OSPF Special Areas
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Foreword
This section focuses on OSPF special areas, including stub
areas, totally stub areas and Not-So-Stubby areas (NSSA).
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Page2
Objectives
Upon completion of this section, you will be able to :
 Understand the concept and configuration for stub areas
 Understand the concept and configuration for totally stub areas
 Understand the concept and configuration for an NSSA
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Stub Areas
Stub Area
Flood AS-external-LSA
Area 0
Area 1
Flood AS-external-LSA
RTB
Area 2
RTA
Connected to another
AS by BGP
A default route is flooded by NetworkSummary-LSA, AS-external-LSA is not flooded
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Network-Summary-LSA in a Stub Area
Area 1
RTA
Area 0
RTC
RTB
10.1.1.0/30
.1
.2
10.2.1.0/30
.1
.2
RTD
10.3.1.0/30
.1
.2
Stub
Area
Loopback0
10.4.1.1/24
The static route destined for
10.4.1.0/24 is configured on RTC
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Network-Summary-LSA in a Stub Area
[RTA]display ospf lsdb
OSPF Process 1 with Router ID 1.1.1.1
Link State Database
Type
Router
Router
Network
Sum-Net
Sum-Net
LinkState ID
2.2.2.2
1.1.1.1
10.1.1.2
0.0.0.0
10.2.1.0
Area: 0.0.0.1
AdvRouter
2.2.2.2
1.1.1.1
2.2.2.2
2.2.2.2
2.2.2.2
RTB only advertises a default
route instead of AS-external-LSA
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Age
165
165
166
228
228
Len
36
36
32
28
28
Sequence
80000004
80000003
80000001
80000001
80000001
Metric
1
1
0
1
1
Inter-area routing information is
still advertised into the stub area
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Totally Stub Area
[RTA]display ospf lsdb
OSPF Process 1 with Router ID 1.1.1.1
Link State Database
Type
Router
Router
Network
Sum-Net
LinkState ID
2.2.2.2
1.1.1.1
10.1.1.1
0.0.0.0
Area: 0.0.0.1
AdvRouter
2.2.2.2
1.1.1.1
1.1.1.1
2.2.2.2
Age
10
10
14
550
Len
36
36
32
28
Sequence
80000008
80000008
80000001
80000001
Only one default route is advertised
Neither inter-area routing information nor
AS-external-LSA is advertised
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Metric
1
1
0
1
Not-So-Stubby Area (NSSA)
Transform type 7 LSA
into type 5 LSA
Configure the static route
destined for 10.4.1.0/24
Area 1
NSSA
Area 0
RTA
10.1.1.0/24
.1
.2
RTC
10.2.1.0/24
.1
.2
RTD
10.3.1.0/24
.1
.2
RTB
Loopback0
10.4.1.1/24
Flood type 5 LSA
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Flood type 7 LSA
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NSSA-LSA
[RTC]display ospf lsdb nssa
OSPF Process 1 with Router ID 3.3.3.3
Area: 0.0.0.1
Link State Database
Type
: NSSA
Ls id
: 10.4.1.0
Adv rtr
: 3.3.3.3
LSA header，LS ID is
Ls age
: 48
Len
: 36
destination network address
Options
: NP
seq#
: 8000000d
chksum
: 0xeea6
Net mask : 255.255.255.0
TOS 0 Metric: 1
E type
: 2
Forwarding Address is the IP
Forwarding Address : 3.3.3.3
address of the stub interface
Tag
: 1
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Transforming NSSA-LSA to AS-External-LSA
[RTB]display ospf lsdb ase self-originate
OSPF Process 1 with Router ID 2.2.2.2
Link State Database
Type
: External
Ls id
: 10.4.1.0
Adv rtr
: 2.2.2.2
Ls age
: 580
Len
: 36
Options
: E
seq#
: 8000000b
chksum
: 0x871c
Net mask : 255.255.255.0
TOS 0 Metric: 1
E type
: 2
Forwarding Address : 3.3.3.3
Tag
: 1
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Modify Advertising Router
as the Router ID of itself
Copy the Forwarding Address of
the original NSSA-LSA
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Using NSSA-LSA to Advertise Default Route
[RTB]display ospf lsdb nssa self-originate
OSPF Process 1 with Router ID 2.2.2.2
Area: 0.0.0.0
Link State Database
Area: 0.0.0.1
Link State Database
Type
: NSSA
Ls id
: 0.0.0.0
Adv rtr
: 2.2.2.2
Ls age
: 1231
Len
: 36
Options
: None
seq#
: 80000001
chksum
: 0x6654
Net mask : 0.0.0.0
TOS 0 Metric: 1
E type
: 2
Forwarding Address : 2.2.2.2
Tag
: 1
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Configuring a Stub Area－Topology
Loopback0
1.1.1.1/32
E0
E0
10.1.1.1/30 .2
RTA
S1
S1
10.2.1.1/30 .2
RTB
Stub Area
RTC
Area 0
Area 1
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ASBR
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Configuring a Stub Area －Configuration of RTA and RTB
[RTA]router id 1.1.1.1
[RTA]ospf
[RTA-ospf-1]area 1
[RTA-ospf-1-area-0.0.0.1]network 1.1.1.1 0.0.0.0
[RTA-ospf-1-area-0.0.0.1]network 10.1.1.0 0.0.0.3
[RTA-ospf-1-area-0.0.0.1]stub
[RTA-ospf-1-area-0.0.0.1]return
<RTA>
Specify area 1 as the stub
area
[RTB]router id 2.2.2.2
[RTB]ospf
[RTB-ospf-1]area 1
[RTB-ospf-1-area-0.0.0.1]network 2.2.2.2 0.0.0.0
[RTB-ospf-1-area-0.0.0.1]network 10.1.1.0 0.0.0.3
[RTB-ospf-1-area-0.0.0.1]stub
[RTB-ospf-1-area-0.0.0.1]quit
[RTB-ospf-1]area 0
[RTB-ospf-1-area-0.0.0.0]network 10.2.1.0 0.0.0.3
[RTB-ospf-1-area-0.0.0.0]return
<RTB>
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Configuring a Stub Area －Configuration of RTC
[RTC]router id 3.3.3.3
[RTC]ospf
[RTC-ospf-1]area 0
[RTC-ospf-1-area-0.0.0.0]network 3.3.3.3 0.0.0.0
[RTC-ospf-1-area-0.0.0.0]
[RTC-ospf-1-area-0.0.0.0]network 10.2.1.0 0.0.0.3
[RTC-ospf-1-area-0.0.0.0]quit
[RTC-ospf-1]
[RTC-ospf-1]import-route direct
Import direct route
[RTC-ospf-1]quit
[RTC]
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Configuring a Stub Area －Checking RTA’s Routing Table
[RTA]display ip routing-table
Routing Tables: Public
Destinations : 12
Routes : 12
Destination/Mask
0.0.0.0/0
1.1.1.1/32
10.1.1.0/30
10.1.1.1/32
10.2.1.0/30
10.5.1.0/30
10.5.1.1/32
10.5.1.2/32
20.1.1.1/32
20.1.1.2/32
127.0.0.0/8
127.0.0.1/32
Proto
Pre
Cost
NextHop
Interface
OSPF
Direct
Direct
Direct
OSPF
Direct
Direct
Direct
Direct
Direct
Direct
Direct
10
0
0
0
10
0
0
0
0
0
0
0
2
0
0
0
1563
0
0
0
0
0
0
0
10.1.1.2
127.0.0.1
10.1.1.1
127.0.0.1
10.1.1.2
10.5.1.1
127.0.0.1
10.5.1.2
127.0.0.1
127.0.0.1
127.0.0.1
127.0.0.1
Ethernet0
InLoopBack0
Ethernet0
InLoopBack0
Ethernet0
Serial1
InLoopBack0
Serial1
InLoopBack0
InLoopBack0
InLoopBack0
InLoopBack0
Only one default route instead of detailed
external routes is included in routing table
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Configuring a Totally Stub Area
[RTB]ospf
[RTB-ospf-1]area 1
[RTB-ospf-1-area-0.0.0.1]stub no-summary
[RTB-ospf-1-area-0.0.0.1]quit
[RTB-ospf-1]quit
[RTB]
No-summary indicates no advertising
Summary-LSAs ( type 3 and type 4 )
into the stub area
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Configuring a Totally Stub Area －Checking RTA’s
Routing Table
[RTA]display ip routing-table
Routing Tables: Public
Destinations : 11
Routes : 11
Destination/Mask
0.0.0.0/0
1.1.1.1/32
10.1.1.0/30
10.1.1.1/32
10.5.1.0/30
10.5.1.1/32
10.5.1.2/32
20.1.1.1/32
20.1.1.2/32
127.0.0.0/8
127.0.0.1/32
Proto
Pre
Cost
NextHop
Interface
OSPF
Direct
Direct
Direct
Direct
Direct
Direct
Direct
Direct
Direct
Direct
10
0
0
0
0
0
0
0
0
0
0
2
0
0
0
0
0
0
0
0
0
0
10.1.1.2
127.0.0.1
10.1.1.1
127.0.0.1
10.5.1.1
127.0.0.1
10.5.1.2
127.0.0.1
127.0.0.1
127.0.0.1
127.0.0.1
Ethernet0
InLoopBack0
Ethernet0
InLoopBack0
Serial1
InLoopBack0
Serial1
InLoopBack0
InLoopBack0
InLoopBack0
InLoopBack0
Only one default route instead of detailed external
routes or internal routes is included in routing table
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Configuring an NSSA－Topology
RTB
Loopback0
.1 10.1.1.0/30 .2
E0
E0
3.3.3.3/32
S0
S0
.1 10.3.1.0/30 .2
Area 1
1.1.1.1/32
Loopback0
S1
S1
.1
10.2.1.0/30
.2
2.2.2.2/32
Loopback0
RTC
Area 0
RTA
Loopback1
20.1.1.1/32
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RTD
Area 2
NSSA Area
Loopback0
4.4.4.4/32
Loopback1
30.1.1.1/32
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Configuring an NSSA － Configuration of
RTA and RTB
[RTA]router id 1.1.1.1
[RTA]ospf
[RTA-ospf-1]area 1
[RTA-ospf-1-area-0.0.0.1]network 1.1.1.1 0.0.0.0
[RTA-ospf-1-area-0.0.0.1]network 10.1.1.0 0.0.0.255
[RTA-ospf-1-area-0.0.0.1]quit
[RTA-ospf-1]import-route direct
[RTA-ospf-1]quit
Import the route
[RTA]
destined for
20.1.1.1/32 as an external route
[RTB]router id 2.2.2.2
[RTB]ospf
[RTB-ospf-1]area 1
[RTB-ospf-1-area-0.0.0.1]network 2.2.2.2 0.0.0.0
[RTB-ospf-1-area-0.0.0.1]network 10.1.1.0 0.0.0.255
[RTB-ospf-1-area-0.0.0.1]quit
[RTB-ospf-1]area 0
[RTB-ospf-1-area-0.0.0.0]network 10.2.1.0 0.0.0.255
[RTB-ospf-1-area-0.0.0.0]return
<RTB>
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Configuring an NSSA － Configuration of
RTC
[RTC]router id 3.3.3.3
[RTC]ospf
[RTC-ospf-1]area 0
[RTC-ospf-1-area-0.0.0.0]network 10.2.1.0 0.0.0.255
[RTC-ospf-1-area-0.0.0.0]quit
[RTC-ospf-1]area 2
[RTC-ospf-1-area-0.0.0.2]network 3.3.3.3 0.0.0.0
[RTC-ospf-1-area-0.0.0.2]network 10.3.1.0 0.0.0.255
[RTC-ospf-1-area-0.0.0.2]nssa default-route-advertise no-summary
[RTC-ospf-1-area-0.0.0.2]return
<RTC>
[RTD]router id 4.4.4.4
[RTD]ospf
[RTD-ospf-1]area 2
[RTD-ospf-1-area-0.0.0.2]network 4.4.4.4 0.0.0.0
[RTD-ospf-1-area-0.0.0.2]network 10.3.1.0 0.0.0.255
[RTD-ospf-1-area-0.0.0.2]nssa
[RTD-ospf-1-area-0.0.0.2]quit
[RTD-ospf-1]import-route direct
[RTD-ospf-1]quit
[RTD]
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Checking RTD’s LSDB
[RTD]display ospf lsdb
OSPF Process 1 with Router ID 4.4.4.4
Link State Database
Type
Router
Router
Sum-Net
NSSA
NSSA
NSSA
NSSA
NSSA
LinkState ID
4.4.4.4
3.3.3.3
0.0.0.0
10.3.1.1
10.3.1.0
30.1.1.1
4.4.4.4
0.0.0.0
Area: 0.0.0.2
AdvRouter
4.4.4.4
3.3.3.3
3.3.3.3
4.4.4.4
4.4.4.4
4.4.4.4
4.4.4.4
3.3.3.3
Age
1075
1072
1081
172
172
176
176
1878
Len
48
48
28
36
36
36
36
36
Sequence
80000006
80000004
80000001
80000002
80000002
80000002
80000002
80000001
Metric
1
1
1
1
1
1
1
1
Due to using default-route-advertise and no-summary
parameter at the same time, the ABR for the NSSA advertises
a type 7 default route and a type 3 default route separately.
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Checking RTD’s Routing Table
[RTD]display ip routing-table
Routing Tables: Public
Destinations : 8
Routes : 8
Destination/Mask
0.0.0.0/0
4.4.4.4/32
10.3.1.0/30
10.3.1.1/32
10.3.1.2/32
30.1.1.1/32
127.0.0.0/8
127.0.0.1/32
Proto
Pre
Cost
NextHop
Interface
OSPF
Direct
Direct
Direct
Direct
Direct
Direct
Direct
10
0
0
0
0
0
0
0
2
0
0
0
0
0
0
0
10.3.1.1
127.0.0.1
10.3.1.2
10.3.1.1
127.0.0.1
127.0.0.1
127.0.0.1
127.0.0.1
Serial0
InLoopBack0
Serial0
Serial0
InLoopBack0
InLoopBack0
InLoopBack0
InLoopBack0
Only one default route，no AS external route is learned from OSPF
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Checking RTC’s Routing Table
[RTC]display ip routing-table
Routing Tables: Public
Destinations : 15
Destination/Mask
Proto Pre
1.1.1.1/32 O_ASE 150
3.3.3.3/32 Direct 0
4.4.4.4/32 O_NSSA 150
10.1.1.0/30 OSPF
10
10.2.1.0/30 Direct 0
10.2.1.1/32 Direct 0
10.2.1.2/32 Direct 0
10.3.1.0/30 Direct 0
10.3.1.1/32 Direct 0
10.3.1.2/32 Direct 0
10.4.1.0/24 Static 60
20.1.1.1/32 O_ASE 150
30.1.1.1/32 O_NSSA 150
127.0.0.0/8
Direct 0
127.0.0.1/32 Direct 0
Routes : 15
Cost
NextHop
1
10.2.1.1
0
127.0.0.1
1
10.3.1.2
65531
10.2.1.1
0
10.2.1.2
0
10.2.1.1
0
127.0.0.1
0
10.3.1.1
0
127.0.0.1
0
10.3.1.2
0
10.3.1.2
1
10.2.1.1
1
10.3.1.2
0
127.0.0.1
0
127.0.0.1
RTC learned the AS external route
destined for 30.1.1.1/32 by NSSA-LSA
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Interface
Serial1
InLoopBack0
Serial0
Serial1
Serial1
Serial1
InLoopBack0
Serial0
InLoopBack0
Serial0
Serial0
Serial1
Serial0
InLoopBack0
InLoopBack0
Checking RTB’s Routing Table
[RTB]display ip routing-table
Routing Tables: Public
Destinations : 14
Destination/Mask
Proto Pre
1.1.1.1/32 O_ASE 150
2.2.2.2/32 Direct 0
4.4.4.4/32 O_ASE 150
10.1.1.0/30 Direct 0
10.1.1.2/32 Direct 0
10.2.1.0/30 Direct 0
10.2.1.1/32 Direct 0
10.2.1.2/32 Direct 0
10.3.1.0/30 OSPF
10
10.3.1.1/32 O_ASE 150
20.1.1.1/32 O_ASE 150
30.1.1.1/32 O_ASE 150
127.0.0.0/8
Direct 0
127.0.0.1/32 Direct 0
Routes : 14
Cost
NextHop
1
10.1.1.1
0
127.0.0.1
1
10.2.1.2
0
10.1.1.2
0
127.0.0.1
0
10.2.1.1
0
127.0.0.1
0
10.2.1.2
1563
10.2.1.2
1
10.2.1.2
1
10.1.1.1
1
10.2.1.2
0
127.0.0.1
0
127.0.0.1
Interface
Ethernet0
InLoopBack0
Serial1
Ethernet0
InLoopBack0
Serial1
InLoopBack0
Serial1
Serial1
Serial1
Ethernet0
Serial1
InLoopBack0
InLoopBack0
All AS external routes are learned by AS-external-LSA
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Page 24
Summary
 What is the difference between a stub
area and a totally stub Area？
 What is the difference between a stub
area and a Not-So-Stubby area？
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Thank You
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