Multiple OSPF Areas

1
Multiple OSPF Areas

• Three issues can overwhelm an OSPF router in a
  heavily populated OSPF network high demand for
  router processing and memory resources, large
  routing tables, and large topology tables.
• Fortunately, OSPF allows large areas to be
  separated into smaller, more manageable areas
  that can exchange summaries of routing
  information rather than exchange every detail.

2
Multiple OSPF Areas

• Just how many routers can an OSPF area support?
  Field studies have shown that a single OSPF area
  should not stretch beyond 50 routers, although
  there is no concrete limit.
• OSPF's capability to separate a large
  internetwork into multiple areas is referred to
  as hierarchical routing. Hierarchical routing
  enables you to separate large internetworks into
  smaller internetworks that are called areas.

3
Multiple OSPF Areas

• Interarea routing is the process of exchanging
  routing information between OSPF areas.
• The hierarchical topology possibilities of OSPF
  have several important advantages
• Reduced frequency of SPF calculations.
• Smaller routing tables.
• Reduced link-state update (LSU) overhead.

4
Multiple OSPF Areas

• Hierarchical routing increases routing efficiency
  because it allows you to control the type of
  routing information that flows into and out of an
  area.

5
OSPF Routing Types

• Four different types of OSPF routers exist,
• Internal router- routers that have all their
  interfaces within the same area are called
  internal routers. Internal routers in the same
  area have identical link-state databases and run
  a single copy of the routing algorithm.

6
OSPF Routing Types

• Backbone router- Routers that are attached to the
  backbone area of the OSPF network are called
  backbone routers. They have at least one
  interface connected to Area 0 (the backbone
  area). These routers maintain OSPF routing
  information using the same procedures and
  algorithms as internal routers.

7
OSPF Routing Types

• Area Border Router (ABR) - ABRs are routers with
  interfaces attached to multiple areas. They
  maintain separate link-state databases for each
  area to which they are connected, and they route
  traffic destined to or arriving from other areas.
  ABRs are exit points for the area, which means
  that routing information destined for another
  area can travel there only via the local area's
  ABR.

8
OSPF Routing Types

• ABRs summarize information about the attached
  areas from their link-state databases and
  distribute the information into the backbone. The
  backbone ABRs then forward the information to all
  other connected areas. An area can have one or
  more ABRs.

9
OSPF Routing Types

• Autonomous System Boundary Router (ASBR) - ASBRs
  are routers that have at least one interface
  connected to an external internetwork (another
  autonomous system), such as a non-OSPF network.
  These routers can import non-OSPF network
  information to the OSPF network, and vice versa
  (this is referred to as redistribution.

10
OSPF Routing Types

• A router can be more than one router type. For
  example, if a router interconnects to Area 0 and
  Area 1, as well as to a non-OSPF network, it
  would be both an ABR and an ASBR.

11
OSPF Area Types

• Multiarea OSPF is scalable because a router's
  link-state database can include multiple types of
  LSAs. DRs (Designated Routers) and routers that
  reside in multiple areas or autonomous systems
  use special LSAs to send or summarize routing
  information.The characteristics that you assign
  to an area control the type of route information
  that it can receive.

12
OSPF Area Types

• For example, you may want to minimize the size
  of routing tables in an OSPF area, in which case
  you can configure the routers to operate in an
  area that does not accept external routing
  information (Type 5 LSAs).
• Read about the 7 different LSAs.

13
OSPF Area Types

• Standard area - A standard area can accept link
  updates and route summaries.
• Backbone area (transit area) - When
  interconnecting multiple areas, the backbone area
  is the central entity to which all other areas
  connect. The backbone area is always Area 0. All
  other areas must connect to this area to exchange
  route information. The OSPF backbone has all the
  properties of a standard OSPF area.

14
OSPF Area Types

• Stub area - A stub area is an area that does not
  accept information about routes external to the
  autonomous system (the OSPF internetwork), such
  as routes from non-OSPF sources. If routers need
  to reach networks outside the autonomous system,
  they use a default route. A default route is
  noted as 0.0.0.0/0.

15
OSPF Area Types

• Totally stubby area - A totally stubby area is an
  area that does not accept external autonomous
  system (AS) routes and summary routes from other
  areas internal to the autonomous system. Instead,
  if the router needs to send a packet to a network
  external to the area, it sends it using a
  0.0.0.0/0 default route. Totally stubby areas are
  a Cisco proprietary feature.

16
OSPF Area Types

• Not-so-stubby area (NSSA) - An NSSA is an 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. 

17
OSPF Area Types

• A key difference among these OSPF area types is
  the way they handle external routes. External
  routes are injected into OSPF by an ASBR. The
  ASBR may learn these routes from RIP or some
  other routing protocol.You can configure an ASBR
  to send out two types of external routes into
  OSPF Type 1 (denoted in the routing table as E1)
  and Type 2 (E2).

18
OSPF Area Types

• Depending on the type, OSPF calculates the cost
  of external routes differently, as follows
• E1 - If a packet is an E1, then the metric is
  calculated by adding the external cost to the
  internal cost of each link that the packet
  crosses. You use this packet type when you have
  multiple ASBRs advertising a route to the same
  autonomous system.

19
OSPF Area Types

• E2 - If a packet is an E2, then the packet will
  always have the external cost assigned, no matter
  where in the area it crosses (this is the default
  setting on ASBRs). You use this packet type if
  only one router is advertising a route to the
  autonomous system. Type 2 routes are preferred
  over Type 1 routes unless two equal cost routes
  exist to the destination.

20
Configuring OSPF Across Multiple Areas

• This section summarizes how the different types
  of OSPF routers flood information and how they
  build their routing tables when operating within
  a multiarea environment.
• In Chapter 4, you saw that a packet destined for
  a network within an area is merely forwarded from
  one internal router to another until it reaches
  the destination network.

21
Configuring OSPF Across Multiple Areas

• However, what if a packet must traverse multiple
  areas?
• For the OSPF routers to make routing decisions,
  they must build sufficient routing tables by
  exchanging LSUs. The LSU exchange process within
  a single OSPF area relies on just two LSA
  types-Type 1 and Type 2. To distribute routing
  information to multiple areas efficiently, Type 3
  and Type 4 LSAs must be used by ABRs.

22
Flooding LSUs to Multiple Areas

• An ABR is responsible for generating routing
  information about each area to which it is
  connected and flooding the information through
  the backbone area to the other areas to which the
  backbone is connected. The general process for
  flooding follows these steps

23
Flooding LSUs to Multiple Areas

• The routing processes occur within the area. The
  entire area must be synchronized before the ABR
  can begin sending summary LSAs to other areas.

24
Flooding LSUs to Multiple Areas

• The ABR reviews the resulting link-state database
  and generates summary LSAs (Type 3 or Type 4). By
  default, the ABR sends summary LSAs for each
  network that it knows about. To reduce the number
  of summary LSA entries, you can configure route
  summarization so that a single IP address can
  represent multiple networks. To use route
  summarization, your areas need to use contiguous
  IP addressing.

25
Flooding LSUs to Multiple Areas

• The summary LSAs are placed in an LSU and
  distributed through all ABR interfaces, with the
  following exceptions
• If the interface is connected to a neighboring
  router that is in a state below the exchange
  state, then the summary LSA is not forwarded.

26
Flooding LSUs to Multiple Areas

• If the interface is connected to a totally stubby
  area, then the summary LSA is not forwarded.
• If the summary LSA includes a Type 5 (external)
  route and the interface is connected to a stub or
  totally stubby area, then the LSA is not sent to
  that area.

27
Configuring OSPF Across Multiple Areas

• After an ABR or ASBR receives summary LSAs, it
  adds them to its link-state databases and floods
  them to the local area. The internal routers then
  assimilate the information into their databases.

28
Configuring OSPF Across Multiple Areas

• Remember that OSPF enables you to configure
  different area types so that you can reduce the
  number of route entries that internal routers
  maintain. To minimize routing information, you
  can define the area as a stub area, a totally
  stubby area, or an NSSA.

29
Updating the Routing Tables

• The order in which paths are calculated is as
  follows
• All routers first calculate the paths to
  destinations within their area and add these
  entries into the routing table. These are learned
  via Type 1 and Type 2 LSAs.

30
Updating the Routing Tables

• All routers then calculate the paths to the other
  areas within the internetwork. These paths are
  learned via interarea route entries, or Type 3
  and Type 4 LSAs. If a router has an interarea
  route to a destination and an intra-area route to
  the same destination, the intra-area route is
  kept.

31
Updating the Routing Tables

• All routers, except those that are in any of the
  stub area types, then calculate the paths to the
  AS external (Type 5) destinations.

32
Configuring OSPF Components

• Configuring an ABRThere are no special commands
  to make a router an ABR or an ASBR. The router
  becomes an ABR as soon as you configure two of
  its interfaces to operate in different areas.

33
Configuring OSPF Components

• Configuring an ASBRASBRs are created when you
  configure OSPF to import, or redistribute,
  external routes into OSPF. Ex. Redistribute Rip,
  This command tells OSPF to import RIP routing
  information.

34
OSPF Route Summarization

• Recall that summarization is the consolidation of
  multiple routes into one single, supernet
  advertisement.
• Proper summarization requires contiguous
  (sequential) addressing (for example, 200.10.0.0,
  200.10.1.0, 200.10.2.0, and so on). OSPF routers
  can be manually configured to advertise a
  supernet route, which is different from an LSA
  summary route.

35
OSPF Route Summarization

• OSPF supports two types of summarization
• Interarea route summarization - Interarea route
  summarization is done on ABRs and applies to
  routes from within each area. It does not apply
  to external routes injected into OSPF via
  redistribution. To take advantage of
  summarization, network numbers within areas
  should be contiguous.

36
OSPF Route Summarization

• External route summarization - External route
  summarization is specific to external routes that
  are injected into OSPF via redistribution. Here
  again, it is important to ensure that external
  address ranges that are being summarized are
  contiguous. Summarization of overlapping ranges
  from two different routers could cause packets to
  be sent to the wrong destination. Only ASBRs can
  summarize external routes.

37
OSPF Route Summarization

• To configure an ABR to summarize routes for a
  specific area before injecting them into a
  different area, you use the following syntax
• Router(config-router) area area-id range address
  mask. To perform interarea summarization

38
OSPF Route Summarization

• RTB(config) router ospf 1RTB(config-router)
  area 1 range 192.168.16.0 255.255.252.0.
• Note that the area 1 range command in this
  example specifies the area containing the range
  to be summarized before being injected into Area
  0.

39
OSPF Route Summarization

• OSPF Route Summarization
• To configure an ASBR to summarize external routes
  before injecting them into the OSPF domain, you
  use the following syntax
• Router(config-router) summary-address address
  mask

40
OSPF Route Summarization

• RTA(config) router ospf 1RTA(config-router)
  summary-address 200.9.0.0 255.255.0.0

41
OSPF Route Summarization

• Also, note that, depending on your network
  topology, you may not want to summarize area 0
  networks. If you have more than one ABR between
  an area and the backbone area, for example,
  sending a summary LSA with the explicit network
  information will ensure that the shortest path is
  selected. If you summarize the addresses, a
  suboptimal path selection may occur.

42
Using Stub and Totally Stubby Areas

• You can configure an OSPF router interface to
  either operate in a stub area (does not accept
  information about routes external to the AS) or
  as a totally stubby area (does not accept
  external AS routes and summary routes from other
  areas internal to the AS).

43
Using Stub and Totally Stubby Areas

• By configuring an area as stub, you can greatly
  reduce the size of the link-state database inside
  that area and, as a result, reduce the memory
  requirements of area routers. Remember that stub
  areas do not accept Type 5 (that is, external)
  LSAs.

44
Using Stub and Totally Stubby Areas

• Because OSPF routers internal to a stub area will
  not learn about external networks, routing to the
  outside world is based on a 0.0.0.0/0 default
  route. When you configure a stub area, the stub's
  ABR automatically propagates a 0.0.0.0/0 default
  route within the area.

45
Using Stub and Totally Stubby Areas

• Stub areas are typically created when you have a
  hub-and-spoke topology, with the spokes (such as
  branch offices) configured as stub areas.

46
Using Stub and Totally Stubby Areas

• To further reduce the number of routes in a
  table, you can create a totally stubby area,
  which is a Cisco-specific feature. A totally
  stubby area is a stub area that blocks external
  Type 5 LSAs and summary (that is, Type 3 and Type
  4) LSAs from entering the area. This way,
  intra-area routes and the default of 0.0.0.0/0
  are the only routes known to the stub area. ABRs
  inject the default summary link 0.0.0.0/0 into
  the totally stubby area.

47
Using Stub and Totally Stubby Areas

• This is typically a better solution than creating
  stub areas, unless the target area uses a mix of
  Cisco and non-Cisco routers.

48
Stub and Totally Stub Criteria

• An area can be qualified as a stub or totally
  stubby when it meets the following criteria
• There is a single exit point from that area.
• The area is not needed as a transit area for
  virtual links. (Virtual links are discussed at
  the end of this chapter.).

49
Stub and Totally Stub Criteria

• No ASBR is internal to the stub area.
• The area is not the backbone area (Area 0).
• These criteria are important because a
  stub/totally stubby area is configured primarily
  to exclude external routes. If these criteria are
  not met, external links may be injected into the
  area, invalidating their stubby nature.

50
Stub and Totally Stub Criteria

• To configure an area as a stub or totally stubby
  area, use the following syntax on all router
  interfaces that are configured to belong to that
  area
• Router(config-router)area area-id stub  

51
Stub and Totally Stub Criteria

• The optional no-summary keyword is used only on
  ABRs. This keyword configures the ABR to block
  interarea summaries (Type 3 and Type 4 LSAs). The
  no-summary keyword creates a totally stubby area.

52
Stub and Totally Stub Criteria

• The area stub command is configured on each
  router in the stub location, which is essential
  for the routers to become neighbors and exchange
  routing information. When this command is
  configured, the stub routers exchange hello
  packets with the E bit set to 0. The E bit is in
  the Options field of the hello packet. It
  indicates that the area is a stub area.

53
Stub and Totally Stub Criteria

• On ABRs only, you also have the option of
  defining the cost of the default route that is
  automatically injected in the stub/totally stubby
  area. You use the following syntax to configure
  the default route's cost

54
Stub and Totally Stub Criteria

• Router(config-router)area area-id default-cost
  cost

55
Meeting the Backbone Requirements

• OSPF has certain restrictions when multiple areas
  are configured. One area must be defined as Area
  0, the backbone area. It is called the backbone
  because all inter-area communication must go
  through it.

56
Meeting the Backbone Requirements

• Thus, all areas should be physically connected to
  Area 0 so that the routing information injected
  into this backbone can be disseminated to other
  areas. The backbone area must always be
  configured as Area 0. You cannot make any other
  area ID function as the backbone.

57
Virtual Links

• There are situations, however, when a new area is
  added after the OSPF internetwork has been
  designed, and it is not possible to provide that
  new area with direct access to the backbone. In
  these cases, a virtual link can be defined to
  provide the needed connectivity to the backbone
  area.

58
Virtual Links

• The virtual link provides the disconnected area a
  logical path to the backbone. All areas must
  connect directly to the backbone area or through
  a transit area.
• The virtual link has the following two
  requirements

59
Virtual Links

• It must be established between two routers that
  share a common area.
• One of these two routers must be connected to the
  backbone.
• Virtual links serve the following purposes
• They can link an area that does not have a
  physical connection to the backbone. This linking
  could occur, for example, when two organizations
  merge.

60
Virtual Links

• They can patch the backbone if discontinuity in
  Area 0 occurs. Discontinuity of the backbone
  might occur, for example, if two companies merge
  their two separate OSPF networks into a single
  one with a common Area 0. The only alternative
  for the companies is to redesign the entire OSPF
  network and create a unified backbone.

61
Virtual Links

• Another reason for creating a virtual link is to
  add redundancy in cases when router failure might
  cause the backbone to be split into two.

62
Virtual Links

• To configure a virtual link, perform the
  following steps
• router(config-router)area area-id virtual-link
  router-id
• If you do not know the neighbor's Router ID, you
  can Telnet to it and type the show ip ospf
  command.

63
Virtual Links

• Area 3 does not have a direct physical connection
  to the backbone (Area 0), which is an OSPF
  requirement because the backbone is a collection
  point for LSAs. ABRs forward summary LSAs to the
  backbone, which in turn forwards the traffic to
  all areas. All interarea traffic transits the
  backbone.

64
Virtual Links

• To provide connectivity to the backbone, a
  virtual link must be configured between R2 and
  R1. Area 1 will be the transit area and R1 will
  be the entry point into area 0. R2 will have a
  logical connection to the backbone through the
  transit area.

65
Virtual Links

• Both sides of the virtual link must be
  configured, as follows
• R2(config-router)area 1 virtual-link 10.3.10.5
  --- With this command, area 1 is defined to be
  the transit area and the router ID of the other
  side of the virtual link is configured

66
Virtual Links

• R1(config-router)area 1 virtual-link
  10.7.20.123 --- With this command, area 1 is
  defined to be the transit area and the router ID
  of the other side of the virtual link is
  configured.