3Com MSR 50 Series Configuration Manual page 925

NBMA (Non-Broadcast Multi-Access): when the link layer protocol is Frame
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Relay, ATM or X.25, OSPF considers the network type as NBMA by default.
Packets on these networks are sent to unicast addresses.
P2MP (point-to-multipoint): by default, OSPF considers no link layer protocol as
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P2MP, which is a conversion from other network types such as NBMA in
general. On P2MP networks, packets are sent to multicast addresses
(224.0.0.5).
P2P (point-to-point): when the link layer protocol is PPP or HDLC, OSPF
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considers the network type as P2P. On P2P networks, packets are sent to
multicast addresses (224.0.0.5).
NBMA network configuration principle
Typical NBMA networks are ATM and Frame Relay networks.
You need to perform some special configuration on NBMA interfaces. Since these
interfaces cannot broadcast hello packets for neighbor location, you need to
specify neighbors manually and configure whether the neighbors have the DR
election right.
An NBMA network is fully meshed, which means any two routers in the NBMA
network have a direct virtual link for communication. If direct connections are not
available between some routers, the type of interfaces associated should be
configured as P2MP, or as P2P for interfaces with only one neighbor.
Differences between NBMA and P2MP networks:
NBMA networks are fully meshed, non-broadcast and multi access. P2MP
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networks are not required to be fully meshed.
It is required to elect the DR and BDR on NBMA networks, while DR and BDR
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are not available on P2MP networks.
NBMA is the default network type, while P2MP is a conversion from other
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network types such as NBMA in general.
On NBMA networks, packets are unicast, and neighbors are configured
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manually on routers. On P2MP networks, packets are multicast.
DR and BDR DR/BDR introduction
On broadcast or NBMA networks, any two routers exchange routing information
with each other. If n routers are present on a network, n(n-1)/2 adjacencies are
required. Any change on a router in the network generates traffic for routing
information synchronization, consuming network resources. The Designated
Router is defined to solve the problem. All other routers on the network send
routing information to the DR, which is responsible for advertising link state
information.
If the DR fails to work, routers on the network have to elect another DR and
synchronize information with the new DR. It is time-consuming and prone to
routing calculation errors. The Backup Designated Router (BDR) was introduced to
reduce the synchronization period.
The BDR is elected along with the DR and establishes adjacencies for routing
information exchange with all other routers. When the DR fails, the BDR will
Introduction to OSPF 925