User guide

E
XTREME
W
ARE
S
OFTWARE
U
SER
G
UIDE
12-7
O
VERVIEW OF
OSPF
The backbone allows summary information to be exchanged between ABRs. Every ABR
hears the area summaries from all other ABRs. The ABR then forms a picture of the
distance to all networks outside of its area by examining the collected advertisements,
and adding in the backbone distance to each advertising router.
When a VLAN is configured to run OSPF, you must configure the area for the VLAN. If
you want to configure the VLAN to be part of a different OSPF area, use the following
command:
config ospf vlan <name> area <areaid>
If this is the first instance of the OSPF area being used, you must create the area first
using the following command:
create ospf area <areaid>
S
TUB
A
REAS
OSPF allows certain areas to be configured as stub areas. A stub area is connected to
only one other area. The area that connects to a stub area can be the backbone area.
External route information is not distributed into stub areas. Stub areas are used to
reduce memory consumption and computation requirements on OSPF routers.
N
OT
-S
O
-S
TUBBY
-A
REAS
(NSSA)
NSSAs are similar to the existing OSPF stub area configuration option, but have the
following two additional capabilities:
• External routes originating from an ASBR connected to the NSSA can be advertised
within the NSSA.
• External routes originating from the NSSA can be propagated to other areas,
including the backbone area.
The CLI command to control the NSSA function is similar to the command used for
configuring a stub area, as follows:
config ospf area <area_id> nssa {summary | nosummary} stub-default-cost
<cost> {translate}
The
translate
option determines whether type 7 LSAs are translated into type 5 LSAs.
When configuring an OSPF area as an NSSA, the
translate
should only be used on
NSSA border routers, where translation is to be enforced. If
translate
is not used on
any NSSA border router in a NSSA, one of the ABRs for that NSSA is elected to