10G Core Routing Switch User Manual
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bandwidth resources. To solve this problem, the OSPF protocol regulates that a DR must be elected in a multi-access
network and only the DR (and the BDR in the following content) can be the neighbor of other routers in this network.
Two non-DR routers or BDR routers cannot be neighbors and exchange routing information. Which router can be the
DR in its segment is not manually specified. Instead, DR is elected by all the
routers in the segment.
Backup Designated Router (BDR)
If the DR fails for some faults, a new DR must be elected and synchronized with the other routers on the segment. This
process will take a relatively long time, during which, the route calculation is incorrect. To shorten the process, BDR is
brought forth in OSPF.In fact, BDR is a backup for DR. DR and BDR are elected in the meantime. The adjacencies are
also established between the BDR and all the routers on the segment, and routing information is also exchanged
between them. After the existing DR fails, the BDR will become a DR immediately.
III. Area
The network size grows increasingly larger. If all the routers on a huge network are running OSPF, the large number of
routers will result in an enormous LSDB, which will consume an enormous storage space, complicate the SPF algorithm,
and add the CPU load as well. Furthermore, as a network grows larger, the topology becomes more likely to take
changes. Hence, the network will always in “turbulence”, and a great deal of OSPF packets will be generated and
transmitted in the network. This will lower the network bandwidth utility. In addition, each change will cause all the
routes on the network to recompute the route.
OSPF solves the above problem by partition an AS into different areas. Areas logically group the routers. The borders of
areas are formed by routers. Thus, some routers may belong to different areas. A router connects the backbone area
and a non-backbone area is called Area Border Router (ABR). An ABR can connect to the backbone area physically or
logically.
IV. Backbone area and virtual link
Backbone Area
After the area division of OSPF, not all the areas are equal. In which, an area is different from all the other areas. Its
area-id is 0 and it is usually called the backbone area.
Virtual link
Since all the areas should be connected to the backbone area, virtual link is adopted so that the physically separated
areas can still maintain the logic connectivity to the backbone area.
V. Route summary
AS is divided into different areas that are interconnected via OSPF ABRs. The routing information between areas can be
reduced through route summary. Thus, the size of routing table can be reduced and the calculation speed of the router
can be improved.After calculating an intra-area route of an area, the ABR summarizes multiple OSPF routes into an LSA
and sends it outside the area according to the configuration of summary.
OSPF Configuration
In various configurations, you must first enable OSPF, specify the interface and area ID before configuring other
functions. But the configuration of the functions related to the interface is not restricted by whether the OSPF is
enabled or not. It should be noted that after OSPF is disabled, the OSPF-related interface parameters also become
invalid.
