Interior Gateway Protocols RIP, OSPF continued

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Interior Gateway Protocols (RIP, OSPF)continued

• Sandeep Nijsure
• Madhuri Raju Chamarthi

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Open Shortest Path First (OSPF)

• Interior Gateway Protocol (IGP) for TCP/IP based
  internets, i.e. used within an Autonomous System
  (AS).
• Uses Link-state routing
• Open stands for publicly available
  specification.

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A comparison of routing philosophies
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Mapping between AS configuration and resulting
directed graph

• Two routers joined by a point-to-point link are
  represented in the graph as being directly
  connected by a pair of edges, one in each
  direction (e.g. routers 6 and 10)
• When multiple routers are attached to a network
  (such as a LAN or packet-switching network), the
  directed graph shows all routers bi-directionally
  connected to the network vertex (e.g. routers
  1,2,3, and 4 all connect to network 3) Note that
  cost from network vertex to router is always
  zero.
• If a single router is attached to a network, the
  network will appear in the graph as a stub
  connection (e.g. network 7)
• An end system, called a host can be directly
  connected to a router (why should it be?), in
  which case it is depicted in the corresponding
  graph (e.g. host 1)
• If a router is connected to other autonomous
  systems, then the path cost to each network in
  the other system must be obtained by some
  exterior routing protocol (like BGP). Each such
  network is represented on the graph by a stub and
  an edge to the router with the known path cost
  (e.g. networks 12 through 15).

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SPF tree for router RT6 (which is at the root)
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RT6s routing table listing local destinations
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RT6s routing table listing external destinations
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Concept of Designated Router (DR)

• A multi-access network is a network that supports
  the attachment of multiple (more than two)
  routers. Each pair of routers on such a network
  is assumed to be able to communicate directly.
  The network might be broadcast (e.g. Ethernet) or
  non-broadcast.
• In such a network, each router is connected to
  each other. Each will generate a reachability
  message for each reachable neighbor. Thus if K
  routers are up in a network, each will generate
  K-1 messages, thus a total of K(K-1). This is
  superfluous.
• Also, if each router exchanges topological
  databases with all others, it will create a high
  network traffic. This is superfluous, too.
• So OSPF allows such a network to have a
  designated router. It exchanges topological
  databases with all routers, so that it always has
  the full information. It also sends link state
  advertisements which describe all the routers
  attached to that network.
• Individual routers then do not have to indicate
  the reachability of each of their neighbors.
• These routers are said to have an adjacency
  with the DR. They only indicate that they are
  connected to DR. All OSPF packets except Hello
  are communicated only over such adjacencies.
  Adjacency is thus more important than
  neighborhood.

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OSPF areas
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OSPF areas

• Partitioning the networks and routers in an AS
  into subsets called areas.
• Knowledge of an areas topology remains hidden
  from other areas. The topological databases have
  to be maintained only inside the area, so the
  link state advertisements are flooded only inside
  that area.
• Less network traffic.
• Less CPU power required in each router.
• Thus, permits growth and makes the networks in an
  AS easier to manage.
• The areas are connected by a backbone over which
  the area border routers communicate.
• 2 types of routers
• Internal routers having interfaces only in one
  area (e.g. RT1, RT2) or only in backbone (RT6).
• Area border routers which attach to multiple
  areas, including the backbone (RT3 and RT4).

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What does a OSPF router do?

• It first initializes the routing protocol data
  structures
• Waits for indication from the lower-level
  protocols that its interfaces are functional.
• Uses OSPFs Hello protocol to acquire neighbors.
  Sends Hello packets to its neighbors and in turn
  receives their Hello packets.
• Attempts to form adjacencies with some of its
  newly acquired neighbors. Topological databases
  are synchronized between pairs of adjacent
  routers.
• Periodically advertises its link state. Also
  advertises when the link state changes. The link
  state reflects its interfaces to networks,
  adjacencies to other routers connected by
  point-to-point network, and adjacencies to
  Designated router in multi-access networks .
• Sends link state advertisements received from
  other routers over its adjacencies.

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What does a OSPF router do? (contd..)

• If it is an AS boundary router
• Originates AS external link advertisements for
  each known AS external destination.
• If it is an Area Border Router
• Originates a summary links advertisement
  describing each known inter-area destination for
  routers inside the area.
• Originates a summary links advertisement
  describing each of the networks inside the area,
  for use of other area border routers
• If it is a Designated router for a multi-access
  network
• Originates a network link advertisement giving
  the network no. of the multi-access network, and
  the router IDs of the routers attached to it.

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The OSPF packet header

• Type The OSPF packet types are as follows. The
  format of each of these packet types is described
  in a succeeding section.
• Type Description
• 1 Hello
• 2 Database Description
• 3 Link State Request
• 4 Link State Update
• 5 Link State Acknowledgment
• Packet length The length of the protocol packet
  in bytes. This length includes the standard OSPF
  header.
• Router ID The Router ID of the packets source.
  In OSPF, the source and destination of a routing
  protocol packet are the two ends of an
  (potential) adjacency.
• Area ID A 32 bit number identifying the area that
  this packet belongs to. All OSPF packets are
  associated with a single area. Most travel a
  single hop only.
• Autype It can be no authentication or a simple
  password

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Hello Message

• Network mask The network mask associated with
  this interface.
• HelloInt The number of seconds between this
  routers Hello packets.
• Rtr Pri This routers Router Priority. Used in
  (Backup) Designated Router election. If set to 0,
  the router will be ineligible to become (Backup)
  Designated Router.
• Deadint The number of seconds before declaring a
  silent router down.
• Neighbor The Router IDs of each router from whom
  valid Hello packets have been seen recently on
  the network. Recently means in the last DeadInt
  seconds. (This is to ensure bi-directional
  communication.)

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Database Description Message

• Options The optional capabilities supported by
  the router
• I-bit The init bit. When 1, this packet is the
  first one in the sequence of Database
  description packets
• M-bit The more bit. When 1, it indicates that
  more Database description packets to follow
• MS-bit The Master/Slave bit. When set to 1, it
  indicates that the router is the master during
  the database exchange process. Otherwise, the
  router is the slave.
• Note that only the header of the link state is
  sent, and not the entire link info.

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Link State Request Message

• Link State type
• 1 Router link
• 2 Network link
• 3 Summary link (link to IP network)
• Inside ABRs area or outside.
• 4 Summary link (link to AS border router)
• 5 External link (link to network in another AS)

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Link State Update Message

• Routers send the status of links with a link
  status update message. These are flooded.
• Link state update message is also sent in
  response to a link state request message after
  the database exchange process.
• Each link state update message has to be
  acknowledged, using Link state acknowledgement
  packet, which is very similar to the above format.

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Link State Advertisement Header

• There are 5 distinct types of link state
  advertisements (described before), which have the
  20 byte header in common. The part after the
  header is different for each. The most important
  data in that part is cost of the link.
• LS age The time in seconds since the Link State
  advertisement was originated.
• Link State ID The contents of this field depend
  on the advertisements LS type. For example, in
  network links advertisements the Link State ID is
  set to the IP interface address of the networks
  Designated Router (from which the networks IP
  address can be derived). In a summary links adv.,
  this field is set to the network no. of the IP
  network being described.
• LS sequence number Detects old or duplicate link
  state advertisements. Successive instances of a
  link state advertisement are given successive LS
  sequence numbers.
• Advertising Router The Router ID of the router
  that originated the link state advertisement.

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Some facilities provided by OSPF

• Type of service routing. Instead of providing the
  link cost in terms of just one metric like hop
  count, different costs can be specified for
  different types of service, e.g. a satellite link
  will have a low cost for the bandwidth metric,
  but a high cost for delay metric. Routers may
  keep different routing tables for different types
  of service, and choose the route depending on
  type of service field in IP header.
• OSPF has provision for authentication scheme like
  a simple password for an area. This guards
  against routers inadvertently coming up in the
  area. They must first be configured with their
  attached networks passwords before they can join
  the routing domain. This also guards against
  imposters. Different areas can use different
  authentication schemes.

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Advantages of OSPF over RIP

• Network bandwidth is saved because routing
  information is not sent frequently as in RIP.
• Suitable for autonomous systems of length more
  than 15.
• OSPF converges much faster than RIP. It also
  avoids routing loops
• Though slow convergence problem is solved by use
  of triggered updates, but still the update
  avalanche problem does not arise.
• Even though RIP operates over UDP, it does not
  check whether neighbors have received the updates
  sent, so RIP packets may get lost and timeouts
  may occur. OSPF checks this through the use of
  acknowledgements between neighbors.

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Drawbacks of OSPF

• Link state advertisements which are flooded
  through the autonomous system take up
  considerable amount of bandwidth, which is
  directly proportional to the connectivity of the
  network.
• Complex to implement than RIP.
• Since the shortest path calculations for the
  entire AS take place inside a router, they
  require more CPU power than RIP routers. Since
  the entire topological database is to be stored,
  they need more router memory.(This drawback can
  be reduced when areas are used)