Path Cost Metrics; Areas And Area Border Routers - Juniper J2300 User Manual

In LANs, the election of the designated router takes place when the OSPF
network is initially established. When the first OSPF links are active, the router
with the highest router identifier (defined by the
the loopback address) is elected designated router. The router with the second
highest router identifier is elected the backup designated router (BDR). If the
designated router fails or loses connectivity, the BDR assumes its role and a new
BDR election takes place between all the routers in the OSPF network.

Path Cost Metrics

Once the topology is shared across the network, OSPF uses it to route packets
between network nodes. Each path between neighbors is assigned a cost based on
the throughput, round-trip time, and reliability of the link. The sum of the costs
across a particular path between hosts determines the overall cost of the path.
Packets are then routed along the shortest path using the shortest path first (SPF)
algorithm. If multiple equal-cost paths exist between a source and destination
address, OSPF routes packets along each path alternately, in round-robin fashion.
OSPF allows you to manually assign a cost (or metric) to a particular path
segment to control the flow of packets across the network.

Areas and Area Border Routers

The OSPF networks in an AS are administratively grouped into areas. Each area
within an AS operates like an independent network and has a unique 32-bit area ID,
which functions like a network address. Within an area, the topology database
contains only information about the area, LSAs are flooded only to nodes within the
area, and routes are computed only within the area. Subnetworks are divided into
other areas, which are connected to form the whole of the main network.
The central area of an AS, called the backbone area, has a special function and
is always assigned the area ID
in dotted decimal notation, but they are not IP addresses. Area IDs need only
be unique within an AS. All other networks or areas in the AS must be directly
connected to the backbone area by a router that has interfaces in more than one
area. These connecting routers are called area border routers (ABRs). Figure 64
shows an OSPF topology of three areas connected by two area border routers.
router-id
. Area IDs are unique numeric identifiers,
0.0.0.0
Routing Overview
configuration value or
271
OSPF Overview