Huawei Quidway S3900 Series Operation Manual page 316

Operation Manual – Routing Protocol
Quidway S3900 Series Ethernet Switches-Release 1510
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:
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.
NBMA sends protocol packets in unicast and neighbors should be configured
manually, while P2MP sends protocol packets in multicast.
III. 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.
In fact, a BDR provides backup for a DR. DR and BDR are elected at the same time.
Adjacencies are also established between the BDR and all the other routers on the
segment, and routing information is also exchanged between them. Once the DR
becomes invalid, the BDR becomes a DR. Since no re-election is needed and the
adjacencies already exist, the switchover process is very short. Now, a new BDR
should be elected. Although this election process will also take quite a long time, route
calculation will not be affected.
Neither neighboring relationship is established nor routing information is exchanged
between DR Others (routers other than DR and BDR). This reduces the number of
adjacencies among routers on the broadcast or NBMA network.
Huawei Technologies Proprietary
4-5
Chapter 4 OSPF Configuration