Multi-Area OSPF

1
Multi-Area OSPF

• Multi-area OSPF networks can be difficult to
  design, and typically demand more administrative
  attention than any other popular interior gateway
  protocol.

2
OSPF Advantages

• OSPF routers in a heavily populated OSPF network
  can be overwhelmed by high demand for router
  processing and memory resources, large routing
  tables, and large topology tables.
• OSPF allows splitting the network into manageable
  pieces, or areas, thereby allowing tremendous
  scalability.

3
Hierarchical Routing

• OSPF's capability to separate a large
  internetwork into multiple smaller internetworks,
  called areas, is referred to as hierarchical
  routing.
• With this technique interarea routing occurs,
  which is simply the process of exchanging routing
  information between OSPF areas.
• Interarea routing allows OSPF to summarize and
  contain area-specific information so that many of
  the smaller internal routing operations are
  restricted to one area.

4
Hierarchical Routing Advantages

• The hierarchical topology possibilities of
  OSPF yield several important advantages
• Reduced frequency of SPF calculations
• Smaller routing tables
• Reduced LSU overhead

5
OSPF Router Types

• There are four different types of OSPF Routers
• Internal router - routers that have all their
  interfaces within the same area with identical
  Link State databases and running the same routing
  algorithm.
• Backbone router - Routers attached to the
  backbone area of the OSPF network with at least
  one interface connected to Area 0.
• Area Border Router - routers with interfaces
  attached to multiple areas that maintain separate
  link-state databases for each area they are
  connected to. Areas can have one or more ABRs.
• Autonomous System Boundary Router - ASBRs -
  routers that have at least one interface
  connected to an external internetwork (another
  autonomous system), such as a non-OSPF network.
• A router can be more than one router type.
  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.

6
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. 

7
LSA types

• LSA1s build the Router Link States
• LSA2s build the Net Link States (O)
• LSA3s build the Summary Net Link States (O IA)
• LSA4s build the AS Summary Net Link States (IA)
• LSA5s build external route information (E1 or E2)

8
Stub and Totally Stub 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).
• 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.
• Stub areas are typically created when you have a
  hub-and-spoke topology, with the spokes (such as
  branch offices) configured as stub areas. Branch
  offices may not need to know about every network
  at the headquarters site but can instead use
  default routes to get there.

9
Is it Stubby? Or Totally Stubby?

• An area qualifies as 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.
• No ASBR is internal to the stub area.
• The area is not the backbone area (Area 0).
• All these criteria are important because a
  stub/totally stubby area is configured primarily
  to exclude external routes.

10
Configuring Stub/Totally Stub Areas(See
curriculum 5.4.4 and 5.4.5)

• To configure a stub or totally stubby area, use
  the following on all router interfaces in that
  area
• Router(config-router)area area-id stub  
• An optional no-summary keyword is added only on
  ABRs. This configures the ABR to block interarea
  routes (Type 3 and Type 4 LSAs). The no-summary
  creates a totally stubby area.
• The area stub command is configured on each
  router in the stub location and is essential for
  the routers to become neighbors and exchange
  routing information.
• On ABRs only, there is an option of defining a
  cost of the default route to be automatically
  injected in the stub/totally stubby area
• Router(config-router)area area-id default-cost
  cost

11
Not-so-stubby Areas (NSSAs)

• NSSAs are a relatively new, standards-based OSPF
  enhancement.
• Generally, NSSAs are the area located between a
  RIP/IGRP network and Area 0 of an OSPF network.
• This area allows LSA5s in, in a limited fashion.
  Data updates about the RIP or IGRP networks come
  in as LSA7s and are then switched into Area 0 via
  an ABR as LSA5s.

12
Configuring NSSAs

• To configure an area as a NSSA, you must
  configure all OSPF router interfaces that belong
  to the area using the following command syntax
• Router(config-router)area area-id nssa
  no-summary
• The no-summary keyword is used on the ABR and
  typically makes the NSSA totally stubby.
• To control the summarization or filtering during
  the translation using the following syntax
• Router(config)summary-address prefix mask
  not-advertise tag tag
• The not-advertise keyword is used to suppress
  routes that match the prefix/mask pair. This
  keyword applies to OSPF only. The tag value can
  also be assigned but is not required.

13
Configuring NSSAs (continued)

• The ABR does not generate default routes in
  an NSSA. To force the ABR to generate the
  default route, use this command only on the ABR
  for the NSSA
• router ospf pid
• area id nssa default-information originate
• To verify that NSSA is defined on a given
  router, you can use the show ip ospf command

14
Virtual Links

• If 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, a virtual link can be
  defined to provide the needed connectivity to the
  backbone area. Because all areas must be
  connected to Area 0, the virtual link provides
  the disconnected area a logical path to the
  backbone.
• The virtual link has the 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.

15
Reasons for Virtual Links

• They can link an area that does not have a
  physical connection to the backbone. For example,
  when two organizations merge.
• They can patch the backbone if discontinuity in
  Area 0 occurs. 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.
• Virtual links can add redundancy in cases when
  router failure might cause the backbone to be
  split into two.

16
Configuring Virtual Links

• On each router that will use the virtual
  link, create the "virtual link" configuration.
  The routers that make the links are the ABR that
  connects the remote area to the transit area and
  the ABR that connects the transit area to the
  backbone area
• router(config-router)area area-id virtual-link
  router-id

17
Summarization

• Summarization is the consolidation of multiple
  routes into one single, supernet advertisement to
  conserve bandwidth, CPU, and memory resources .
• Summarization is done only on the ABR and ASBR.

18
Summarization commands

• On the ABR
• (config-router)area id range address mask
• On the ASBR
• (config-router)summary-address address mask