Operation Manual – IPv4 Routing
H3C S3610&S5510 Series Ethernet Switches
Chapter 3 OSPF Configuration
3-10
z
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).
z
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).
II. 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:
z
NBMA networks are fully meshed, non-broadcast and multi access. P2MP
networks are not required to be fully meshed.
z
It is required to elect the DR and BDR on NBMA networks, while DR and BDR are
not available on P2MP networks.
z
NBMA is the default network type, while P2MP is a conversion from other network
types, such as NBMA in general.
z
On NBMA networks, packets are unicast, and neighbors are configured manually
on routers. On P2MP networks, packets are multicast.
3.1.4 DR and BDR
I. 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) is 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 become