© 2009 cisco systems, inc. all rights reserved. route v1.0—3-1 implementing a scalable multiarea...

© 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


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


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


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.


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


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)


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


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)#


NBMA Topology

A single interface interconnects multiple sites

NBMA topologies support multiple routers, but without broadcasting capabilities

Five modes of OSPF operation are available


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.


Frame Relay Topologies


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


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


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


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)#


NBMA Configuration Example


The show ip ospf neighbor Command


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


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)#


Point-to-Point Subinterface Example


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.


Multipoint Subinterface Example


Point-to-Point Mode

Leased-line emulation

Automatic configuration of adjacency

DR is not used

Only a single subnet is used


Point-to-Point Configuration Example


Point-to-Point Verification Example


Point-to-Multipoint Mode

Fixes partial-mesh and star topologies

Automatic configuration of adjacency

DR is not used

Only a single subnet is used


Point-to-Multipoint Configuration Example


Point-to-Multipoint Verification Example


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)#


OSPF ModeNBMA Preferred

TopologySubnet Address

Hello Timer

AdjacencyRFC or Cisco

BroadcastFull- or partial-

meshSame 10 sec

Automatic, DR/BDR elected

Cisco

Nonbroadcast (NBMA)

Full- or partial-mesh

Same 30 secManual

configuration, DR/BDR elected

RFC

Point-to-multipoint

Partial-mesh or star

Same 30 secAutomatic, no DR/BDR

RFC

Point-to-multipoint

nonbroadcast

Partial-mesh or star

Same 30 secManual

configuration, no DR or BDR

Cisco

Point-to-pointPartial-mesh or

star, using subinterface

Different for each subinterface

10 secAutomatic,

no DR or BDRCisco

OSPF over NBMA Topology Summary


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.


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.

© 2009 cisco systems, inc. all rights reserved. route v1.0—3-1 implementing a scalable multiarea...

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