H3C S7500 Series Operation Manual page 323

Operation Manual – Routing Protocol
H3C S7500 Series Ethernet Switches
An NBMA network is fully connected, non-broadcast, and multi-accessible,
whereas a P2MP network is not necessarily fully connected.
DR and BDR must be elected on an NBMA network, while on a P2MP network
there are no such routers.
NBMA is a default network type. A P2MP network, however, must be
compulsorily changed from another network type. The most common practice is
to change an NBMA network into a P2MP network.
NBMA sends protocol packets in unicast and neighbors must 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 must be established. In this case, any route change 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 in
the network. The process will take quite a long time and in the process, route
calculation is incorrect. To shorten the process, BDR is introduced in OSPF.
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 fails,
the BDR becomes the DR immediately. This is because no re-election is needed and
the adjacencies already exist. Now, a new BDR needs to be elected. This process will
also take quite a long time, but it will not affect route calculation.
With DR and BDR, routers other than DR and BDR (called DR Others) needs not build
adjacencies between them, nor will they exchange routing information. This reduces
the number of adjacencies among routers on the broadcast or NBMA network.
In Figure
lines represent adjacencies established. The figure shows that, with the DR/BDR
mechanism adopted, seven adjacencies suffice for the five routers.
4-2, the solid lines represent physical Ethernet connections and the dotted
4-5
Chapter 4 OSPF Configuration