Multi Area OSPF
Carl Bergenhem - 030404
carl@bergenhem.com
Session Number
Version 2002-1
© 2002, Svenska-CNAP Halmstad University
1
Issues with large OSPF nets
• Frequent SPF calculations
• Large routing table
• Large link-state table (topological
database)
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Issues with large OSPF nets
Version 2002-1
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OSPF uses “Areas”
• Hierarchical
– separate large internetworks (autonomous system)
into smaller internetworks, called areas.
• Routing between the areas (called inter-area
routing),
• Routing within an area: intra-area
• But many of the smaller internal routing
operations are restricted within an area.
• Hide unnecessary information, e.g. summarise
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OSPF Areas
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OSPF Router Types
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OSPF Router Types
Internal: Routers with all their interfaces within the
same area
Backbone: Routers with at least one interface
connected to area 0
ASBR: (Autonomous System Boundary Router):
Routers that have at least one interface connected
to an external internetwork (another autonomous
system)
ABR: (Area Border Router): Routers with interfaces
attached to multiple areas.
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LSA
• Link State Advertisement
– Sent in a LSU (Link State Update)
• Flooding = information distribution within an area
• Different types of information
– External route (outside the OSPF AS)
– Summary routes
– Inter-area routes (from different area but same OSPF AS)
– Intra-area routes (from same area and same OSPF AS)
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LSA Types
• See online material: chpt: 5.1.3
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LSA Types
LSA1 contain the Router Link States, from all routers
LSA2 contain the Net Link States, from the DR
Intra-area routes (O)
LSA3 contain the Summary Net Link States, from ABR
(describes routes within AS)
LSA4 contain the AS Summary Net Link States, from ASBR
(describes routes to ASBR)
Inter-area routes (O IA)
LSA5 contain external route information (E1 or E2),from ASBR
LSA7 contain NSSA External Link Entry, from ASBR
translated into LSA5 at the ABR
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LSA Types (con’t)
• Type 6 MOSPF (Multicast OSPF) Not
supported by Cisco.
– MOSPF enhances OSPF by letting routers use
their link-state databases to build multicast
distribution trees for the forwarding of
multicast traffic.
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LSA Types (con’t)
• Type 7
NSSA External Link Entry
– Originated by an ASBR connected to an
NSSA.

Version 2002-1
Type 7 messages can be flooded throughout
NSSAs and translated into LSA Type 5
messages by ABRs. Routes learned via Type-7
LSAs are denoted by either a “N1” or and “N2”
in the routing table. (Compare to E1 and E2).
© 2002, Svenska-CNAP / Halmstad University.
12
Area Types
• Standard
(accept all LSAs)
• Backbone
(accept all LSAs)
• Stub area
(not accept all LSAs)
– Stub
– Totally Stubby Area (TSA)
– Not-so-stubby-area (NSSA)
• Difference in what LSAs they accept......
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Area Types
Default route 
External (type 5) routes
Accepted  Summary (type 3,4) routes
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© 2002, Svenska-CNAP / Halmstad University.
 Default route
 Default route
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Area Types - differences
– What LSAs that may be flooded=>

What info an area can know (hide info!!)
– External routes are injected into OSPF by
ASBR: LSA5
Version 2002-1

Type 1 (E1): cost metric increments as route is
passed through OSPF domain

Type 2 (E2): cost metric remains the same as
route is passed through OSPF domain
© 2002, Svenska-CNAP / Halmstad University.
15
Configuring Areas
• stub = Stub area
– LSA3,4 OK,
LSA5 not OK
• stub + no summary = Totally stubby area
– LSA3,4 and 5 not OK
• NSSA = Not so stubby area
– LSA3,4 OK,
LSA5 not OK,
LSA7 OK
• NSSA + no summary = NSSA totally stubby area..
– LSA3,4 and 5 not OK
Version 2002-1
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LSA7 OK
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Multi-area Example
ABR
RIP
ASBR
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Stub Example
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Stub Example
ABR
ASBR
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Stub Example
ABR
No Type
5 LSAs
ASBR
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Stub Example
ABR
No Type
5 LSAs
Route to 0.0.0.0/0
via RTB
ASBR
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LSA Types
LSA1 contain the Router Link States, from all routers
LSA2 contain the Net Link States, from the DR
Intra-area routes (O)
LSA3 contain the Summary Net Link States, from ABR
(desc. routes within AS)
LSA4 contain the AS Summary Net Link States, from ASBR
(desc. routes to ASBR)
Inter-area routes (O IA)
LSA5 contain external route information (E1 or E2),from ASBR
LSA7 contain NSSA External Link Entry, from ASBR
translated into LSA5 at the ABR
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Totally Stubby Example
no summary
ABR
Totally Stubby Area
No Type 3, 4,
or 5 LSAs
Route to 0.0.0.0/0
via RTB
(no more IA
routes)
ASBR
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© 2002, Svenska-CNAP / Halmstad University.
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LSA Types
LSA1 contain the Router Link States, from all routers
LSA2 contain the Net Link States, from the DR
Intra-area routes (O)
LSA3 contain the Summary Net Link States, from ABR
(desc. routes within AS)
LSA4 contain the AS Summary Net Link States, from ASBR
(desc. routes to ASBR)
Inter-area routes (O IA)
LSA5 contain external route information (E1 or E2),from ASBR
LSA7 contain NSSA External Link Entry, from ASBR
translated into LSA5 at the ABR
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NSSA Example
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NSSA Example
ABR
No Type 5
LSAs
3,4 OK
Type 7 OK
ASBR
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NSSA Example
no summary
ABR
No Type 3, 4
or 5 LSAs
Type 7 OK
ASBR
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LSA Types (con’t)
• Type 7
NSSA External Link Entry
– Originated by an ASBR connected to an
NSSA.

Version 2002-1
Type 7 messages can be flooded throughout
NSSAs and translated into LSA Type 5
messages by ABRs. Routes learned via Type-7
LSAs are denoted by either a “N1” or and “N2”
in the routing table. (Compare to E1 and E2).
© 2002, Svenska-CNAP / Halmstad University.
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Multi Area OSPF
LSA 1 Router LSA
Loop 162.10.5.1/16
Normal Areas
.2
RIP
AS-Remote
RIP
10
.0.
OSPF
Area 51
.0
.10 .1
0
1
2.
19 /24
.2
0.0
/
.1
8
ASBR
.3
Loop 1.5.202.206/24
OSPF
Area 0
206.202.0.0/24
.4
RouterE
Loop 1.10.202.206/24
Switch
.1
.1
RouterF
Loop 2.10.202.206/24
192.10.5.0/24
.2
RouterA
Loop 1.0.202.206/24
.1
OSPF
Area 1
LSA 1
Originated
RouterB
Loop 2.0.202.206/24
206.202.1.0/24
LSA 1’s are flooded out
other interfaces within
the same area.
Version 2002-1
© 2002, Svenska-CNAP / Halmstad University.
LSA 1
flooded
Switch
206.202.2.0/24
.2
.1
RouterC
Loop 1.2.202.206/24
LSA 1
.2Orig.
LSA 1 LoopRouterD
2.2.202.206
flooded
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Multi Area OSPF
LSA 2 Network LSA
Loop 162.10.5.1/16
Normal Areas
.2
RIP
AS-Remote
RIP
10
.0.
OSPF
Area 51
.0
.10 .1
0
1
2.
19 /24
.2
.1
8
OSPF
Area 0
206.202.0.0/24
.4
LSA 2
Switch
.1
192.10.5.0/24
LSA 2’s being sent
ASBR
.3
Loop 1.5.202.206/24
RouterE
Loop 1.10.202.206/24
.1
RouterF
Loop 2.10.202.206/24
RouterA
Loop 1.0.202.206/24
No LSA 2’s for
Area 51 – No DR
elected because
no other routers
on the broadcast
segment
Version 2002-1
0.0
/
© 2002, Svenska-CNAP / Halmstad University.
.2
.1
RouterB
Loop 2.0.202.206/24
LSA 2
flooded
206.202.1.0/24
OSPF
Area 1
Switch
206.202.2.0/24
.2
.1
RouterC
Loop 1.2.202.206/24
.2
LSA 2
RouterD
Loop 2.2.202.206
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Multi Area OSPF
LSA 3 Summary LSA
Loop 162.10.5.1/16
Normal Areas
.2
RIP
AS-Remote
RIP
10
.0.
OSPF
Area 51
LSA 30
.
.10 .1
0
1
2.
19 /24
LSA 1
.1
RouterF
Loop 2.10.202.206/24
192.10.5.0/24
.1
8
ASBR
.3
Loop 1.5.202.206/24
OSPF
Area 0
206.202.0.0/24
.4
RouterE
Loop 1.10.202.206/24
.2
0.0
/
LSA 1
Switch
LSA 1
.1
RouterA
Loop 1.0.202.206/24
.2
.1
RouterB
Loop 2.0.202.206/24
OSPF
Area 1
LSA 3
206.202.1.0/24
Switch
206.202.2.0/24
.2
LSA 1’s are sent as
LSA 3’s into other
areas.
Version 2002-1
© 2002, Svenska-CNAP / Halmstad University.
.1
RouterC
Loop 1.2.202.206/24
.2
RouterD
Loop 2.2.202.206
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Multi Area OSPF
LSA 3 Summary LSA
Loop 162.10.5.1/16
Normal Areas
.2
RIP
AS-Remote
RIP
10
.0.
OSPF
Area 51
LSA
3
0
.
.10 .1
0
1
2.
19 /24
0.0
/
ASBR
.3
Loop 1.5.202.206/24
OSPF
LSA 3 Area 0
206.202.0.0/24
.4
RouterE
Loop 1.10.202.206/24
.2
LSA 1
LSA 1
.1
RouterF
Loop 2.10.202.206/24
192.10.5.0/24
.1
8
LSA 3
Switch
.1
RouterA
Loop 1.0.202.206/24
.2
.1
LSA 1
RouterB
Loop 2.0.202.206/24
LSA 3
206.202.1.0/24
.2
Version 2002-1
© 2002, Svenska-CNAP / Halmstad University.
Switch
206.202.2.0/24
LSA 1
LSA 1’s are sent as
LSA 3’s into other
areas.
OSPF
Area 1
.1
RouterC
Loop 1.2.202.206/24
.2
LSA 1
RouterD
Loop 2.2.202.206
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LSA 4 ASBR Summary LSA
Loop 162.10.5.1/16
.2
RIP
AS-Remote
RIP
10
.0.
OSPF
Area 51
LSA 4
LSA 4
.0
.10 .1
0
2.1
19 /24
.2
OSPF
Area 0
.4
0.0
/
.1
8
Normal Areas
ASBR
.3
Loop 1.5.202.206/24
LSA 5’s flooded
206.202.0.0/24
RouterE
Loop 1.10.202.206/24
Switch
.1
.1
RouterF
Loop 2.10.202.206/24
192.10.5.0/24
RouterA
Loop 1.0.202.206/24
.2
.1
RouterB
Loop 2.0.202.206/24
OSPF
Area 1
LSA 4
Switch
206.202.1.0/24
206.202.2.0/24
.2
.2
LSA 4
.1
RouterC
Loop 1.2.202.206/24
Version 2002-1
© 2002, Svenska-CNAP / Halmstad University.
LSA 4
RouterD
Loop 2.2.202.206
33
LSA 5 AS External LSA
Loop 162.10.5.1/16
.2
RIP
AS-Remote
RIP
10
.0.
OSPF
Area 51
LSA 5
OSPF
Area 0
LSA 5
.0
.10 .1
0
2.1
19 /24
.2
.4
0.0
/
Normal Areas
.1
8
ASBR
.3
Loop 1.5.202.206/24
LSA 5’s flooded
206.202.0.0/24
RouterE
Loop 1.10.202.206/24
Switch
.1
.1
RouterF
Loop 2.10.202.206/24
192.10.5.0/24
RouterA
Loop 1.0.202.206/24
.2
.1
RouterB
Loop 2.0.202.206/24
OSPF
Area 1
LSA 5
ASBR
router ospf 1
redistribute rip metric 500
network 206.202.0.0 0.0.0.255 area 0
ip route 0.0.0.0 0.0.0.0 10.0.0.2
Version 2002-1
© 2002, Svenska-CNAP / Halmstad University.
Switch
206.202.1.0/24
206.202.2.0/24
.2
.2
.1
LSA 5
RouterC
Loop 1.2.202.206/24
LSA 5
RouterD
Loop 2.2.202.206
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Stub Areas
Loop 162.10.5.1/16
.2
RIP
AS-Remote
RIP
10
.0.
LSA 3
OSPF
Area 51
LSA 4
LSA 5
OSPF
Area 0
LSA 4
.0
.10 .1
0
2.1
19 /24
.2
LSA 5
192.10.5.0/24
.1
8
ASBR
.3
Loop 1.5.202.206/24
206.202.0.0/24
.4
RouterE
Loop 1.10.202.206/24
.1
RouterF
Loop 2.10.202.206/24
0.0
/
LSA 3
.2 LSA 4
LSA 5
OSPF
Switch
LSA.1 5
RouterA
Loop 1.0.202.206/24
.1
RouterB
Loop 2.0.202.206/24
Blocked X
Area 1
Switch
Default
206.202.2.0/24
route
to
Stub Area
.2
.2
ABR
.1
RouterC
injected LoopRouterD
Loop 1.2.202.206/24
2.2.202.206
206.202.1.0/24

All routers in the area must be configured as “stub” including the ABR:
router ospf 1
network 206.202.0.0 0.0.0.255 area 0
network 206.202.1.0 0.0.0.255 area 1
Version 2002-1area 1 stub
© 2002, Svenska-CNAP / Halmstad University.
35
Totally Stubby Areas
Loop 162.10.5.1/16
.2
RIP
AS-Remote
RIP
10
.0.
LSA 3
OSPF
Area 51
LSA 4
LSA 5
OSPF
Area 0
LSA 4
.0
.10 .1
0
2.1
19 /24
.2
RouterF
Loop 2.10.202.206/24
192.10.5.0/24
.1
8
ASBR
.3
Loop 1.5.202.206/24
206.202.0.0/24
.4
RouterE
Loop 1.10.202.206/24
.1
0.0
/
LSA 5
LSA 3
.2 LSA 4
LSA 5
OSPF
Switch
LSA.1 5
RouterA
Loop 1.0.202.206/24
X X Blocked
.1
RouterB
Loop 2.0.202.206/24
Blocked X
Area 1
Switch
Default
206.202.2.0/24
route
to
Stub Area
.2
.2
ABR
.1
RouterC
injected LoopRouterD
Loop 1.2.202.206/24
2.2.202.206
206.202.1.0/24

All routers in the area must be configured as “stub” except the ABR “stub no
summary”:
router ospf 1
network 206.202.0.0 0.0.0.255 area 0
network 206.202.1.0 0.0.0.255 area 1
Version 2002-1
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/ Halmstad University.
area 1 stub
no-summary
36
Virtual Links
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37
Virtual Links
• A virtual link has the following two
requirements:
– It must be established between two routers
that share a common area.
– One of these two routers must be connected
to the backbone.
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38
Virtual Links
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39
Virtual Links
RTA(config)#router ospf 1
RTA(config-router)#network 192.168.0.0 0.0.0.255 area 51
RTA(config-router)#network 192.168.1.0 0.0.0.255 area 3
RTA(config-router)#area 3 virtual-link 10.0.0.2
...
RTB(config)#router ospf 1
RTB(config-router)#network 192.168.1.0 0.0.0.255 area 3
RTB(config-router)#network 192.168.2.0 0.0.0.255 area 0
RTB(config-router)#area 3 virtual-link 10.0.0.1
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40
OSPF verification commands
• show ip ospf ?
– list of possible commands
– border-routers
– database
– neighbours
• etc.
• Be curious!!!!
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• Reserve slides
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OSPF Area Types
• Backbone area (Area 0) – A transit area, the central entity to which
all other areas connect, all other areas must connect to this area to
exchange route information. Accepts all LSAs.
• Stub area - Area that does not accept information about routes
external to the autonomous system (AS), such as routes from nonOSPF sources. If routers need to reach networks outside the AS,
they use a default route. Does not accept LSAs.
• Totally stubby area - Area that does not accept external AS routes
and summary routes from other areas internal to the AS. Does not
accept external or summary LSAs. A totally proprietary Cisco
feature that uses a default route to reach networks outside the AS.
• Not-so-stubby areas - Area that is similar to a stub area but allows
for importing external routes as Type 7 LSAs and translation of
specific Type 7 LSA routes into Type 5 LSAs.
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LSA Types
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