3Com SuperStack 4 Configuration Manual page 123

To ensure the stability of OSPF, you must determine the division of router IDs and
manually configure them when implementing network planning.
Configuring the Network Type on the OSPF Interface
The route calculation of OSPF is based upon the topology of the adjacent network
of the local router. Each router describes the topology of its adjacent network and
transmits it to all the other routers.
OSPF divides networks into four types by link layer protocol:
Broadcast — If Ethernet or FDDI is adopted, OSPF defaults the network type to
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broadcast.
Non-Broadcast Multi-access (NBMA) — If Frame Relay, ATM, HDLC or X.25 is
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adopted, OSPF defaults the network type to NBMA.
Point-to-Multipoint (P2MP) — OSPF will not default the network type of any
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link layer protocol to P2MP. The usual policy is to change a partially connected
NBMA network to a P2MP network, if the NBMA network is not fully
connected.
Point-to-point (P2P) — If PPP, LAPB or POS is adopted, OSPF defaults the
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network type to P2P.
As you configure the network type, consider the following points:
NBMA means that a network is non-broadcast and multi-accessible. ATM is a
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typical example. You can configure the polling interval for hello packets before
the adjacency of the neighboring routers is formed.
Configure the interface type to nonbroadcast on a broadcast network without
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multi-access capability.
Configure the interface type to P2MP if not all the routers are directly
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accessible on an NBMA network.
Change the interface type to P2P if the router has only one peer on the NBMA
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network.
The differences between NBMA and P2MP are listed below:
In OSPF, NBMA refers to the networks that are fully connected, non-broadcast
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and multi-accessible. However, a P2MP network is not required to be fully
connected.
DR and BDR are required on a NBMA network but not on a P2MP network.
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NBMA is the default network type. For example, if ATM is adopted as the link
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layer protocol, OSPF defaults the network type on the interface to NBMA,
regardless of whether the network is fully connected. P2MP is not the default
network type. No link layer protocols will be regarded as P2MP. You must
change the network type to P2MP manually. The most common method is to
change a partially connected NBMA network to a P2MP network.
NBMA forwards packets by unicast and requires neighbors to be configured
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manually. P2MP forwards packets by multicast.
OSPF Configuration 135