3com 5500-SI Manual De Usuario
OSPF Configuration 237
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Backup Designated Router (BDR)
If the DR fails, 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, OSPF creates a BDR as
backup for the DR. A new 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 immediately becomes a DR.
on the segment. This process will take a relatively long time, during which the
route calculation is incorrect. To shorten the process, OSPF creates a BDR as
backup for the DR. A new 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 immediately becomes a DR.
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Area
If all routers on a large network are running OSPF, the large number of routers
results in an enormous LSD, which consumes storage space, complicates the SPF
algorithm, and adds CPU load. Furthermore, as a network grows larger, the
topology becomes more likely to change. Hence, the network is always in
“turbulence”, and a large number of OSPF packets are generated and transmitted
in the network. This shrinks network bandwidth. In addition, each change causes
all the routers on the network to recalculate the routes.
results in an enormous LSD, which consumes storage space, complicates the SPF
algorithm, and adds CPU load. Furthermore, as a network grows larger, the
topology becomes more likely to change. Hence, the network is always in
“turbulence”, and a large number of OSPF packets are generated and transmitted
in the network. This shrinks network bandwidth. In addition, each change causes
all the routers on the network to recalculate the routes.
OSPF solves this problem by dividing an AS into different areas. Areas logically
group the routers, which form the borders of each area. Thus, some routers may
belong to different areas. A router that connects the backbone area and a
non-backbone area is called an area border router (ABR). An ABR can connect to
the backbone area physically or logically.
group the routers, which form the borders of each area. Thus, some routers may
belong to different areas. A router that connects the backbone area and a
non-backbone area is called an area border router (ABR). An ABR can connect to
the backbone area physically or logically.
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Backbone Area
After the area division of OSPF, one area is different from all the other areas. Its
area-id is 0 and it is usually called the backbone area.
area-id is 0 and it is usually called the backbone area.
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Virtual link
As all the areas should be connected to the backbone area, virtual link is adopted
so that the physically separated areas can still maintain logical connectivity to the
backbone area.
so that the physically separated areas can still maintain logical connectivity to the
backbone area.
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Route Summary
An AS is divided into different areas that are interconnected using OSPF ABRs. The
routing information between areas can be reduced by use of a 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 the summary.
routing information between areas can be reduced by use of a 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 the summary.
Configuring OSPF
You must first enable OSPF then specify the interface and area ID before configuring
other functions. However, the configuration of functions that are related to the
interface does not depend on whether OSPF is enabled. If OSPF is disabled, the
OSPF-related interface parameters become invalid.
other functions. However, the configuration of functions that are related to the
interface does not depend on whether OSPF is enabled. If OSPF is disabled, the
OSPF-related interface parameters become invalid.
OSPF configuration includes tasks that are described in the following sections:
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