Open Shortest Path First OSPF

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Open Shortest Path FirstOSPF

• OSPF Overview
• OSPF Operation
• By Grace Deng Oct.16.2003

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OSPF OverviewHistory

• Development began 1987 by IETF
• Goala link state protocol more efficient and
  scaleable than RIP
• Latest revision is RFC 2328April 1998

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OSPF OverviewOSPF versus RIP

• OSPF
• Link state
• Efficient routing updates (sends changes only)
• No hop count limit
• Fast Convergence
• Supports VLSM
• Path selection based on bandwidth

• RIP
• Distance vector
• Copies entire routing table
• Hop count limit of 15
• Hold-down timers to prevent routing loops
• Does not advertise sub-net masks
• Uses only hop count as metric

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OSPF Overview Concepts

• OSPF is a Link-State Routing Protocol
• Uses IP as transport, IP protocol 89
• Uses multicast addresses in neighbor maintenance
  and flooding of LSAs
• 224.0.0.5 All OSPF Routers
• 224.0.0.6 All DRouters
• Employs Dijkstras Shortest Path First (SPF)
  algorithm to calculate the path tree

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OSPF Overview Concepts (cont.)

• Uses Metricspath cost
• Typically faster convergence than DVRPs
• Support for CIDR, VLSM, Authentication,
  Multi-path and IP unnumbered
• Relatively low steady state bandwidth requirements

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OSPF Overview Terminology
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OSPF Overview Terminology

• Link
• Link state
• Link State (LS) or topological database
• Area
• OSPF Metric Cost
• Routing table
• Adjacencies database

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OSPF Overview Topology/Link State Database

• A router has a separate Link State (LS) or
  topological database for each area to which it
  belongs
• All routers belonging to the same area should
  have identical databases
• SPF calculation is performed independently for
  each area
• LSA flooding is bounded by area

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OSPF Overview Areas

• OSPF uses a 2 level hierarchical model
• Areas labeled with a 32-bit number
• Can be defined using single decimal or IP
  address format value
• (i.e. Area 0.0.0.0 or Area 0)
• Area 0 reserved for the backbone area
• All areas must connect to area 0

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OSPF Overview OSPF Metric

• Cost applied on all router link paths
• 16-bit positive number 165,535
• The lower the more desirable
• Relevant going out an interface only
• Route decisions made on total cost of path

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OSPF Overview OSPF Packet Types
OSPF Packet format
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OSPF Packet Types (cont.)
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OSPF Overview Router ID

• Routers are identified by a unique 32-bit ID
• RID highest IP address configured on any active
  loopback interface
• RID if no loopback exists, highest IP address
  configured on any active physical interface
• RID can be configured with
• router-id ltip addressgt

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OSPF Overview OSPF Hello Packets

• Multicast 224.0.0.5 on all router interfaces
• Hello interval 10 sec. LAN, 30 sec. NBMA
• Used to form adjacencies between routers

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OSPF Overview Database Descriptor Packets (DDP)

• Contain link state database headers
• Describe the current LS database
• Exchange stage

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OSPF Overview Link State Request Update Packets

• Request for specific parts of database
• Send only database updates requested
• Loading Stage, labeled Full when complete

Link State Request
Link State Update
Link State Request
Link State Update
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OSPF Operation

• Network changes generate link-state
  advertisements (LSA)
• Cost change to an interface
• Link being added or deleted from topology
• All routers exchange LSAs to build and maintain a
  consistent database
• The protocol remains relatively quiet during
  steady-state conditions.

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OSPF Operation Steps to OSPF Operation

• 1. Establishing router adjacencies
• 2. Electing DR and BDR
• 3. Discovering Routes
• 4. Choosing Routes
• 5. Maintaining Routing Information

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OSPF Operation OSPF States

• OSPF router interfaces can be in one of seven
  states
• Down State
• Init State
• Two-way State
• ExStart State
• Exchange State
• Loading State
• Full Adjacency State

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OSPF Operation Steps to OSPF Operation with OSPF
States

• 1. Establishing router adjacencies
• Down State
• Init State
• Two-way State
• (ExStart State unless DR/BDR election needed)
• 2. Electing DR and BDR
• ExStart State with DR and BDR
• Two-way State with all other routers

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OSPF Operation Steps to OSPF Operation with OSPF
States

• 3. Discovering Routes
• ExStart State
• Exchange State
• Loading State
• Full State
• 4. Choosing Routes
• 5. Maintaining Routing Information

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OSPF Operation 1. Establishing Adjacencies (1)

• Initially, an OSPF router interface is in the
  down state.not exchanged information with any
  neighbor.

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OSPF Operation 1. Establishing Adjacencies (2)

• Init State
• Init State - OSPF routers send Type 1 Hello
  packets at regular intervals (10 sec.) to
  establish neighbors.
• When a router receives its first Hello packet, it
  enters the init state, meaning the router is
  ready to take the relationship to the next level.

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OSPF Operation 1. Establishing Adjacencies (3)

• From init state to the two-way state
• RTB receives Hello packets from RTA and RTC (its
  neighbors), and sees its own Router ID (10.6.0.1)
  in the Neighbor ID field.
• RTB declares takes the relationship to a new
  level, and declares a two-way state between
  itself and RTA, and itself and RTC.

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OSPF Operation 1. Establishing Adjacencies (4)

• Two-way state to ExStart state?
• RTB now decides who to establish a full adjacency
  with depending upon the type of network that the
  particular interfaces resides on.
• If the interface is on a point-to-point link, the
  routers becomes adjacent with its sole link
  partner (aka soul mates), and take the
  relationship to the next level by entering the
  ExStart state.
• If the interface is on a multi-access link
  (Ethernet, Frame Relay, ) RTB must enter an
  election process to see who it will establish a
  full adjacency with, and remains in the two-way
  state. (Next!)

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OSPF Operation Designated Router

• Reduce OSPF traffic on multiaccess links
• Routers form FULL adjacencies with DR/BDR
• Store and distribute neighbors LSDBs
• Backup DR for redundancy
• OSPF priority used in DR selection
• Range 1255 default 1, 0 for non-candidate.
  Priority carried in Hello packet
• ip ospf priority ltvaluegt

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OSPF Operation Function of DR/BDR
DR
BDR
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OSPF Operation 2.Electing a DR and BDR (1)

• On point-to-point links adjacencies are
  established with all neighbors, because there is
  only one neighbor.
• On multi-access networks,OSPF elects a DR and BDR
  to limit the number of adjacencies.
• Reduce routing update traffic

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OSPF Operation 2.Electing a DR and BDR (2)

• DR - Designated Router
• BDR Backup Designated Router
• DRs serve as collection points for Link State
  Advertisements (LSAs)
• A BDR back ups the DR.
• If the IP network is multi-access, the OSPF
  routers will elect 1 DR and 1 BDR (unless there
  is only 1 router on the network).

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OSPF Operation 2.Electing a DR and BDR (3)

• The formation of an adjacency between every
  attached router would create many unncessary LSA
  (Link State Advertisements), n(n-1)/2
  adjacencies.
• Flooding on the network itself would be chaotic.
• To prevent this problem, a Designated Router is
  elected on multi-access networks.

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OSPF Operation 2.Electing a DR and BDR (4)

• All other routers, DRother, establish
  adjacencies with only the DR and BDR.
• DRother routers multicast LSAs to only the DR
  and BDR
• (224.0.0.6 - all DR routers)
• DR sends LSA to all adjacent neighbors
• (224.0.0.5 - all OSPF routers)

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OSPF Operation 2.Electing a DR and BDR (5)

• Once a DR is established, a new router that
  enters the network with a higher priority or
  router id will NOT become the DR or BDR. (Bug in
  early IOS 12.0)
• If DR fails, BDR takes over as DR and selection
  process for new BDR begins.
• State of the relationship
• DRothers enter ExStart state with DR and BDR and
  two-way state with all other routers

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OSPF Operation 2.Electing a DR and BDR (6)

• DR - Summary
• DR Election
• Router with the highest interface priority
• (priority 0 cannot become DR or BDR)
• Router with the highest router ID.
• Loopback address used first
• IP Address on active interface used second
• BDR is the second highest

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OSPF Operation 2.Electing a DR and BDR (7)

• DR - Summary
• Adjacencies and multicasting
• All other routers, DRother, establish adjacencies
  with only the DR and BDR.
• All routers continue to multicast Hello packets
  to AllSPFRouters (224.0.0.5) so they can track
  neighbors.
• But updates (LSAs) are multicast to DR and BDR
  only (224.0.0.6 - AllDRrouters) and in turn
• DR floods updates (LSAs) to all adjacent
  neighbors (224.0.0.5 - AllSPFRrouters)

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OSPF Operation 2.Electing a DR and BDR (8)

• BDR-summary
• Listens, but doesnt act.
• If LSA is sent, BDR sets a timer.
• If timer expires before it sees the reply from
  the DR, it becomes the DR and takes over the
  update process.
• The process for a new BDR begins.

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OSPF Operation 3. Discovering Routes and
reaching Full State
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OSPF Operation 4. Choosing routes (1)
Dijkstra - Shortest Path First (SPF) Algorithm

• Link state database
• Created with Link State Packets (LSPs) from each
  router
• TENT database
• Tentative triples (ID, path cost, direction)
• PATH database
• Best path triples (ID, path cost, direction)
• Forwarding database
• The Routing Table

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OSPF Operation 4. Choosing routes (2)
Dijkstra (SPF) Overview (Cont.)

• All routers exchange Link State Packets (LSPs)
• Each router starts with itself as root
• Tent is built from LSPs
• Path is created by examining and comparing TENT
  triples
• Once path is final the forwarding table is
  populated

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OSPF Operation 4. Choosing routes (3)
Link State Packet (LSP) Data
B
C
D
E
F
G
A
B/4 G/2
A/4 C/1
B/1 D/4 E/2
C/4 E/1
C/2 D/1 F/2
E/2 G/2
A/2 F/2
Lowest cost best
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OSPF Operation 5. Maintaining routes
Router 2, Area 1
Router 1, Area 1
LSA

• ..

Link State Table
ACK

• Every router in area receives the new LSA via
  flooding
• Each router computes shortest path routing table
  when a link changes State.

Dijkstra Algorithm
Old Routing Table
New Routing Table
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Issues with large OSPF nets

• Large routing table
• Large link-state table
• Frequent SPF calculations

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reference

• RFC 1403, "BGP OSPF Interaction", K. Varadhan,
  1993.
• RFC 1584, "Multicast Extensions to OSPF", J. Moy,
  March 1994.
• RFC 1850, "OSPF Version 2 Management Information
  Base", F. Baker and R. Coltun, Nov 1995.
• RFC 2328, "OSPF Version 2", J. Moy, April 1998,
  also STD 54.
• RFC 2370, "The OSPF Opaque LSA Option", R.
  Coltun, July 1998.
• http//www2.rad.com/networks/1995/ospf/ospf.htm,
  OSPF, B. Daniel, B. Omer, R. Carmel.
• Internetworking with TCP/IP (Vol I) - Comer
• www.et.fnt.hvu.nl/docenten/cuiterwijk/ccnp/guides,
  The Technology Innovation Centre Brimingham.