H3C Technologies H3C SecPath F1000-E - Manual (Page 303)

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generally 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 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 unicast.
• 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).
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 (DR) 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 the new DR in a very short period by avoiding
adjacency establishment and DR reelection. Meanwhile, other routers elect another BDR, which requires
a relatively long period but has no influence on routing calculation.
Other routers, also known as DRothers, establish no adjacency and exchange no routing information
with each other, thus reducing the number of adjacencies on broadcast and NBMA networks.
In the following figure, real lines are Ethernet physical links, and dashed lines represent adjacencies.
With the DR and BDR in the network, only seven adjacencies are enough.
Figure 6 DR and BDR in a network