3Com Switch 7754, Configuration Manual

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304 C
HAPTER
34: OSPF C
ONFIGURATION
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Non-broadcast multi-access (NBMA): If Frame Relay, ATM, or X.25 is adopted,
OSPF defaults the network type to NBMA. In an NBMA network, protocol
packets are sent in unicast.
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Point-to-multipoint (P2MP): OSPF will not default the network type of any link
layer protocol to P2MP. A P2MP network must be compulsorily changed from
another network type. The common practice is to change an NBMA network
into a P2MP network. In a P2MP network, protocol packets are sent in
multicast (224.0.0.5).
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Point-to-point (P2P): If PPP or HDLC is adopted, OSPF defaults the network type
to P2P. In a P2P network, protocol packets are sent in multicast (224.0.0.5).
Principles for configuring an NBMA network
An NBMA network is a non-broadcast and multi-accessible network. ATM and
frame relay networks are typical NBMA networks.
Some special configurations need to be done on an NBMA network. In an NBMA
network, an OSPF router cannot discover an adjacent router by broadcasting Hello
packets. Therefore, you must manually specify an IP address for the adjacent
router and whether the adjacent router has the right to vote for a DR.
An NBMA network must be fully connected. That is, any two routers in the
network must be directly reachable to each other through a virtual circuit. If two
routers in the network are not directly reachable to each other, you must configure
the corresponding interface type to P2MP. If a router in the network has only one
peer, you can change the corresponding interface type to P2P.
The differences between NBMA and P2MP are as follows:
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An NBMA network is fully connected, non-broadcast, and multi-accessible,
whereas a P2MP network is not necessarily fully connected.
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DR and BDR are required to be elected on an NBMA network but not on a
P2MP network.
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NBMA is a default network type. A P2MP network, however, must be
compulsorily changed from another network type. The more common practice
is to change an NBMA network into a P2MP network.
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NBMA sends protocol packets in unicast and neighbors should be configured
manually, while P2MP sends protocol packets in multicast.
DR and BDR
In a broadcast network or an NBMA network, routing information needs to be
transmitted between any two routers. If there are n routers in the network, n x
(n-1)/2 adjacencies need to be established. In this case, the route changes on any
router will result in multiple transmissions, which waste bandwidth. To solve this
problem, DR is defined in OSPF so that all routers send information to the DR only
and the DR broadcasts the network link states in the network.
If the DR fails, a new DR must be elected and synchronized with the other routers
on the network. The process takes quite a long time; in the process, route
calculation is incorrect. To shorten the process, BDR is introduced in OSPF.