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Open Shortest Path First OSPF

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Open Shortest Path First OSPF OSPF Overview OSPF Operation By Grace Deng Oct.16.2003 OSPF Overview History Development began 1987 by IETF Goal a link state protocol ... – PowerPoint PPT presentation

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Title: Open Shortest Path First OSPF


1
Open Shortest Path First OSPF
  • OSPF Overview
  • OSPF Operation
  • By Grace Deng Oct.16.2003

2
OSPF Overview History
  • Development began 1987 by IETF
  • Goala link state protocol more efficient and
    scaleable than RIP
  • Latest revision is RFC 2328 April 1998

3
OSPF Overview OSPF 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

4
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

5
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

6
OSPF Overview Terminology
7
OSPF Overview Terminology
  • Link
  • Link state
  • Link State (LS) or topological database
  • Area
  • OSPF Metric Cost
  • Routing table
  • Adjacencies database

8
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

9
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

10
(No Transcript)
11
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

12
OSPF Overview OSPF Packet Types
OSPF Packet format
13
OSPF Packet Types (cont.)
14
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

15
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

16
OSPF Overview Database Descriptor Packets (DDP)
  • Contain link state database headers
  • Describe the current LS database
  • Exchange stage

17
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
18
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.

19
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

20
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

21
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

22
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

23
OSPF Operation 1. Establishing Adjacencies (1)
  • Initially, an OSPF router interface is in the
    down state.not exchanged information with any
    neighbor.

24
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.

25
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.

26
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!)

27
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

28
OSPF Operation Function of DR/BDR
DR
BDR
29
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

30
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).

31
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.

32
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)

33
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

34
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

35
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)

36
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.

37
OSPF Operation 3. Discovering Routes and
reaching Full State
38
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

39
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

40
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
41
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
42
Issues with large OSPF nets
  • Large routing table
  • Large link-state table
  • Frequent SPF calculations

43
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.
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