Information Technology Reference
In-Depth Information
router (BDR) but not to each other. All routers reply to the DR and BDR using the multi-
cast address 224.0.0.6. The section “OSPF DRs,” covers the DR concept.
Point-to-Point
Network
Nonbroadcast
Multiple-Access
Network
Area 2
Area 4
Figure 11-1
OSPF Networks
On OSPF point-to-multipoint nonbroadcast networks, it is necessary to configure the set
of neighbors that are directly reachable over the point-to-multipoint network. Each neigh-
bor is identified by its IP address on the point-to-multipoint network. Nonbroadcast
point-to-multipoint networks do not elect DRs, so the DR eligibility of configured neigh-
bors is undefined. OSPF communication in point-to-point networks use unicast or multi-
cast addresses for neighbor communication.
OSPF virtual links unicast OSPF packets. Later, the section “Virtual Links” discusses vir-
tual links.
OSPFv2 Areas
As a network grows, the initial flooding and database maintenance of LSAs can burden a
router's CPU. OSPF uses areas to reduce these effects. An area is a logical grouping of
routers and links that divides the network. Routers share link-state information with only
the routers in their areas. This setup reduces the size of the database and the cost of com-
puting the SPF tree at each router.
Key
To p i c
Each area is assigned a 32-bit integer number. Area 0 (or 0.0.0.0) is reserved for the back-
bone area. Every OSPF network should have a backbone area. The backbone area must ex-
ist in any internetwork using OSPF over multiple areas as a routing protocol. As you can
see in Figure 11-2, communication between Area 1 and Area 2 must flow through Area 0.
This communication can be internal to a single router that has interfaces directly con-
nected to Areas 0, 1, and 2.
Intra-area traffic is packets passed between routers in a single area.
