IP Switching and Routing Essential Chapter 4 Link State Routing and OSPF

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IP Switching and Routing EssentialChapter 4Link
State Routing and OSPF

• Shuhei Tanigawa
• 2005/6/20

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OSPF and Network Organization

• The OSPF protocol employs all of the principle of
  a link state protocol.
• In principle, OSPF treat the entire Internet just
  like the U.S highway example.
• Every router could exchange information with
  every other router.

This approach is not practical.
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OSPF and Network Organization

• OSPF establishes hierarchies within the network.
• Autonomous systems
• Areas
• Backbones
• Stub areas
• Not so stubby areas

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Autonomous Systems (AS)

• The Internet consists of many AS.
• An autonomous system is a collection of many
  computer systems, routers, and other network
  devices.
• The equipment constituting an autonomous system
  shares a single administrative entity.
• An educational institution, a business, a network
  provider, and so on.

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Autonomous Systems(AS)

• Each AS forms a self-contained network, with its
  own administration and management.
• Networks provide more value when they connect
  more users.

Increasing that connectivity means connecting
ASes
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Autonomous Systems (AS)

• ASes define the limits of OSPFs interactions.
• Routers within an AS can exchange routing
  information using OSPF.
• Routers describe their links with LSA packets,
  and they can flood those packets throughout the
  ASes.

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Autonomous Systems(AS)

• At the AS boundary, no OSPF traffic flows.
• Routers which exist on the boundary of an AS are
  AS boundary routers.

AS2
AS1
ATM Network
AS3
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Autonomous Systems(AS)

• An AS boundary router can learn about the network
  beyond the autonomous systems.
• It may learn information from routing protocols
  other than OSPF, or it may learn from manual
  configuration.
• Router can use OSPF to distribute that
  information within the AS.
• It does so by building special LSAs that called
  external LSAs.

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Areas

• ASes provide some relief to arouting protocol
  such as OSPF.
• Sometimes, even an AS by itself is too big and
  unwieldy.

OSPF offers another mechanism that provides
additional hierarchy to its networks.
That mechanism is the area.
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Areas

• Areas are arbitrary collections of networks,
  hosts, and routers.
• All systems within an area must be connected
  together.
• The routers that connect different areas are
  known as area border routers.

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Areas

• Within each area, OSPF functions normally.
• Routers construct and flood LSAs listing all of
  their neighbors.
• they distribute these LSAs to all other routers
  in the area.
• Area border routers construct special LSAs that
  summarize the information within their areas.
  

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The Backbone

• The backbone is a special area within an AS.
• It serves as the hub of the AS.
• All other areas in the AS must connect to the
  backbone.
• Areas can also connect to each other directly,
  without going through the backbone.
• Routers within the backbone is are as known as
  backbone routers.

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The Backbone
FDDI
FDDI
This router is still a backbone router, but it is
not an area border router.
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The Backbone

• The OSPF protocol considers the backbone a
  particularly important area.
• It provides special features to compensate for a
  break in the backbone connectivity.

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The Backbone
area1
FDDI
FDDI
area3
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Special OSPF Routers

• AS boundary routers
• A router with at least one interface outside of
  the OSPF AS.
• Area border router
• A router with interfaces in at least two
  different OSPF areas
• Backbone router
• A router within the OSPF backbone area.

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Stub Areas

• The OSPF protocol can reduce its traffic
  requirements.
• External LSAs
• Summary LSAs
• The LSAs simply indicate which destinations are
  reachable.
• OSPF can still consume considerable network
  bandwidth.

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Stub Areas

• OSPF defines stub areas.
• A stub area is a special OSPF area.
• It has only one area border router.
• Within the stub area, no summary or external LSAs
  circurate.
• Each router in the network learns only the
  location of the area border router.
• Any packets for a destination outside the area
  are simply forwarded to that exit point.

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Stub Areas

• Stub areas are restricted in two ways.
• Virtual links cannot pass through a stub area.
• A virtual link requires two separate connections.
• A stub area has no AS border router.
• One characteristic of stub area is that they do
  not flood external LSAs.

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Not So Stubby Areas

• Stub areas are quite effective in reducing the
  burden on OSPF.
• Routers within a stub area learn about each
  other.
• they learn how to get out of the area through
  the single area border router.

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Not So Stubby Areas

• In exchange, the routers forfeit the ability to
  exchange routes learned from any source other
  than OSPF.
• All such routes are classified as external
  routes.
• OSPF does not propagate them within a stub area.
• This restriction is sometimes too severe.
• It makes difficult to coordinate additional
  routing protocols (such as the RIP) within an
  area.

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Not So Stubby Areas

• A NSSA provides most of benefits of stub areas.
• It has a little more flexibility.
• An NSSA allows routers to exchange some
  information from other source, without incurring
  the cost of becoming a full OSPF area.

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Not So Stubby Areas

• An NSSA area allows the distribution of special
  NSSA LSAs.
• These are very similar to external LSAs.
• The only difference is how they are disseminated.
• External LSAs are not flooded in a not so stubby
  area.
• NSSA LSAs are flooded within a single NSSA.

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To be continued

• Special Networks
• Multicast routing
• OSPF Message Format