NetWork Types 大揭密:(

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OSPF Network Types

 

OSPF Network Types.. 1

Introduction. 1

Broadcast1

NBMA (Non-Broadcast Multi-Access)2

Point-to-Point2

Point-to-Multipoint2

 

 

Introduction

 

In order to correctly build the network topology, OSPF maintains network types, i.e. network segments are described not only with an IP address and mask, but also with the type of network they represent. Four network types are supported by OSPF:

 

o        Broadcast

 

o        NBMA (Non-Broadcast Multiple Access)

 

o        Point-to-Point

 

o        Point-to-Multipoint

 

Based on the Layer 2 interface type (Ethernet, PPP, Frame Relay, ATM, etc.) there are defaults for the OSPF network types. For example, the RS defaults to OSPF type Broadcast for Ethernet interfaces. 

 

In order for two OSPF routers to establish adjacency over a common link, they must agree on the OSPF network type of that link. Different default values for OSPF Network types is the most common reason for interoperability problems. When the two routers have a mismatch in their default setting, one end must be re-configured to match the other. Here we document the default settings and possible values for the RS platform.

 

Broadcast

 

For the purposes of OSPF, Broadcast interface is an interface that has native Layer 2 broadcast/multicast capabilities.   The typical example of a broadcast/multicast network is an Ethernet, in which IP Multicast packets are transmitted as Ethernet frames with multicast destination MAC addresses. 

 

On a broadcast network, OSPF will send its HELLOs to IP multicast address of 224.0.0.5. Those HELLOs will be heard by every OSPF router on the network. Therefore no special configuration is required for neighbor discovery – neighbor discovery is automatic.

 

RS1(config)# ip-router global set trace-state on

RS1(config)# ospf trace hello detail

-04-24 14:27:59 OSPF SEND: 10.0.0.1 -> 224.0.0.5: Version 2, Type Hello (1), Length 48

-04-24 14:27:59         Router ID 1.1.1.1, Area 0.0.0.0, Authentication <None> (0)

-04-24 14:27:59         Authentication data: 00000000 00000000

-04-24 14:27:59         Mask 255.255.255, Options <E> (2), Priority 1, Neighbours 1

-04-24 14:27:59         Intervals: Hello 10s, Dead Router 40s, Designated Router 10.0.0.1, Backup 10.0.0.2

-04-24 14:27:59         Neighbours: 2.2.2.2

 

Designated Router (DR) and Backup Designated Router (BDR) are selected on Broadcast networks.

 

On the RS router, Broadcast network type is by default assigned to the following interfaces:

 

o        Ethernet

o        ATM

o        Frame Relay

 

NBMA (Non-Broadcast Multi-Access)

 

The NBMA network type was created so that OSPF can correctly model Frame Relay and X.25 environments, where the network is multi-access, but has no native Layer 2 broadcast/multicast capabilities. Instead of multicasting its HELLOs, OSPF uses unicast to send them: In addition, the HELLO timer is extended to 30 seconds and the Dead Router timer is extended to 120 sec.

 

RS1(config)# ip-router global set trace-state on

RS1(config)# ospf trace hello detail

-04-24 15:13:03 OSPF SEND: 10.0.0.1 -> 10.0.0.2: Version 2, Type Hello (1), Length 44

-04-24 15:13:03         Router ID 1.1.1.1, Area 0.0.0.0, Authentication <None> (0)

-04-24 15:13:03         Authentication data: 00000000 00000000

-04-24 15:13:03         Mask 255.255.255, Options <E> (2), Priority 1, Neighbours 0

-04-24 15:13:03         Intervals: Hello 30s, Dead Router 120s, Designated Router 10.0.0.1, Backup 0.0.0.0

 

Because NBMA networks have no multicast/broadcast capabilities, all OSPF packets are sent as unicast, and therefore OSPF neighbors must be manually configured:

 

ospf add nbma-neighbor 10.0.0.2 to-interface TO-RS2

Designated Router (DR) and Backup Designated Router (BDR) are selected on Broadcast networks.

NBMA networks have one significant drawback – they require full-mesh connectivity between all OSPF routers. This is in most cases impractical and prone to unpredictable failure modes, and therefore NBMA network type is usually avoided. Point-to-Multipoint is used instead.

On the RS router, none of the interfaces default to NBMA network type. 

Point-to-Point

 

The point-to-point interface type is reserved for point-to-point links, i.e. links that can only have two routers attached to them. Typical examples of those are PPP, Cisco HDLC, or POS. The neighbor address of the link is usually discovered via a Layer 2 specific mechanism, such as IPCP or Inverse ARP. OSPF however uses a multicast destination IP address for its HELLOs, and leaves it up to the specific Data Link layer to handle those. These multicasts are typically transmitted as Layer 2 unicasts, using the known mapping mentioned above.

 

RS1(config)# ip-router global set trace-state on

RS1(config)# ospf trace hello detail

-04-24 16:07:46 OSPF RECV: 10.0.0.2 -> 224.0.0.5: Version 2, Type Hello (1), Length 48

-04-24 16:07:46         Router ID 2.2.2.2, Area 0.0.0.0, Authentication <None> (0)

-04-24 16:07:46         Authentication data: 00000000 00000000

-04-24 16:07:46         Mask 255.255.255.252, Options <E> (2), Priority 0, Neighbours 1

-04-24 16:07:46         Intervals: Hello 10s, Dead Router 40s, Designated Router 0.0.0.0, Backup 0.0.0.0

-04-24 16:07:46         Neighbours: 1.1.1.1

 

Because the Layer 3 multicasts are transmitted with Layer 2 unicasts using Layer 2-specific mechanisms, neighbor discovery is automatic.

 

Designated Router (DR) and Backup Designated Router (BDR) are NOT selected on Point-to-Point networks.

On the RS router, the following interfaces default to Point-to-Point network type: 

o               WAN (Frame Relay and PPP)

o               POS

Point-to-Multipoint

 

The Point-to-Multipoint interface type was originally created to be configured on networks that otherwise would have been NBMA, but cannot satisfy the full-mesh connectivity requirement of NBMA. Point-to-Multipoint networks are modeled as a collection of Point-to-Point networks, and just like Point-to-Point, don’t have DR/BDR

 

On the RS router, none of the interfaces default to Point-to-Multipoint network type, however all can be configured as Point-to-Multipoint:

ospf add interface <NAME> type point-to-multipoint

To read more on the specifics of Point-to-Multipoint interface type, click here. 

 

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Last Updated June 5, 2002