Configuring The Network Type On The Ospf Interface - 3Com 8800 Configuration Manual

3Com Switch 8800 Configuration Guide

19.2.11 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
broadcast.
Non-Broadcast Multi-access (nbma): If Frame Relay, ATM, HDLC or X.25 is
adopted, OSPF defaults the network type to NBMA.
Point-to-Multipoint (p2mp): OSPF will not default the network type of any link layer
protocol to p2mp. A p2mp network is always changed from another type of
network. The general undertaking is to change a partially connected NBMA
network to p2mp network if the NBMA network is not fully connected.
Point-to-point (p2p): If PPP or LAPB is adopted, OSPF defaults the network type
to p2p.
NBMA means that a network is non-broadcast and multi-accessible. ATM is a typical
example for it. The user can configure the polling interval to specify the interval for
sending polling hello packets before the adjacency of the neighboring routers is formed.
Set the network type to NBMA if routers not supporting multicast addresses exist in a
broadcast network.
Set the interface type to p2mp if not all the routers are directly accessible on an NBMA
network.
Change the interface type to p2p if the router has only one peer on the NBMA network.
The differences between NBMA and p2mp are listed below:
With OSPF, NBMA refers to the networks that are fully connected, non-broadcast
and multi-accessible. However, a p2mp network is not necessarily fully
connected.
DR and BDR are required on a NBMA network but not on p2mp network.
NBMA is the default network type. For example, if ATM is adopted as the link 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 are regarded as p2mp. You must change the network type to
p2mp by force. The commonest undertaking is to change a partially connected
NBMA network to a p2mp network.
NBMA forwards packets by unicast and requires configuring neighbors manually.
p2mp forwards packets by multicast.
Perform the following configuration in interface view.
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Chapter 19 OSPF Configuration