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If you configure an NSSA totally stub area using
no summary
command, inter-area routes are not allowed in the
NSSA area. When redistribution takes place in the situations where there is no need to inject external routes into the
NSSA, you can prevent the router from creating Type 7 LSAs for NSSA using the
translate-never
command. This
situation can occur when an Autonomous System Boundary Router (ASBR) is also an NSSA ABR. On the other
hand, the
translate-always
command enables the router to redistribute all external routes as Type 7 LSAs, which are
translated into Type 5 LSAs by the NSSA ABR and then leaked into the OSPF domain.
5.7.9 Configure Route Summarization between OSPF Areas
Route summarization causes a single summary route to be advertised to other areas by an ABR.
In OSPF, an ABR will advertise networks in one area into another area. If the network numbers in an area are
assigned in a way such that they are contiguous, you can configure the ABR to advertise a summary route that
covers all the individual networks within the area that fall into the specified range.
To define an address range, perform the following task in router configuration mode.
Command Purpose
area
{
area-id
|
area-address
}
range
prefix-length
[
not-advertised
]
Define an address range where a single route will be
advertised.
area
area-address
range
prefix
{suppress
|
substitute}
prefix
Announce an address range where a route will not be
injected.
5.7.10 Create Virtual Links
In OSPF, all areas must be connected to a backbone area. If there is a break in backbone continuity, or the backbone
is purposefully portioned, you can establish a virtual link. The virtual link must be configured in both routers. The
configuration information in each router consists of the other virtual endpoint, and the non-backbone area that the two
routers have in common (called the transit area). Note that virtual link cannot be configured through stub areas.
To create a virtual link, perform the following task in router configuration mode.
Command Purpose
area
area-id
virtual-link
router-id
[
hello-
interval
seconds
]
[
retransmit-interval
seconds
] [
transmit-
delay
seconds
]
[
dead-interval
seconds
]
[[
authentication-key
key
] |
[
message-digest-key
keyed
md5
key]]
Establish a virtual link.
5.7.11 Control Default Metrics
OSPF calculates the OSPF metric for an interface according to the bandwidth of the interface. For example, a 64K
link gets a metric of 1562, while a T1 link gets a metric of 64.
If you have multiple links with high bandwidth, you might want to specify a larger number to differentiate the cost on
those links. To do so, perform the following task in router configuration mode.
