3Com SuperStack 4 Configuration Manual page 120

132 C 6: IP R
HAPTER OUTING
Configuring OSPF
P O
ROTOCOL PERATION
the segment, and routing information is also exchanged between them. After
the existing DR fails, the BDR will immediately becomes a DR.
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.
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.
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.
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.
Route Summary
■
An AS is divided into different areas that are interconnected via 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.
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.
OSPF configuration includes tasks that are described in the following sections:
Enabling OSPF and Entering OSPF View
■
Entering OSPF Area View
■
Specifying the Interface
■
Configuring a Router ID
■
Configuring the Network Type on the OSPF Interface
■
Configuring the Cost for Sending Packets on an Interface
■
Setting the Interface Priority for DR Election
■
Setting the Peer
■
Setting the Interval of Hello Packet Transmission
■