IP Routing: OSPF Configuration Guide, Cisco IOS Release 15SY
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Cisco Open Short Shortest Path First version 3 (OSPFv3) allows
you to configure a Not-So-Stubby Area (NSSA). An NSSA is similar to a stub
area, except that an NSSA allows you to import autonomous system (AS) external
routes within an NSSA using redistribution. This feature adds support for the
OSPFv3 NSSA specification described by RFC 3101. RFC 3101 replaced and is
backward compatible with RFC 1587.
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Information About
Configuring NSSA for OSPFv3
RFC 1587 Compliance
RFC 3101 compliance is automatically enabled on the devices. Use
the compatible
rfc1587 command in router configuration mode to revert to
route selection that is based on RFC 1587.
When you
configure the device to be compatible with RFC 1587, the device performs the following actions:
Reverts the
route selection process to RFC 1587.
Configures Autonomous
System Border Router (ASBR) to configure the P (propagate bit) and zero-forwarding
address.
Disables always translating Area Border Router (ABR).
ABR as OSPFv3 NSSA LSA
Translator
Use the
Not-So-Stubby Area (NSSA) for Open Shortest Path First version 3 (OSPFv3)
feature to simplify administration in a network that connects a central site
that uses OSPFv3 to a remote site that uses a different routing protocol.
When the NSSA
feature is not implemented, the connection between the border device at the
corporate site and the remote device is not established as an OSPFv3 stub area
due to following reasons:
Routes for the remote site
are not redistributed into the stub area.
Two routing protocols must
be maintained.
A protocol such as
Routing Information Protocol (RIP) for IPv6 is run to handle the
redistribution. By implementing NSSA, you can extend OSPFv3 to include the
remote connection by defining the area between the border device at the
corporate site and the remote device as an NSSA.
As with OSPFv3 stub
areas, NSSA areas cannot be injected with distributed routes via a Type 5 Link
State Advertisement (LSA). Route redistribution into an NSSA area is possible
only with a Type 7 LSA. An NSSA Autonomous System Border Router (ASBR)
generates the Type 7 LSA , and an NSSA Area Border Router (ABR) translates the
Type 7 LSA into a Type 5 LSA. These LSAs can be flooded throughout the OSPFv3
routing domain. Route summarization and filtering are supported during the
translation.
Route summarization
is the consolidation of advertised addresses. This feature enables an ABR to
advertise a single summary route to other areas. 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.
When routes from
other protocols are redistributed into an OSPFv3 area, each route is advertised
individually in an external LSA. However, you can configure the Cisco IOS
software to advertise a single route with a specified network address and mask
for all the redistributed routes that are covered by a specified network
address and mask. Thus, the size of the OSPFv3 link-state database decreases.
RFC 3101 allows you
to configure an NSSA ABR device as a forced NSSA LSA translator.
Note
Even a forced
translator might not translate all LSAs; translation depends on the content of
each LSA.
The figure below
shows a network diagram in which OSPFv3 Area 1 is defined as the stub area. The
Enhanced Interior Gateway Routing Protocol (EIGRP) routes are not propagated
into the OSPFv3 domain because routing redistribution is not allowed in the
stub area. However, once OSPFv3 Area 1 is defined as an NSSA, an NSSA ASBR can
include the EIGRP routes to the OSPFv3 NSSA by generating Type 7 LSAs.
Figure 1. OSPFv3 NSSA
The redistributed
routes from the RIP device are not allowed into OSPFv3 Area 1 because NSSA is
an extension to the stub area. The stub area characteristics still exist,
including the exclusion of Type 5 LSAs.
The figure below
shows the OSPFv3 stub network with NSSA Area 1. The redistributed routes that
Device 4 is propagating from the two RIP networks are translated into Type 7
LSAs by NSSA ASBR Device 3. Device 2, which is configured to be the NSSA ABR,
translates the Type 7 LSAs back to Type 5 so that they can be flooded through
the rest of the OSPFv3 stub network within OSPFv3 Area 0.
Figure 2. OSPFv3 NSSA
Network with NSSA ABR and ASBR Devices
How to Configure NSSA
for OSPFv3
Configuring an OSPFv3 NSSA
Area and Its Parameters
Device(config-router)# area 1 nssa default-information-originate nssa-only
Configures an
NSSA area and sets the default advertisement to this NSSA area.
In the
example, area 1 is configured as an NSSA area.
The
nssa-only
keyword instructs the device to instigate Type-7 LSA with cleared P-bit,
thereby, preventing LSA translation to Type 5 on NSSA ABR device.
Step 5
address-family {ipv4 |
ipv6} [unicast]
Example:
Device(config-router)# address-family ipv4 unicast
OR
Device(config-router)# address-family ipv6 unicast
Enables
address family configuration mode for Open Shortest Path First version 3
(OSPFv3).
The
address-family ipv4
unicast command configures an IPv4 address family.
The
address-family ipv6
unicast command configures an IPv6 address family.
(For
IPv4 address family only) Defines an IPv4 summary prefix and address mask in
Open Shortest Path First version 3 (OSPFv3) and summarizes all routes
redistributed from other routing protocols.
(For
IPv6 address family only) Defines an IPv6 summary prefix in Open Shortest Path
First version 3 (OSPFv3) and summarizes all routes redistributed from other
routing protocols.
The
nssa-only
keyword instructs the device to instigate Type-7 LSA with cleared P-bit,
thereby, preventing LSA translation to Type 5 on NSSA ABR router.
Step 7
exit
Example:
Device(config-router-af)# exit
Exits address-family router configuration mode and returns to the
router configuration mode.
Redistributes routes from one routing domain into another routing domain.
In the
example, Routing Information Protocol (RIP) subnets are redistributed into the
OSPFv3 domain.
Step 9
end
Example:
Device(config-router)# end
Exits
router configuration mode and returns to privileged EXEC mode.
Configuring an NSSA ABR as
a Forced NSSA LSA Translator for OSPFv3
SUMMARY STEPS
enable
configureterminal
router
ospfv3process-id
areaarea-idnssatranslatetype7always
areaarea-idnssatranslatetype7suppress-fa
end
DETAILED STEPS
Command or Action
Purpose
Step 1
enable
Example:
Device> enable
Enables
privileged EXEC mode.
Enter
your password if prompted.
Step 2
configureterminal
Example:
Device# configure terminal
Enters global
configuration mode.
Step 3
router
ospfv3process-id
Example:
Device(config)# router ospfv3 1
Enables
OSPFv3 routing and enters router configuration mode.
The
process-id
argument identifies the OSPFv3 process. The range is from 1 to 65535.
Step 4
areaarea-idnssatranslatetype7always
Example:
Device(config-router)# area 10 nssa translate type7 always
Configures a
Not-So-Stubby Area Area Border Router (NSSA ABR) device as a forced NSSA Link
State Advertisement (LSA) translator.
Note
You can use
the
always
keyword to configure an NSSA ABR device as a forced NSSA LSA translator. This
command can be used if RFC 3101 is disabled and RFC 1587 is used.
Step 5
areaarea-idnssatranslatetype7suppress-fa
Example:
Device(config-router)# area 10 nssa translate type7 suppress-fa
OR
Device (config-router)# address-family [ipv4|ipv6] unicast
Device (config-router-af)# area 10 nssa translate type7 suppress-fa
Device (config-router-af)# exit
Allows the ABR
to suppress the forwarding address in translated Type 5 LSA.
Note
You can
configure this command in both router configuration mode and address-family
configuration mode.
Step 6
end
Example:
Device(config-router)# end
Exits
router configuration mode and returns to privileged EXEC mode.
Disabling RFC 3101
Compatibility and Enabling RFC 1587 Compatibility
SUMMARY STEPS
enable
configure
terminal
router
ospfv3process-id
compatiblerfc1587
end
DETAILED STEPS
Command or Action
Purpose
Step 1
enable
Example:
Device> enable
Enables
privileged EXEC mode.
Enter your password if
prompted.
Step 2
configure
terminal
Example:
Device# configure terminal
Enters global
configuration mode.
Step 3
router
ospfv3process-id
Example:
Device(config)# router ospfv3 1
Enables
OSPFv3 routing and enters router configuration mode.
The
process-id argument identifies the
OSPFv3 process.
Step 4
compatiblerfc1587
Example:
Device(config-router)# compatible rfc1587
Changes the
method used to perform route selection to RFC 1587 compatibility and disables
RFC 3101.
Step 5
end
Example:
Device(config-router)# end
Exits router
configuration mode and returns to privileged EXEC mode.
Configuration Examples
for Configuring NSSA for OSPFv3
Example: NSSA for
OSPFv3
Use the
show ospfv3
command to confirm that the device is acting as an Autonomous System Border
Router (ASBR) and that the Open Shortest Path First version 3 (OSPFv3) Area 1
has been configured as a Not-So-Stubby Area (NSSA) area.
Device# show ospfv3
OSPFv3 1 address-family ipv4
Router ID 3.3.3.3
Supports NSSA (compatible with RFC 1587)
It is an autonomous system boundary router
Redistributing External Routes from,
static
Router is not originating router-LSAs with maximum metric
Initial SPF schedule delay 5000 msecs
Minimum hold time between two consecutive SPFs 10000 msecs
Maximum wait time between two consecutive SPFs 10000 msecs
Minimum LSA interval 5 secs
Minimum LSA arrival 1000 msecs
LSA group pacing timer 240 secs
Interface flood pacing timer 33 msecs
Retransmission pacing timer 66 msecs
Number of external LSA 0. Checksum Sum 0x000000
Number of areas in this router is 1. 0 normal 0 stub 1 nssa
Graceful restart helper support enabled
Reference bandwidth unit is 100 mbps
RFC1583 compatibility enabled
Area 1
Number of interfaces in this area is 1
It is a NSSA area
Configured to translate Type-7 LSAs, inactive (RFC3101 support disabled)
Perform type-7/type-5 LSA translation, suppress forwarding address
Area has no authentication
SPF algorithm last executed 00:00:07.160 ago
SPF algorithm executed 3 times
Area ranges are
Number of LSA 3. Checksum Sum 0x0245F0
Number of opaque link LSA 0. Checksum Sum 0x000000
Number of DCbitless LSA 0
Number of indication LSA 0
Number of DoNotAge LSA 0
Flood list length 0
The table below
describes the significant
show ip ospf display fields and their
descriptions.
Table 1. show ospfv3
Field Descriptions
Field
Description
Supports
NSSA (compatible with RFC 1587)
Specifies
that RFC 1587 is active or that the OSPFv3 NSSA area is RFC 1587 compatible.
Configured
to translate Type-7 LSAs, inactive (RFC3101 support disabled)
Specifies
that the OSPFv3 NSSA area has an ABR device configured to act as a forced
translator of Type 7 LSAs. However, it is inactive because RFC 3101 is
disabled.
The output of the
router LSA in LSDB shows Nt-Bit if it is set in the header of LSA.
Router Link States (Area 1)
LS age: 94
Options: (N-Bit, R-bit, DC-Bit, AF-Bit, Nt-Bit)
LS Type: Router Links
Link State ID: 0
Advertising Router: 2.2.2.2
LS Seq Number: 80000002
Checksum: 0x8AD5
Length: 56
Area Border Router
AS Boundary Router
Unconditional NSSA translator
Number of Links: 2
The
“Unconditional NSSA translator” line indicates that the status of the NSSA ASBR
router is as a forced NSSA LSA translator.
Additional References for
Configuring NSSA for OSPFv3
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Feature Information for
Configuring NSSA for OSPFv3
The following table provides release information about the feature or features described in this module. This table lists
only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise,
subsequent releases of that software release train also support that feature.
Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco
Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.
Table 2. Feature Information for
Configuring NSSA for OSPFv3
Feature
Name
Releases
Feature
Information
OSPFv3
Support for NSSA
Cisco Open Short Shortest Path First version 3 (OSPFv3) allows
you to configure a Not-So-Stubby Area (NSSA). An NSSA is similar to a stub
area, except that an NSSA allows you to import autonomous system (AS) external
routes within an NSSA using redistribution. This feature adds support for the
OSPFv3 NSSA specification described by RFC 3101. RFC 3101 replaced and is
backward compatible with RFC 1587.
The
following commands were introduced or modified:
area nssa
translate,
compatible
rfc1587,
show ospfv3.
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