Classification Of Ospf Networks; Dr And Bdr - H3C LS-3100-52P-OVS-H3 Operation Manual

the same destination have the same cost, then take the cost from the router to the ASBR into
consideration.

Classification of OSPF Networks

OSPF network types
OSPF classifies networks into four types upon the link layer protocol:
Broadcast: When the link layer protocol is Ethernet or FDDI, OSPF considers the network type
broadcast by default. On Broadcast networks, hello packets, LSU packets and LSAck packets are
sent to multicast addresses 224.0.0.5 (reserved for OSPF routers) and 224.0.0.6 (reserved for
OSPF DRs), while DD packets and LSR packets are sent in unicast.
NBMA (Non-Broadcast Multi-Access): When the link layer protocol is Frame 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 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 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 networks are not
required to be fully meshed.
It is required to elect the DR and BDR on NBMA networks, while DR and BDR are not available on
P2MP networks.
NBMA is the default network type, while P2MP is a conversion from other network types, such as
NBMA in general.
On NBMA networks, packets are unicast, and neighbors are configured 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.
1-8