1. Carl Bergenhem - 030404 carl@bergenhem.com
Multi Area OSPF
Carl Bergenhem

2. Issues with large OSPF nets
Frequent SPF calculations
Large routing table
Large link-state table (topological database)

3. Issues with large OSPF nets

4. 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

5. OSPF Areas

6. OSPF Router Types

7. 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.

8. 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)

9. LSA Types
See online material: chpt: 5.1.3

10. 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

11. 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.

12. LSA Types (con’t) Type 7 NSSA External Link Entry
Originated by an ASBR connected to an NSSA.
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).

13. 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......

14. External (type 5) routes Summary (type 3,4) routes
Area Types
Default route  Accepted 
External (type 5) routes Summary (type 3,4) routes
 Default route  Default route

15. 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
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

16. 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 LSA7 OK

17. Multi-area Example
ABR
RIP
ASBR

18. Stub Example

19. Stub Example
ABR
ASBR

20. Stub Example
ABR
No Type 5 LSAs
ASBR

21. Stub Example
ABR
No Type 5 LSAs
Route to /0 via RTB
ASBR

22. 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

23. Totally Stubby Example
no summary
ABR
Totally Stubby Area
No Type 3, 4, or 5 LSAs
Route to /0 via RTB
(no more IA routes)
ASBR

24. 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

25. NSSA Example

26. NSSA Example
ABR
No Type 5 LSAs
3,4 OK
Type 7 OK
ASBR

27. NSSA Example
no summary
ABR
No Type 3, 4 or 5 LSAs
Type 7 OK
ASBR

28. LSA Types (con’t) Type 7 NSSA External Link Entry
Originated by an ASBR connected to an NSSA.
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).

29. LSA 1 Router LSA Multi Area OSPF Normal Areas LSA 1 Originated
LSA 1 flooded
LSA 1’s are flooded out other interfaces within the same area.
LSA 1 flooded

30. LSA 2 Network LSA Multi Area OSPF Normal Areas LSA 2’s being sent
No LSA 2’s for Area 51 – No DR elected because no other routers on the broadcast segment
LSA 2 flooded
LSA 2

31. LSA 3 Summary LSA Multi Area OSPF Normal Areas LSA 3 LSA 1 LSA 1 LSA 1
LSA 1’s are sent as LSA 3’s into other areas.

32. LSA 3 Summary LSA Multi Area OSPF Normal Areas LSA 3 LSA 3 LSA 3 LSA 1
LSA 1’s are sent as LSA 3’s into other areas.

33. LSA 4 ASBR Summary LSA Normal Areas LSA 4 LSA 5’s flooded LSA 4 LSA 4

34. LSA 5 AS External LSA Normal Areas LSA 5 LSA 5’s flooded LSA 5 LSA 5
ASBR
router ospf 1
redistribute rip metric 500
network area 0
ip route
LSA 5
LSA 5

35. Stub Areas Stub Area LSA 3 LSA 4 LSA 4 LSA 3 LSA 5 LSA 5 LSA 4 LSA 5
Blocked X
Default route to ABR injected
Stub Area
All routers in the area must be configured as “stub” including the ABR:
router ospf 1
network area 0
network area 1
area 1 stub

36. X Blocked X Totally Stubby Areas Stub Area LSA 3 LSA 4 LSA 4 LSA 3
Default route to ABR injected
Stub Area
All routers in the area must be configured as “stub” except the ABR “stub no summary”:
router ospf 1
network area 0
network area 1
area 1 stub no-summary

37. Virtual Links

38. 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.

39. Virtual Links

40. Virtual Links RTA(config)#router ospf 1
RTA(config-router)#network area 51
RTA(config-router)#network area 3
RTA(config-router)#area 3 virtual-link
...
RTB(config)#router ospf 1
RTB(config-router)#network area 3
RTB(config-router)#network area 0
RTB(config-router)#area 3 virtual-link

41. OSPF verification commands
show ip ospf ?
list of possible commands
border-routers
database
neighbours
etc.
Be curious!!!!

42. Reserve slides

43. 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 non-OSPF 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. 

44. LSA Types