0/0.1 from LOADING to FULL, Loading Done OSPF: Build router LSA for area 0, router ID , seq 0x

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Creation of Adjacencies for Broadcast Mode RouterA# debug ip ospf adj OSPF: Interface FastEthernet0/0 going Up OSPF: Build router LSA for area 0, router ID ,seq 0x OSPF: 2 Way Communication to on FastEthernet0/0, state 2WAY OSPF: end of Wait on interface FastEthernet0/0 OSPF: Neighbor change Event on interface FastEthernet0/0 OSPF: DR/BDR election on FastEthernet0/0 OSPF: Elect BDR OSPF: Elect DR DR: (Id) BDR: (Id) OSPF: Rcv DBD from on FastEthernet0/0 seq 0x14B 7 opt 0x52 flag 0x7 len 32 mtu 1500 state EXSTART OSPF: First DBD and we are not SLAVE-if)# OSPF: Send DBD to on FastEthernet0/0 seq 0xDCE opt 0x52 flag 0x7 len 32 OSPF: Retransmitting DBD to on FastEthernet0/0[1] OSPF: Rcv DBD from on FastEthernet0/0 seq 0xDCE opt 0x52 flag 0x2 len 152 mtu 1500 state EXSTART

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Summary OSPF defines three types of networks: point-to-point, broadcast, and NBMA. On point-to-point links, adjacency is dynamic, uses multicast addresses, and has no DR or BDR. On broadcast links, 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. The router with the highest OSPF priority is selected as the DR. The router with the second-highest priority value is the BDR. By default on NBMA links, adjacency requires manual definition of neighbors for the DR and BDR because OSPF will consider the network similar to broadcast media.

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v 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. In nonbroadcast mode, a DR and BDR are elected, and neighbors must be statically configured. In point-to-multipoint mode, no DR and BDR are needed and neighbors are automatically discovered. In point-to- multipoint nonbroadcast mode, no DR and BDR are needed, but neighbors must be statically configured. A physical interface can be split into multiple logical interfaces called subinterfaces. Each subinterface requires an IP subnet. Using the debug ip ospf adj command enables you to see OSPF packet exchanges and the status of neighbor adjacencies.

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v