Ospf Network Type; Dr/Bdr - 3Com 5500-EI PWR Install Manual

OSPF Network Type

Four OSPF network types
OSPF divides networks into four types by link layer protocols:
Broadcast: If Ethernet or FDDI is adopted, OSPF defaults the network type to broadcast. In a
broadcast network, protocol packets are sent in multicast (224.0.0.5 and 224.0.0.6) by default.
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.
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 to the multicast address (224.0.0.5) by default, but protocol packets can be sent in the
unicast address as needed.
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 neighbor, you can change the corresponding interface type to P2P.
The differences between NBMA and P2MP are as follows:
An NBMA network is fully connected, non-broadcast, and multi-accessible, whereas a P2MP
network is not necessarily fully connected.
DR and BDR are required to be elected on an NBMA network but not on a P2MP network.
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.
Since NBMA interfaces send packets to unicast addresses, you need to configure neighbors
manually. By default, P2MP sends protocol packets to multicast addresses.

DR/BDR

DR/BDR introduction
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 × (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
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