Week 4: RIP vs' OSPF

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Week 4RIP vs. OSPF

• ???
• lyyu_at_cs.ecnu.edu.cn

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Outline

• RIPv2
• Introduction to RIP
• Limitations of the Protocol
• Protocol Specification
• Example Discussion
• Message Format
• OSPF
• Introduction to OSPF
• Classification of routers
• Example
• Bringing Up Adjacencies

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Introduction to RIP

• The Internet is organized as a collection of
  Autonomous Systems (AS), each of which will, in
  general, be administered by a single entity.
• Each AS will have its own routing technology,
  which may differ among ASs.

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Introduction to RIP

• The routing protocol used within an AS is
  referred to as an Interior Gateway Protocol
  (IGP).
• A separate protocol, called an Exterior Gateway
  Protocol (EGP), is used to transfer routing
  information among the ASs.
• RIP was designed to work as an IGP in
  moderate-size ASs. It is not intended for use in
  more complex environments.

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Introduction to RIP

• RIP is a routing protocol based on the
  Bellman-Ford (or distance vector) algorithm.
• This algorithm has been used for routing
  computations in computer networks since the early
  days of the ARPANET.

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Limitations of RIP

• The protocol is limited to networks whose longest
  path (the networks diameter) is 15 hops.
• The protocol depends upon "counting to infinity"
  to resolve certain unusual situations.
• This protocol uses fixed "metrics" to compare
  alternative routes.

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Protocol Specification

• Each router that implements RIP is assumed to
  have a routing table.
• This table has one entry for every destination
  that is reachable throughout the system operating
  RIP.

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Protocol Specification

• Distance vector algorithms are based on the
  exchange of only a small amount of information.
• Each entity (router or host) that participates in
  the routing protocol is assumed to keep
  information about all of the destinations within
  the system.
• Generally, information about all entities
  connected to one network is summarized by a
  single entry, which describes the route to all
  destinations on that network.

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Protocol Specification

• Each entry contains at least the following
  information
• address in IP implementations of these
  algorithms, this will be the IP address of the
  host or network.
• router the first router along the route to the
  destination. (next-hop, gateway)
• interface the physical network which must be
  used to reach the first router.
• metric a number, indicating the distance to the
  destination. (cost)
• timer the amount of time since the entry was
  last updated.

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Protocol Specification

• Distance vector algorithms get their name from
  the fact that it is possible to compute optimal
  routes when the only information exchanged is the
  list of these distances.
• Furthermore, information is only exchanged among
  entities that are adjacent, that is, entities
  that share a common network.

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Protocol Specification

• The most important information exchanged by the
  hosts and routers is carried in update messages.
• Each entity that participates in the routing
  scheme sends update messages that describe the
  routing database as it currently exists in that
  entity.
• It is possible to maintain optimal routes for the
  entire system by using only information obtained
  from neighboring entities.

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Protocol Specification

• The metric for a directly-connected network is
  set to 1.
• Every 30 seconds, the RIP process is awakened to
  send an unsolicited Response message containing
  the complete routing table to every neighboring
  router.

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Protocol Specification

• Input processing of Response Message

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Protocol Specification

• Input processing of Response Message

• Ignore the response not from the RIP port
• Ignore the response not from a valid neighbor

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Protocol Specification

• Input processing of Response Message

• is the destination address valid (e.g., unicast
  not net 0 or 127)
• is the metric valid (i.e., between 1 and 16,
  inclusive)

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Protocol Specification

• Input processing of Response Message

• The new metric
• MIN (the old metric 1, 16)

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Protocol Specification

• Input processing of Response Message

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Check the routing table
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Example
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Example

• Route table while router A is finishing booting

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Example

• Route table after exchanging route information
  with Router C

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Discussion

• all networks have cost 1, except for the direct
  link from C to D, which has cost 10
• D directly connected, metric 1
• B route via D, metric 2
• C route via B, metric 3
• A route via B, metric 3
• What if the link from B to D fails?

x
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Discussion
x

• time ------gt

Worst case counting to infinity
You should now see why "infinity" is chosen to be
as small as possible.
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Discussion

• Solution
• Split horizon (????) never claim reachability
  for a destination network to the neighbor(s) from
  which the route was learned
• Split horizon with poisoned reverse (????)
  includes such routes in updates, but sets their
  metrics to infinity
• Triggered updates (????) whenever a router
  changes the metric for a route, it is required to
  send update messages almost immediately

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Protocol Specification

• When will the router send response messages that
  contain all or part of the routing table
• By input processing, when a Request is received
• By the regular routing update router (every 30
  seconds)
• By triggered updates

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Message format

• RIP-1/RIP-2 (UDP 520)

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Message format

• Authentication

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RIP vs. OSPF

• RIP
• has very little overhead in terms of bandwidth
  used and configuration and management time
• very easy to implement
• - is limited to networks whose longest path is 15
  hops
• - uses fixed "metrics" to compare alternative
  routes
• - slow convergence

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Introduction to OSPF

• OSPF the Open Shortest Path First TCP/IP
  internet routing protocol.
• OSPF is classified as an Interior Gateway
  Protocol (IGP).
• This means that it distributes routing
  information between routers belonging to a single
  Autonomous System.
• The OSPF protocol is based on link-state or SPF
  technology.

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Introduction to OSPF

• In a link-state routing protocol, each router
  maintains a database describing the Autonomous
  Systems topology.
• Each participating router has an identical
  database.
• Each individual piece of this database is a
  particular routers local state (e.g., the
  routers usable interfaces and reachable
  neighbors).
• The router distributes its local state throughout
  the Autonomous System by flooding.

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Introduction to OSPF

• From the link-state database, each router
  constructs a tree of shortest paths with itself
  as root.
• This shortest-path tree gives the route to each
  destination in the Autonomous System.
• OSPF allows sets of networks to be grouped
  together. Such a grouping is called an area.
• All OSPF protocol exchanges are authenticated.

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Classification of routers

• Internal routers A router with all directly
  connected networks belonging to the same area.
• Area border routers A router that attaches to
  multiple areas.
• Backbone routers A router that has an interface
  to the backbone area.
• AS boundary routers A router that exchanges
  routing information with routers belonging to
  other Autonomous Systems.

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Backbone routers RT3, RT4, RT5, RT6 , RT, RT10
and RT11
Area border routers RT3, RT4, RT7, RT10 and RT11
AS boundary routers RT5 and RT7
Internal routers RT1, RT2, RT5, RT6, RT8, RT9
and RT12
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• The resulting directed graph

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• The SPF tree for Router RT6

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Bringing Up Adjacencies

• OSPF creates adjacencies between neighboring
  routers for the purpose of exchanging routing
  information.
• Not every two neighboring routers will become
  adjacent.

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Bringing Up Adjacencies

• The Hello Protocol
• The Hello Protocol is responsible for
  establishing and maintaining neighbor
  relationships.
• It also ensures that communication between
  neighbors is bidirectional.
• Hello packets are sent periodically out all
  router interfaces.

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Bringing Up Adjacencies

• The Hello Protocol
• On broadcast and NBMA networks, the Hello
  Protocol elects a Designated Router for the
  network.
• The Hello Protocol works differently on broadcast
  networks, NBMA networks and Point-to-MultiPoint
  networks.

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Bringing Up Adjacencies

• The Designated Router
• Every broadcast and NBMA network has a Designated
  Router.
• The DR performs two main functions
• Becomes adjacent to all other routers on the
  network.
• Originates a network-LSA on behalf of the network.

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Bringing Up Adjacencies

• The Synchronization of Databases
• If an adjacency is to be formed, the first step
  is to synchronize the neighbors link-state
  databases.
• In a link-state routing algorithm, it is very
  important for all routers link-state databases
  to stay synchronized.

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Bringing Up Adjacencies

• The Synchronization of Databases
• OSPF simplifies this by requiring only adjacent
  routers to remain synchronized.
• The synchronization process begins as soon as the
  routers attempt to bring up the adjacency.

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Bringing Up Adjacencies

• The Backup Designated Router
• In order to make the transition to a new
  Designated Router smoother, there is a Backup
  Designated Router for each broadcast and NBMA
  network.
• The Backup Designated Router is also adjacent to
  all routers on the network, and becomes
  Designated Router when the previous Designated
  Router fails.

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Bringing Up Adjacencies

• The Backup Designated Router
• The Backup Designated Router does not generate a
  network-LSA for the network.
• The Backup Designated Router is also elected by
  the Hello Protocol.