OSPF Network Types

Introduction

OSPF Network Types

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.

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.

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.

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:

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