Improving Routing
Performance in a
Complex Enterprise
Network
Implementing a Scalable Multiarea Network OSPFBased Solution
© 2009 Cisco Systems, Inc. All rights reserved.
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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
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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
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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.
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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
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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)
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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
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Setting the Priority for DR Election
DR(config-if)#
ip ospf priority 3
 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.
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NBMA Topology
 A single interface interconnects multiple sites
 NBMA topologies support multiple routers, but without
broadcasting capabilities
 Five modes of OSPF operation are available
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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.
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Frame Relay Topologies
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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
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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
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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
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Nonbroadcast Mode Operation
 Neighbors must be statically configured
 The OSPF network type must be defined
R1(config-router)#
neighbor 192.168.1.2 priority 0
 Use this command to statically define neighbor relationships in an
NBMA network.
R1(config-if)#
ip ospf network non-broadcast
 This command defines the OSPF non-broadcast network type.
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NBMA Configuration Example
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The show ip ospf neighbor Command
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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
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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.
R1(config)#
interface serial 0/0/0.1 point-to-point
 This shows how to configure a point-to-point subinterface.
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Point-to-Point Subinterface Example
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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.
R1(config)#
interface serial 0/0/0.1 multipoint
 This shows how to configure a multipoint subinterface.
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Multipoint Subinterface Example
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Point-to-Point Mode
 Leased-line emulation
 Automatic configuration of adjacency
 DR is not used
 Only a single subnet is used
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Point-to-Point Configuration Example
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Point-to-Point Verification Example
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Point-to-Multipoint Mode
 Fixes partial-mesh and star topologies
 Automatic configuration of adjacency
 DR is not used
 Only a single subnet is used
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Point-to-Multipoint Configuration
Example
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Point-to-Multipoint Verification Example
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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
R1(config-if)#
ip ospf network point-to-multipoint non-broadcast
 Defines the OSPF network type
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OSPF over NBMA Topology Summary
OSPF Mode
NBMA Preferred
Topology
Subnet Address
Hello
Timer
Adjacency
RFC or
Cisco
Broadcast
Full- or partialmesh
Same
10 sec
Automatic,
DR/BDR elected
Cisco
Nonbroadcast
(NBMA)
Full- or partialmesh
Same
30 sec
Manual
configuration,
DR/BDR elected
RFC
Point-tomultipoint
Partial-mesh
or star
Same
30 sec
Automatic,
no DR/BDR
RFC
Point-tomultipoint
nonbroadcast
Partial-mesh
or star
Same
30 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
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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.
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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.
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