Defining Areas; Assigning The Area Index - Lenovo CN4093 Application Manual

Defining Areas

Assigning the Area Index

© Copyright Lenovo 2015
If you are configuring multiple areas in your OSPF domain, one of the areas must
be designated as area 0, known as the backbone. The backbone is the central OSPF
area and is usually physically connected to all other areas. The areas inject routing
information into the backbone which, in turn, disseminates the information into
other areas.
Since the backbone connects the areas in your network, it must be a contiguous
area. If the backbone is partitioned (possibly as a result of joining separate OSPF
networks), parts of the AS will be unreachable, and you will need to configure
virtual links to reconnect the partitioned areas (see
Up to three OSPF areas can be connected to the CN4093 with Lenovo N/OS
software. To configure an area, the OSPF number must be defined and then
attached to a network interface on the switch. The full process is explained in the
following sections.
An OSPF area is defined by assigning two pieces of information: an area index and
an area ID. The commands to define and enable an OSPF area are as follows:
CN 4093(config)# router ospf
CN 4093(config-router-ospf)# area
CN 4093(config-router-ospf)# area
CN 4093(config-router-ospf)# exit
Note: The aindex option above is an arbitrary index used only on the switch and
does not represent the actual OSPF area number. The actual OSPF area number is
defined in the areaid portion of the command as explained in the following
sections.
The aindex <area index> option is actually just an arbitrary index (0-2) used only
by the CN4093. This index does not necessarily represent the OSPF area number,
though for configuration simplicity, it should where possible.
For example, both of the following sets of commands define OSPF area 0 (the
backbone) and area 1 because that information is held in the area ID portion of the
command. However, the first set of commands is easier to maintain because the
arbitrary area indexes agree with the area IDs:
 Area index and area ID agree
area 0 area-id 0.0.0.0
area 1 area-id 0.0.0.1
Area index set to an arbitrary value

area 1 area-id 0.0.0.0
area 2 area-id 0.0.0.1
"Virtual Links" on page
<area index> <n.n.n.n>
area-id
<area index>
enable
(Use index 0 to set area 0 in ID octet format)
(Use index 1 to set area 1 in ID octet format)
(Use index 1 to set area 0 in ID octet format)
(Use index 2 to set area 1 in ID octet format)
457).
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Chapter 30: OSPF