Your SlideShare is downloading. ×
Presentation Ospf Ranjeetv0.2
Upcoming SlideShare
Loading in...5
×

Thanks for flagging this SlideShare!

Oops! An error has occurred.

×

Introducing the official SlideShare app

Stunning, full-screen experience for iPhone and Android

Text the download link to your phone

Standard text messaging rates apply

Presentation Ospf Ranjeetv0.2

1,440
views

Published on

OSPF Routing Protocol

OSPF Routing Protocol


0 Comments
0 Likes
Statistics
Notes
  • Be the first to comment

  • Be the first to like this

No Downloads
Views
Total Views
1,440
On Slideshare
0
From Embeds
0
Number of Embeds
1
Actions
Shares
0
Downloads
42
Comments
0
Likes
0
Embeds 0
No embeds

Report content
Flagged as inappropriate Flag as inappropriate
Flag as inappropriate

Select your reason for flagging this presentation as inappropriate.

Cancel
No notes for slide

Transcript

  • 1. 1
    Introduction to
    OSPF Protocol Design
    By:
    Ranjeet Saini
    Engineering Test Lead
    Agnity Technologies, Inc.
    March, 2008
  • 2. 2
    Agenda
    References & Standard
    II. Terminology
    OSPF Format
    IV. OSPF Algorithm
  • 3. 3
    References & Standard
    RFC 2328 OSPF Version 2 April, 1998
    by John Moy
    2. RFC 2370 The OSPF Opaque LSA Option April, 1998
    by R. Coltun
    Understanding TCP/IP December, 1995
    SynOptics Communications
    4. RFC 1349 Type of Service in the Internet Protocol July, 1992
  • 4. 4
    References & Standard
    II. Terminology
    OSPF Format
    IV. OSPF Algorithm
  • 5. 5
    Internet Routing
    Rate %
    Rate %
    Rate %
    Rate %
    Rate %
    Rate %
    Rate %
    Rate %
    Rate %
    Rate %
    Rate %
    Rate %
    1 2 3 4 5 6 7 8
    1 2 3 4 5 6 7 8
    1 2 3 4 5 6 7 8
    1 2 3 4 5 6 7 8
    1 2 3 4 5 6 7 8
    1 2 3 4 5 6 7 8
    1 2 3 4 5 6 7 8
    1 2 3 4 5 6 7 8
    1 2 3 4 5 6 7 8
    1 2 3 4 5 6 7 8
    1 2 3 4 5 6 7 8
    1 2 3 4 5 6 7 8
    30+
    90+
    1
    X 2X 3X 4X 5X 6X 7X 8MDI-X-or-8MDI
    30+
    90+
    1
    X 2X 3X 4X 5X 6X 7X 8MDI-X-or-8MDI
    30+
    90+
    1
    X 2X 3X 4X 5X 6X 7X 8MDI-X-or-8MDI
    30+
    90+
    1
    X 2X 3X 4X 5X 6X 7X 8MDI-X-or-8MDI
    30+
    90+
    1
    X 2X 3X 4X 5X 6X 7X 8MDI-X-or-8MDI
    30+
    90+
    1
    X 2X 3X 4X 5X 6X 7X 8MDI-X-or-8MDI
    30+
    90+
    1
    X 2X 3X 4X 5X 6X 7X 8MDI-X-or-8MDI
    30+
    90+
    1
    X 2X 3X 4X 5X 6X 7X 8MDI-X-or-8MDI
    30+
    90+
    1
    X 2X 3X 4X 5X 6X 7X 8MDI-X-or-8MDI
    30+
    90+
    1
    X 2X 3X 4X 5X 6X 7X 8MDI-X-or-8MDI
    30+
    90+
    1
    X 2X 3X 4X 5X 6X 7X 8MDI-X-or-8MDI
    30+
    90+
    1
    X 2X 3X 4X 5X 6X 7X 8MDI-X-or-8MDI
    Fast SwitcHub-8mi
    20
    70
    Fast SwitcHub-8mi
    20
    70
    Fast SwitcHub-8mi
    20
    70
    Fast SwitcHub-8mi
    20
    70
    Fast SwitcHub-8mi
    20
    70
    Fast SwitcHub-8mi
    20
    70
    Fast SwitcHub-8mi
    20
    70
    Fast SwitcHub-8mi
    20
    70
    Fast SwitcHub-8mi
    20
    70
    Fast SwitcHub-8mi
    20
    70
    Fast SwitcHub-8mi
    20
    70
    Fast SwitcHub-8mi
    20
    70
    10
    Collision
    50
    10
    Collision
    50
    10
    Collision
    50
    10
    Collision
    50
    10
    Collision
    50
    10
    Collision
    50
    10
    Collision
    50
    10
    Collision
    50
    10
    Collision
    50
    10
    Collision
    50
    10
    Collision
    50
    10
    Collision
    50
    5
    35
    5
    35
    5
    35
    5
    35
    5
    35
    5
    35
    5
    35
    5
    35
    5
    35
    5
    35
    5
    35
    5
    35
    20
    20
    20
    20
    20
    20
    20
    20
    20
    20
    20
    20
    1
    1
    1
    1
    1
    1
    1
    1
    1
    1
    1
    1
    100
    M
    10
    100
    M
    10
    100
    M
    10
    100
    M
    10
    100
    M
    10
    100
    M
    10
    100
    M
    10
    100
    M
    10
    100
    M
    10
    100
    M
    10
    100
    M
    10
    100
    M
    10
    Tx/Rx
    5
    Tx/Rx
    5
    Tx/Rx
    5
    Tx/Rx
    5
    Tx/Rx
    5
    Tx/Rx
    5
    Tx/Rx
    5
    Tx/Rx
    5
    Tx/Rx
    5
    Tx/Rx
    5
    Tx/Rx
    5
    Tx/Rx
    5
    Full Duplex
    PWR
    1
    Full Duplex
    PWR
    1
    Full Duplex
    PWR
    1
    Full Duplex
    PWR
    1
    Full Duplex
    PWR
    1
    Full Duplex
    PWR
    1
    Full Duplex
    PWR
    1
    Full Duplex
    PWR
    1
    Full Duplex
    PWR
    1
    Full Duplex
    PWR
    1
    Full Duplex
    PWR
    1
    Full Duplex
    PWR
    1
    Util%
    Util%
    Util%
    Util%
    Util%
    Util%
    Util%
    Util%
    Util%
    Util%
    Util%
    Util%
    Demo
    Demo
    Demo
    Demo
    Demo
    Demo
    Demo
    Demo
    Demo
    Demo
    Demo
    Demo
    Forward%
    Filter%
    Config
    Link
    SNMP
    Forward%
    Filter%
    Config
    Link
    SNMP
    Forward%
    Filter%
    Config
    Link
    SNMP
    Forward%
    Filter%
    Config
    Link
    SNMP
    Forward%
    Filter%
    Config
    Link
    SNMP
    Forward%
    Filter%
    Config
    Link
    SNMP
    Forward%
    Filter%
    Config
    Link
    SNMP
    Forward%
    Filter%
    Config
    Link
    SNMP
    Forward%
    Filter%
    Config
    Link
    SNMP
    Forward%
    Filter%
    Config
    Link
    SNMP
    Forward%
    Filter%
    Config
    Link
    SNMP
    Forward%
    Filter%
    Config
    Link
    SNMP
    Status
    Status
    Status
    Status
    Status
    Status
    Status
    Status
    Status
    Status
    Status
    Status
    Select/Link
    Select/Link
    Select/Link
    Select/Link
    Select/Link
    Select/Link
    Select/Link
    Select/Link
    Select/Link
    Select/Link
    Select/Link
    Select/Link
    Diag
    Diag
    Diag
    Diag
    Diag
    Diag
    Diag
    Diag
    Diag
    Diag
    Diag
    Diag
    Full/Half
    10
    M/ 100 M
    Full/Half
    10
    M/ 100 M
    Full/Half
    10
    M/ 100 M
    Full/Half
    10
    M/ 100 M
    Full/Half
    10
    M/ 100 M
    Full/Half
    10
    M/ 100 M
    Full/Half
    10
    M/ 100 M
    Full/Half
    10
    M/ 100 M
    Full/Half
    10
    M/ 100 M
    Full/Half
    10
    M/ 100 M
    Full/Half
    10
    M/ 100 M
    Full/Half
    10
    M/ 100 M
    Core System
    Area Border Router
    Area Border Router
    R7
    R8
    R1
    R2
    Area Border Router
    R9
    R3
    R4
    R5
    Autonomous System #1
    Autonomous System #3
    R6
    Autonomous System #2
  • 6. 6
    Exterior Gateway Protocol
    Rate %
    Rate %
    Rate %
    Rate %
    Rate %
    Rate %
    Rate %
    Rate %
    1 2 3 4 5 6 7 8
    1 2 3 4 5 6 7 8
    1 2 3 4 5 6 7 8
    1 2 3 4 5 6 7 8
    1 2 3 4 5 6 7 8
    1 2 3 4 5 6 7 8
    1 2 3 4 5 6 7 8
    1 2 3 4 5 6 7 8
    30+
    90+
    1
    X 2X 3X 4X 5X 6X 7X 8MDI-X-or-8MDI
    30+
    90+
    1
    X 2X 3X 4X 5X 6X 7X 8MDI-X-or-8MDI
    30+
    90+
    1
    X 2X 3X 4X 5X 6X 7X 8MDI-X-or-8MDI
    30+
    90+
    1
    X 2X 3X 4X 5X 6X 7X 8MDI-X-or-8MDI
    30+
    90+
    1
    X 2X 3X 4X 5X 6X 7X 8MDI-X-or-8MDI
    30+
    90+
    1
    X 2X 3X 4X 5X 6X 7X 8MDI-X-or-8MDI
    30+
    90+
    1
    X 2X 3X 4X 5X 6X 7X 8MDI-X-or-8MDI
    30+
    90+
    1
    X 2X 3X 4X 5X 6X 7X 8MDI-X-or-8MDI
    Fast SwitcHub-8mi
    20
    70
    Fast SwitcHub-8mi
    20
    70
    Fast SwitcHub-8mi
    20
    70
    Fast SwitcHub-8mi
    20
    70
    Fast SwitcHub-8mi
    20
    70
    Fast SwitcHub-8mi
    20
    70
    Fast SwitcHub-8mi
    20
    70
    Fast SwitcHub-8mi
    20
    70
    10
    Collision
    50
    10
    Collision
    50
    10
    Collision
    50
    10
    Collision
    50
    10
    Collision
    50
    10
    Collision
    50
    10
    Collision
    50
    10
    Collision
    50
    5
    35
    5
    35
    5
    35
    5
    35
    5
    35
    5
    35
    5
    35
    5
    35
    20
    20
    20
    20
    20
    20
    20
    20
    1
    1
    1
    1
    1
    1
    1
    1
    100
    M
    10
    100
    M
    10
    100
    M
    10
    100
    M
    10
    100
    M
    10
    100
    M
    10
    100
    M
    10
    100
    M
    10
    Tx/Rx
    5
    Tx/Rx
    5
    Tx/Rx
    5
    Tx/Rx
    5
    Tx/Rx
    5
    Tx/Rx
    5
    Tx/Rx
    5
    Tx/Rx
    5
    Full Duplex
    PWR
    1
    Full Duplex
    PWR
    1
    Full Duplex
    PWR
    1
    Full Duplex
    PWR
    1
    Full Duplex
    PWR
    1
    Full Duplex
    PWR
    1
    Full Duplex
    PWR
    1
    Full Duplex
    PWR
    1
    Util%
    Util%
    Util%
    Util%
    Util%
    Util%
    Util%
    Util%
    Demo
    Demo
    Demo
    Demo
    Demo
    Demo
    Demo
    Demo
    Forward%
    Filter%
    Config
    Link
    SNMP
    Forward%
    Filter%
    Config
    Link
    SNMP
    Forward%
    Filter%
    Config
    Link
    SNMP
    Forward%
    Filter%
    Config
    Link
    SNMP
    Forward%
    Filter%
    Config
    Link
    SNMP
    Forward%
    Filter%
    Config
    Link
    SNMP
    Forward%
    Filter%
    Config
    Link
    SNMP
    Forward%
    Filter%
    Config
    Link
    SNMP
    Status
    Status
    Status
    Status
    Status
    Status
    Status
    Status
    Select/Link
    Select/Link
    Select/Link
    Select/Link
    Select/Link
    Select/Link
    Select/Link
    Select/Link
    Diag
    Diag
    Diag
    Diag
    Diag
    Diag
    Diag
    Diag
    Full/Half
    10
    M/ 100 M
    Full/Half
    10
    M/ 100 M
    Full/Half
    10
    M/ 100 M
    Full/Half
    10
    M/ 100 M
    Full/Half
    10
    M/ 100 M
    Full/Half
    10
    M/ 100 M
    Full/Half
    10
    M/ 100 M
    Full/Half
    10
    M/ 100 M
    IGP1
    IGP2
    IGP1
    IGP1
    IGP2
    IGP2
    EGP
    IGP1
    IGP2
    Autonomous System1
    Autonomous System #2
    An External Links Advertisement originates for each destination outside
    The AS, and these advertisements are in turn flooded throughout the AS.
    An External Links Advertisement is used for externally derived routing
    Information obtained by another routing protocol such as EGP or BGP.
  • 7. 7
    Terminology
    Autonomous Systems(AS)
    Each AS is a group of networks & routers administered by single
    authority using a common routing protocol.
    Interior Gateway Protocol(IGP)
    Routers within single AS communicate using one of several dynamic
    routing protocols, known generically as an IGP.
    Exterior Gateway Protocols(EGP)
    Communication between routers belonging to different AS requires
    additional protocol, so-called EGP.
    Open Shortest Path First(OSPF)
    is an Interior Gateway Protocol(IGP) IP routing protocol.
  • 8. 8
    Configurable Metrics
    Rate %
    Rate %
    Rate %
    1 2 3 4 5 6 7 8
    1 2 3 4 5 6 7 8
    1 2 3 4 5 6 7 8
    30+
    90+
    1
    4X 5X 6X 7X 8MDI-X-or-8MDI
    30+
    90+
    1
    4X 5X 6X 7X 8MDI-X-or-8MDI
    30+
    90+
    1
    4X 5X 6X 7X 8MDI-X-or-8MDI
    Fast SwitcHub-8mi
    20
    70
    Fast SwitcHub-8mi
    20
    70
    Fast SwitcHub-8mi
    20
    70
    10
    Collision
    50
    10
    Collision
    50
    10
    Collision
    50
    5
    35
    5
    35
    5
    35
    20
    20
    20
    1
    1
    1
    100
    M
    10
    100
    M
    10
    100
    M
    10
    Tx/Rx
    5
    Tx/Rx
    5
    Tx/Rx
    5
    Full Duplex
    PWR
    1
    Full Duplex
    PWR
    1
    Full Duplex
    PWR
    1
    Util%
    Util%
    Util%
    Demo
    Demo
    Demo
    Forward%
    Filter%
    Config
    Link
    SNMP
    Forward%
    Filter%
    Config
    Link
    SNMP
    Forward%
    Filter%
    Config
    Link
    SNMP
    Status
    Status
    Status
    Select/Link
    Select/Link
    Select/Link
    Diag
    Diag
    Diag
    Full/Half
    10
    M/ 100 M
    Full/Half
    10
    M/ 100 M
    Full/Half
    10
    M/ 100 M
    Hop = 1
    Cost= 10
    Hop = 1
    Cost= 10
    T1
    T1
    56Kb
    Hop = 1
    Cost= 100
    B
    A
    Using RIP, traffic would be routed over slow 56K link since the hop count
    Metric is 1. Using OSPF as the IGP, traffic can be routed over faster T-1
    Link since total cost would be 20.
    For single destination, there may be separate routing table entries for each
    Type Of Service. A metric for TOS 0 must always be specified.
  • 9. 9
    Network Types
    OSPF operates over below physical networks :
    Point-to-point Network A network joining single pair of routers.
    Broadcast Network A network with more than 2 attached routers, and the ability
    to address single physical messages to all of attached routers.
    Non-broadcast Network A network with more than 2 attached router, but having no
    broadcast capability such as X.25 public data network.
    DR
    BDR
  • 10. 10
    Adjacencies
    Rate %
    Rate %
    Rate %
    1 2 3 4 5 6 7 8
    1 2 3 4 5 6 7 8
    1 2 3 4 5 6 7 8
    30+
    90+
    1
    X 2X 3X 4X 5X 6X 7X 8MDI-X-or-8MDI
    30+
    90+
    1
    X 2X 3X 4X 5X 6X 7X 8MDI-X-or-8MDI
    30+
    90+
    1
    X 2X 3X 4X 5X 6X 7X 8MDI-X-or-8MDI
    Fast SwitcHub-8mi
    20
    70
    Fast SwitcHub-8mi
    20
    70
    Fast SwitcHub-8mi
    20
    70
    10
    Collision
    50
    10
    Collision
    50
    10
    Collision
    50
    5
    35
    5
    35
    5
    35
    20
    20
    20
    1
    1
    1
    100
    M
    10
    100
    M
    10
    100
    M
    10
    Tx/Rx
    5
    Tx/Rx
    5
    Tx/Rx
    5
    Full Duplex
    PWR
    1
    Full Duplex
    PWR
    1
    Full Duplex
    PWR
    1
    Util%
    Util%
    Util%
    Demo
    Demo
    Demo
    Config
    Link
    SNMP
    Config
    Link
    SNMP
    Config
    Link
    SNMP
    Filter%
    Filter%
    Filter%
    Status
    Forward%
    Status
    Forward%
    Status
    Forward%
    Select/Link
    Select/Link
    Select/Link
    Diag
    Diag
    Diag
    10
    M/ 100 M
    10
    M/ 100 M
    10
    M/ 100 M
    Full/Half
    Full/Half
    Full/Half
    An “adjacency” is a two-way communication between
    selected neighboring routers for the purpose of exchanging
    routing information through link-state advertisements.
    Router
    Router
    Adjacency
    Router
  • 11. 11
    Routing Area
    Rate %
    Rate %
    Rate %
    Rate %
    Rate %
    1 2 3 4 5 6 7 8
    1 2 3 4 5 6 7 8
    1 2 3 4 5 6 7 8
    1 2 3 4 5 6 7 8
    1 2 3 4 5 6 7 8
    30+
    90+
    1
    X 2X 3X 4X 5X 6X 7X 8MDI-X-or-MDI
    30+
    90+
    1
    X 2X 3X 4X 5X 6X 7X 8MDI-X-or-MDI
    30+
    90+
    1
    X 2X 3X 4X 5X 6X 7X 8MDI-X-or-MDI
    30+
    30+
    90+
    1
    X 2X 3X 4X 5X 6X 7X 8MDI-X-or-MDI
    90+
    1
    X 2X 3X 4X 5X 6X 7X 8MDI-X-or-MDI
    Fast SwitcHub-8mi
    20
    70
    Fast SwitcHub-8mi
    20
    70
    Fast SwitcHub-8mi
    20
    70
    Fast SwitcHub-8mi
    20
    70
    Fast SwitcHub-8mi
    20
    70
    10
    Collision
    50
    10
    Collision
    50
    10
    Collision
    50
    10
    50
    10
    Collision
    50
    Collision
    5
    35
    5
    35
    5
    35
    5
    35
    5
    35
    20
    20
    20
    20
    20
    1
    1
    1
    1
    1
    100
    M
    10
    100
    M
    10
    100
    M
    10
    100
    M
    10
    100
    M
    10
    5
    Tx/Rx
    5
    5
    Tx/Rx
    Tx/Rx
    5
    Tx/Rx
    5
    Tx/Rx
    Util%
    Util%
    Util%
    Util%
    Util%
    Full Duplex
    PWR
    1
    Full Duplex
    PWR
    1
    Full Duplex
    PWR
    1
    Full Duplex
    PWR
    1
    Full Duplex
    PWR
    1
    Demo
    Demo
    SNMP
    Demo
    SNMP
    Demo
    Demo
    Status
    Filter%
    SNMP
    Forward%
    Filter%
    Forward%
    Filter%
    Filter%
    SNMP
    Status
    Filter%
    SNMP
    Forward%
    Forward%
    Config
    Link
    Config
    Link
    Config
    Link
    Config
    Link
    Forward%
    Config
    Link
    Status
    Status
    Status
    Select/Link
    Select/Link
    Select/Link
    Select/Link
    Select/Link
    Diag
    Diag
    Diag
    Diag
    Diag
    Full/Half
    Full/Half
    Full/Half
    Full/Half
    Full/Half
    10
    M/ 100 M
    10
    M/ 100 M
    10
    M/ 100 M
    10
    M/ 100 M
    10
    M/ 100 M
    Area 2
    Area 1
    N9
    N10
    Router
    N1
    N3
    AS 200
    Router
    Router
    AS 100
    Area Border Router
    (ABR)
    N6
    N7
    N8
    N4
    N2
    Router
    Router
    N5
    AS 300
    Note : Each area has its own topological link-state database. All routers within an area contain
    Router LSAs & Network LSAs.
    Router is Autonomous System Boundary Router(ASBR).
  • 12. 12
    Routing Area(Con’t)
    OSPF allows collections of contiguous networks and hosts to be grouped
    together. This group, along with the routers having interfaces connected
    to the networks in this group, is called an “area”.
    A single area limits the boundary for Link-Stat Advertisement(LSA) flooding.
    the Shortest Path First(SPF) tree is computed on a per-area basis, and any
    intra-area destinations are derived from the SPF tree.
    All areas must have at least one route/interface connected to area 0.0.0.0(
    the backbone area).
    An Area Border Router(ABR) is an OSPF router having interface connected
    to multiple areas. ABRs must keep a distinct link-state database for each
    area, and run the SPF algorithm on distinct database.
  • 13. 13
    Routing Area- Backbone Area
    If an AS is divided into areas, the areas must be connected to each other
    Via special area called the “Backbone Area”. Backbone consists of those
    Networks not contained in any area.
    All ABRs in an AS must belong to the Backbone Area. Backbone area is
    assigned an area_ID to 0.0.0.0
    Backbone
    Area
    AS 300
    Area-1
    ABR
    Area-2
    ABR
    Area-3
    ABR
  • 14. 14
    Backbone Area(Con’t)
    An ABR connected to the Backbone executes two copies of OSPF protocol:
    Operates on the interface connected to local area and accept flooded
    advertisements from other routers that are members of the area
    Executes over the interface that connects to the backbone. This second
    copy won’t propagate flooded advertisements from the area across
    the backbone. Instead, it sends Summary Link Advertisements over
    the backbone so that attached area can learn about backbone reachability.
  • 15. 15
    Stub Area
    OSPF Domain
    AS 101
    RIP or
    IGP
    Area
    0.0.0.0
    Stub
    Area 51
    0.0.0.0 Default
    ASBR
    When an OSPF area within an AS has a single entry/exit router that
    is used by all externally addressed traffic, it is possible to block the
    import of the AS External Link Advertisements into the area :
    ~ No LSA type 4 & 5’s
    ~ ASBRs are not supported with stub area
    ~ Virtual links are not supported in stub area
    ~ The ABR must be configured as default router for stub area
    ~ The ASBR can be configured to be disabled for an area
  • 16. 16
    Virtual Link
    A virtual link can be configured to allow the connection of an ABR to backbone
    When the ABR and its are aren’t contiguous to the backbone.
    In below figure, Area 1 cannot directly learn all inter-area from the other areas
    Since it lacks a direct connection to the backbone. Area 1 is connected to the
    Backbone by a virtual link between the ABRs in Area 2. All inter-area routes
    From the backbone are flooded over virtual link to the ABR in Area 1. The ABR
    In Area 1 will summarize all intra-area routes for Area 1 over virtual link for
    Transmission on the backbone.
    AS 200
    Area 1
    Area2
    ABR
    Area 4
    ABR
    Router
    Router
    Router
    ABR
    Area 3
    ABR
    Router
    Backbone
  • 17. 17
    Virtual Link(Con’t)
    Virtual Link
    Area
    0.0.0.0
    Area
    0.0.0.51
    Area
    0.0.0.7
    ABR
    ABR
    Any physical arrangement of areas can be logically attached to the
    Backbone through a virtual link.
    Virtual links allow summary-LSAs to be tunneled across a non-
    Backbone area to exchange the routing information.
  • 18. 18
    Virtual Link(Con’t)
    C
    A
    D
    Virtual link
    Area 51
    Data
    Area
    0.0.0.0
    B
    Area
    0.0.0.7
    Data
    Area 52
    Data
    Virtual link
    The exchange of routing information continues to follow via
    virtual link but the forwarding of data packets does not.
    A data packets from router C destine for router D would go
    through routers A & B, but not through Area 0.
  • 19. 19
    Area Routing Advantages
    Routing Area greatly reduce the amount of routing information
    traffic that must be propagated throughout entire AS
    Areas allow the development of a hierarchy of routing information,
    thus protecting each area from external routing information.
    The area’s information is hidden from routers outside of the area.
    This “information hiding” technique is important from a security
    standpoint, since it prohibits other areas from identifying the
    physical topology of an area.
  • 20. 20
    References & Standard
    II. Terminology
    OSPF Format
    IV. OSPF Algorithm
  • 21. 21
    IPv4 Format
    0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
    Total Length
    0
    Type Of Service
    LEN
    VERS
    Identification
    Fragment Offset
    4
    Flags
    Protocol= 89
    Time To live
    = 1
    8
    Header Checksum
    Source IP Address
    C
    E
    Destination IP Address
    10
    Padding
    Option
    14
    D A T A
    Total: 20 bytes
  • 22. 22
    OSPF Assigned Class D Address
    Multicast Class D addresses assigned for OSPF :
    224.0.0.5 All OSPF routers must be capable of transmitting &
    receiving packets with this destination IP address
    224.0.0.6 All OSPF Designated Routers must be capable of
    receiving packets with this destination address. This
    includes the Backup Designated Router.
    IP Address MAC
    224.0.0.5 01005E-000005
    224.0.0.6 01005E-000006
    Note : OSPF multicast addresses are used on both point-to-point links & multi-access
    networks, but does not use over non-broadcast networks or virtual links.
    To ensure that multicast OSPF messages won’t travel multiple hops, their IP TTL
    must be set to 1.
  • 23. 23
    OSPF Packet Header
    0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
    Packet Length
    Type
    0
    4
    8
    C
    10
    14
    Version No.
    Router ID
    Area ID
    AuType
    Header Checksum
    Authentication
    Type Description
    1 Hello
    2 Database Description
    3 Link State Request
    4 Link State Update
    5 Link State Acknowledgment
    OSPF Packet Header = 24 bytes
  • 24. 24
    OSPF Hello Packet
    Type
    = 1
    Hello
    Packet Length
    Version No.
    Network Mask
    HelloInterval Option Rtr Pri
    RouterDeadInterval
    Designated Router
    Backup Designated Router
    Neighbor
    :
    :
    Router ID
    Area ID
    Entry 1
    (24 bytes)
    Checksum
    AuType
    Authentication
    Network Mask
    HolloInterval Option Rtr Pri
    RouterDeadInterval
    Designated Router
    Backup Designated Router
    Neighbor
    D A T A
    Entry n
    Note: DR/BDR = 0 means no designated router
  • 25. 25
    Hello Message Contents
    The router’s “Router Priority” used to determine the Designated
    Router & Backup Designated Router.
    The hello interval in which the transmitting router sends Hello
    packets on given network.
    The interval(in seconds) in which the transmitting router expects
    to receive Hello packets from a neighbor before determining
    that the neighbor is down.
    A list of routers from which Hello packets have been recently
    received.
    The router’s current choice fro the Designated Router & Backup
    Designated Router. A value of zero in these fields indicates that
    one has not yet been selected.
  • 26. 26
    The OSPF Optional Capability
    6 5 4 3 2 1 0
    - - DC EA N/P MC E -
    AS-external-LSAs are flooded
    Whether IP multicast datagrams are forwarded
    The handling of Type-7 LSAs
    The router’s willingness to receive & forward
    External-attributes-LSAs
    The router’s handling of demand circuits
  • 27. 27
    Designated Router & Backup Designated Router
    Rate %
    Rate %
    Rate %
    Rate %
    Rate %
    Rate %
    Rate %
    1 2 3 4 5 6 7 8
    1 2 3 4 5 6 7 8
    1 2 3 4 5 6 7 8
    1 2 3 4 5 6 7 8
    1 2 3 4 5 6 7 8
    1 2 3 4 5 6 7 8
    1 2 3 4 5 6 7 8
    30+
    90+
    1
    4X 5X 6X 7X 8MDI-X-or-8MDI
    30+
    90+
    1
    4X 5X 6X 7X 8MDI-X-or-8MDI
    30+
    90+
    1
    4X 5X 6X 7X 8MDI-X-or-8MDI
    30+
    90+
    1
    4X 5X 6X 7X 8MDI-X-or-8MDI
    30+
    90+
    1
    4X 5X 6X 7X 8MDI-X-or-8MDI
    30+
    90+
    1
    4X 5X 6X 7X 8MDI-X-or-8MDI
    30+
    90+
    1
    4X 5X 6X 7X 8MDI-X-or-8MDI
    Fast SwitcHub-8mi
    20
    70
    Fast SwitcHub-8mi
    20
    70
    Fast SwitcHub-8mi
    20
    70
    Fast SwitcHub-8mi
    20
    70
    Fast SwitcHub-8mi
    20
    70
    Fast SwitcHub-8mi
    20
    70
    Fast SwitcHub-8mi
    20
    70
    10
    Collision
    50
    10
    Collision
    50
    10
    Collision
    50
    10
    Collision
    50
    10
    Collision
    50
    10
    Collision
    50
    10
    Collision
    50
    5
    35
    5
    35
    5
    35
    5
    35
    5
    35
    5
    35
    5
    35
    20
    20
    20
    20
    20
    20
    20
    1
    1
    1
    1
    1
    1
    1
    100
    M
    10
    100
    M
    10
    100
    M
    10
    100
    M
    10
    100
    M
    10
    100
    M
    10
    100
    M
    10
    Tx/Rx
    5
    Tx/Rx
    5
    Tx/Rx
    5
    Tx/Rx
    5
    Tx/Rx
    5
    Tx/Rx
    5
    Tx/Rx
    5
    Full Duplex
    PWR
    1
    Full Duplex
    PWR
    1
    Full Duplex
    PWR
    1
    Full Duplex
    PWR
    1
    Full Duplex
    PWR
    1
    Full Duplex
    PWR
    1
    Full Duplex
    PWR
    1
    Util%
    Util%
    Util%
    Util%
    Util%
    Util%
    Util%
    Demo
    Demo
    Demo
    Demo
    Demo
    Demo
    Demo
    Forward%
    Filter%
    Config
    Link
    SNMP
    Forward%
    Filter%
    Config
    Link
    SNMP
    Forward%
    Filter%
    Config
    Link
    SNMP
    Forward%
    Filter%
    Config
    Link
    SNMP
    Forward%
    Filter%
    Config
    Link
    SNMP
    Forward%
    Filter%
    Config
    Link
    SNMP
    Forward%
    Filter%
    Config
    Link
    SNMP
    Status
    Status
    Status
    Status
    Status
    Status
    Status
    Select/Link
    Select/Link
    Select/Link
    Select/Link
    Select/Link
    Select/Link
    Select/Link
    Diag
    Diag
    Diag
    Diag
    Diag
    Diag
    Diag
    Full/Half
    10
    M/ 100 M
    Full/Half
    10
    M/ 100 M
    Full/Half
    10
    M/ 100 M
    Full/Half
    10
    M/ 100 M
    Full/Half
    10
    M/ 100 M
    Full/Half
    10
    M/ 100 M
    Full/Half
    10
    M/ 100 M
    DR
    Router
    Router
    Router
    Multi-access Network
    Router
    BDR
    Router
    X 2X 3X
    Each router on the network exchanges link-state information(forms an adjacency)
    Only with the Designated Router(DR) and the Backup Designated Router(BDR).
    Each neighbor exchanged information with a DR & BDR specified, the number of
    Exchanges is reduced to O(2n).
    The router with the highest configured Router Priority is elected DR.
    The BDR is elected at the same time as the DR. The router with second highest
    Router Priority is elected the BDR.
  • 28. 28
    OSPF Database Description Packet
    Type
    = 2
    Packet Length
    Version No.
    Router ID
    Area ID
    0
    4
    8
    C
    10
    14
    18
    Interface MTU Option 0 0 0 0 0 I M MS
    Checksum
    AuType
    DD Sequence number
    Authentication
    Authentication
    An LSA Header
    D A T A
    :
    :
    :
  • 29. 29
    Link State Request Packet
    Type
    = 3
    Packet Length
    Version No.
    Router ID
    Area ID
    LS Type
    0
    4
    8
    Checksum
    AuType
    Link State ID
    Advertising Router
    Authentication
    :
    :
    :
    D A T A
  • 30. 30
    Link State Update Packet
    Type
    = 24
    Packet Length
    Version No.
    Router ID
    Area ID
    # LSA
    0
    4
    8
    C
    10
    14
    18
    Checksum
    AuType
    Authentication
    LSAs
    D A T A
    :
    :
    :
  • 31. 31
    Link State Acknowledgment Packet
    Type
    = 5
    Packet Length
    Version No.
    Router ID
    Area ID
    0
    4
    8
    C
    10
    14
    18
    Checksum
    AuType
    An LSA Header
    Authentication
    :
    :
    :
    D A T A
  • 32. 32
    Forming Adjacency
    IP-172.16.32.1RID-100.100.100.4
    IP-172.16.32.2RID-100.100.100.6
    DownInit
    Down
    Hello (Packet 15 & 17)
    2-WayExStart
    Hello (Packet 16 & 18)
    ExStart
    DB Description (Packet 19 & 21)
    Exchange
    Exchange
    Exchange
    DB Description (Packet 20)
    Link State Request (Packet 22)
    Loading
    Loading
    Link State Update (Packet 25)
    (Adjacency Up)
    Full
    Full
    Full
    Hello
    Hello
  • 33. 33
    OSPF LSA Header
    0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
    0
    4
    8
    C
    10
    LS type
    Option
    LS age
    Link State ID
    Advertising Router
    LS sequence number
    LS Checksum
    Length
    Type Description
    1 Router-LSAs
    2 Network-LSAs
    3 Summary-LSAs (IP network)
    4 Summary-LSAs (ASBR)
    5 AS-external-LSAs
    OSPF LSA Header = 20 bytes
  • 34. 34
    Router-LSAs Format
    LS age
    Options
    LS Type= 1
    Link State ID
    Advertising Router
    LS sequence number
    0 V E B 0 No. of Links
    0
    4
    8
    C
    10
    14
    18
    Length
    Checksum
    Link State ID
    Link Data
    D A T A
    Type # TOS Metric
    :
    :
    Note: V for virtual link;
    E for AS boundary router
    B for Area Border router.
    Each router in an area originates a router-LSA.
  • 35. 35
    Router-LSAs Format(Con’t)
    Type Description
    1 Point-to-point connection to another router
    2 Connection to a transit network
    3 Connection to a stub network
    4 Virtual link
    Type Link ID Link Data
    1 Neighboring router’s Router ID Interface’s MIB ifIndex value
    2 IP address of Designated Router DR IP address
    3 IP network / subnet number Network IP address
    4 Neighboring router’s Router ID Network IP address
  • 36. 36
    Network-LSAs Format
    LS age
    Options
    LS Type=2
    0
    4
    8
    C
    10
    14
    18
    Link State ID
    Advertising Router
    LS sequence number
    Length
    Checksum
    Network Mask
    Attached Router
    Note: the distance from network to all attached routers is 0.
    Network-LSA is originated by the network’s DR.
  • 37. 37
    Summary-LSAs(IP network) Format
    LS age
    Options
    LS Type=3
    Link State ID
    Advertising Router
    Network Mask
    0
    4
    8
    LS sequence number
    0 metric
    Length
    Checksum
    TOS TOS metric
    :
    :
    :
    D A T A
    Note: Link State ID= 0.0.0.0 & Network mask= 0.0.0.0
    if default summary route.
    Metric is the cost of this route.
  • 38. 38
    Summary-LSAs(ASBR) Format
    LS age
    Options
    LS Type=4
    Link State ID
    Advertising Router
    Network Mask
    0
    4
    8
    LS sequence number
    0 metric
    Length
    Checksum
    TOS TOS metric
    :
    :
    :
    D A T A
  • 39. 39
    As-external-LSAs Format
    LS age
    Options
    LS Type= 5
    Link State ID
    Advertising Router
    Network Mask
    0
    4
    8
    C
    10
    14
    LS sequence number
    E 0 metric
    Length
    Checksum
    Forwarding address
    External Route Tag
    D A T A
    E TOS TOS metric
    Forwarding address
    :
    :
    :
  • 40. 40
    Rate %
    Rate %
    Rate %
    1 2 3 4 5 6 7 8
    1 2 3 4 5 6 7 8
    1 2 3 4 5 6 7 8
    30+
    90+
    1
    X 2X 3X 4X 5X 6X 7X 8MDI-X-or-8MDI
    30+
    90+
    1
    X 2X 3X 4X 5X 6X 7X 8MDI-X-or-8MDI
    30+
    90+
    1
    X 2X 3X 4X 5X 6X 7X 8MDI-X-or-8MDI
    Fast SwitcHub-8mi
    20
    70
    Fast SwitcHub-8mi
    20
    70
    Fast SwitcHub-8mi
    20
    70
    10
    Collision
    50
    10
    Collision
    50
    10
    Collision
    50
    5
    35
    5
    35
    5
    35
    20
    20
    20
    1
    1
    1
    100
    M
    10
    100
    M
    10
    100
    M
    10
    Tx/Rx
    5
    Tx/Rx
    5
    Tx/Rx
    5
    Full Duplex
    PWR
    1
    Full Duplex
    PWR
    1
    Full Duplex
    PWR
    1
    Util%
    Util%
    Util%
    Demo
    Demo
    Demo
    Config
    Link
    SNMP
    Config
    Link
    SNMP
    Config
    Link
    SNMP
    Filter%
    Filter%
    Filter%
    Status
    Forward%
    Status
    Forward%
    Status
    Forward%
    Select/Link
    Select/Link
    Select/Link
    Diag
    Diag
    Diag
    10
    M/ 100 M
    10
    M/ 100 M
    10
    M/ 100 M
    Full/Half
    Full/Half
    Full/Half
    Shortest Path Tree
    Network1
    Router1
    Network2
    Cost = 20
    Router2
    Network3
    Cost = 30
    Router3
    Network4
  • 41. 41
    Link State Database
    R1 R2 R3
    (N1-0) (N2-0) (N3-0)
    (N2-0) (N3-0) (N4-0)
    (R2-20) (R1-20) (R2-30)
    (R3-30)
    From the Link-State Database, each router builds a shortest path tree
    using itself as the root.
    Each node of the tree shows the shortest, or best cost path to
    the vertex from the root.
    Each router then build its routing table from the shortest path tree.
  • 42. 42
    Rate %
    Rate %
    Rate %
    1 2 3 4 5 6 7 8
    1 2 3 4 5 6 7 8
    1 2 3 4 5 6 7 8
    30+
    90+
    1
    X 2X 3X 4X 5X 6X 7X 8MDI-X-or-8MDI
    30+
    90+
    1
    X 2X 3X 4X 5X 6X 7X 8MDI-X-or-8MDI
    30+
    90+
    1
    X 2X 3X 4X 5X 6X 7X 8MDI-X-or-8MDI
    Fast SwitcHub-8mi
    20
    70
    Fast SwitcHub-8mi
    20
    70
    Fast SwitcHub-8mi
    20
    70
    10
    Collision
    50
    10
    Collision
    50
    10
    Collision
    50
    5
    35
    5
    35
    5
    35
    20
    20
    20
    1
    1
    1
    100
    M
    10
    100
    M
    10
    100
    M
    10
    Tx/Rx
    5
    Tx/Rx
    5
    Tx/Rx
    5
    Full Duplex
    PWR
    1
    Full Duplex
    PWR
    1
    Full Duplex
    PWR
    1
    Util%
    Util%
    Util%
    Demo
    Demo
    Demo
    Config
    Link
    SNMP
    Config
    Link
    SNMP
    Config
    Link
    SNMP
    Filter%
    Filter%
    Filter%
    Status
    Forward%
    Status
    Forward%
    Status
    Forward%
    Select/Link
    Select/Link
    Select/Link
    Diag
    Diag
    Diag
    10
    M/ 100 M
    10
    M/ 100 M
    10
    M/ 100 M
    Full/Half
    Full/Half
    Full/Half
    Shortest Path Tree & Routing Table
    Network1
    R1
    20
    Router1
    R2
    N2
    N1
    30
    Network2
    N3
    Cost = 20
    Router2
    R3
    Network3
    N4
    Cost = 30
    Router3
    Routing Table for R1
    Destination Next Hop Metric
    N1 Direct 0
    N2 Direct 0
    N3 R2 20
    N4 R2 50
    Network4
  • 43. 43
    Rate %
    Rate %
    Rate %
    1 2 3 4 5 6 7 8
    1 2 3 4 5 6 7 8
    1 2 3 4 5 6 7 8
    30+
    90+
    1
    X 2X 3X 4X 5X 6X 7X 8MDI-X-or-8MDI
    30+
    90+
    1
    X 2X 3X 4X 5X 6X 7X 8MDI-X-or-8MDI
    30+
    90+
    1
    X 2X 3X 4X 5X 6X 7X 8MDI-X-or-8MDI
    Fast SwitcHub-8mi
    20
    70
    Fast SwitcHub-8mi
    20
    70
    Fast SwitcHub-8mi
    20
    70
    10
    Collision
    50
    10
    Collision
    50
    10
    Collision
    50
    5
    35
    5
    35
    5
    35
    20
    20
    20
    1
    1
    1
    100
    M
    10
    100
    M
    10
    100
    M
    10
    Tx/Rx
    5
    Tx/Rx
    5
    Tx/Rx
    5
    Full Duplex
    PWR
    1
    Full Duplex
    PWR
    1
    Full Duplex
    PWR
    1
    Util%
    Util%
    Util%
    Demo
    Demo
    Demo
    Config
    Link
    SNMP
    Config
    Link
    SNMP
    Config
    Link
    SNMP
    Filter%
    Filter%
    Filter%
    Status
    Forward%
    Status
    Forward%
    Status
    Forward%
    Select/Link
    Select/Link
    Select/Link
    Diag
    Diag
    Diag
    10
    M/ 100 M
    10
    M/ 100 M
    10
    M/ 100 M
    Full/Half
    Full/Half
    Full/Half
    Shortest Path Tree & Routing Table(Con’t)
    R2
    Network1
    Router1
    20
    30
    R1
    N3
    N2
    R3
    Network2
    Cost = 20
    Router2
    N4
    N1
    Network3
    Cost = 30
    Routing Table for R2
    Router3
    Destination Next Hop Metric
    N1 R1 20
    N2 Direct 0
    N3 Direct 0
    N4 R3 30
    Network4
  • 44. 44
    Rate %
    Rate %
    Rate %
    1 2 3 4 5 6 7 8
    1 2 3 4 5 6 7 8
    1 2 3 4 5 6 7 8
    30+
    90+
    1
    X 2X 3X 4X 5X 6X 7X 8MDI-X-or-8MDI
    30+
    90+
    1
    X 2X 3X 4X 5X 6X 7X 8MDI-X-or-8MDI
    30+
    90+
    1
    X 2X 3X 4X 5X 6X 7X 8MDI-X-or-8MDI
    Fast SwitcHub-8mi
    20
    70
    Fast SwitcHub-8mi
    20
    70
    Fast SwitcHub-8mi
    20
    70
    10
    Collision
    50
    10
    Collision
    50
    10
    Collision
    50
    5
    35
    5
    35
    5
    35
    20
    20
    20
    1
    1
    1
    100
    M
    10
    100
    M
    10
    100
    M
    10
    Tx/Rx
    5
    Tx/Rx
    5
    Tx/Rx
    5
    Full Duplex
    PWR
    1
    Full Duplex
    PWR
    1
    Full Duplex
    PWR
    1
    Util%
    Util%
    Util%
    Demo
    Demo
    Demo
    Config
    Link
    SNMP
    Config
    Link
    SNMP
    Config
    Link
    SNMP
    Filter%
    Filter%
    Filter%
    Status
    Forward%
    Status
    Forward%
    Status
    Forward%
    Select/Link
    Select/Link
    Select/Link
    Diag
    Diag
    Diag
    10
    M/ 100 M
    10
    M/ 100 M
    10
    M/ 100 M
    Full/Half
    Full/Half
    Full/Half
    Shortest Path Tree & Routing Table(Con’t)
    R3
    Network1
    Router1
    30
    R2
    20
    N3
    N4
    Network2
    Cost = 20
    Router2
    R1
    N2
    N1
    Network3
    Cost = 30
    Routing Table for R3
    Router3
    Destination Next Hop Metric
    N1 R2 50
    N2 R2 30
    N3 Direct 0
    N4 Direct 0
    Network4
  • 45. 45
    When Router re-calculate SPF tree?
    • When an intra-area route has been added, deleted or modified
    • 46. When an inter-area route has been added, deleted or modified
    • 47. When a router has an interface that becomes active in an area
    • 48. When a router’s virtual link changes
    • 49. When an external route changes
    • 50. When a router that was an ASBR is no longer an ASBR
  • 46
    References & Standard
    II. Terminology
    OSPF Format
    IV. OSPF Algorithm
  • 51. 47
    Features of Link-State Algorithm
    OSPF uses a Link-State Routing Algorithm.
    All routers maintain identical routing tables.
    The database of each router describes complete topology of
    the router’s domain. The router’s domain may be the
    entire AS, or an area within the AS.
    Each router uses the database to calculate a set of shortest
    paths to all destinations. The routing table is built from
    these calculation.
  • 52. 48
    Rate %
    Rate %
    Rate %
    Rate %
    Rate %
    1 2 3 4 5 6 7 8
    1 2 3 4 5 6 7 8
    1 2 3 4 5 6 7 8
    1 2 3 4 5 6 7 8
    1 2 3 4 5 6 7 8
    30+
    90+
    1
    X 2X 3X 4X 5X 6X 7X 8MDI-X-or-MDI
    30+
    90+
    1
    X 2X 3X 4X 5X 6X 7X 8MDI-X-or-MDI
    30+
    90+
    1
    X 2X 3X 4X 5X 6X 7X 8MDI-X-or-MDI
    30+
    30+
    90+
    1
    X 2X 3X 4X 5X 6X 7X 8MDI-X-or-MDI
    90+
    1
    X 2X 3X 4X 5X 6X 7X 8MDI-X-or-MDI
    Fast SwitcHub-8mi
    20
    70
    Fast SwitcHub-8mi
    20
    70
    Fast SwitcHub-8mi
    20
    70
    Fast SwitcHub-8mi
    20
    70
    Fast SwitcHub-8mi
    20
    70
    10
    Collision
    50
    10
    Collision
    50
    10
    Collision
    50
    10
    50
    10
    Collision
    50
    Collision
    5
    35
    5
    35
    5
    35
    5
    35
    5
    35
    20
    20
    20
    20
    20
    1
    1
    1
    1
    1
    100
    M
    10
    100
    M
    10
    100
    M
    10
    100
    M
    10
    100
    M
    10
    5
    Tx/Rx
    5
    5
    Tx/Rx
    Tx/Rx
    5
    Tx/Rx
    5
    Tx/Rx
    Util%
    Util%
    Util%
    Util%
    Util%
    Full Duplex
    PWR
    1
    Full Duplex
    PWR
    1
    Full Duplex
    PWR
    1
    Full Duplex
    PWR
    1
    Full Duplex
    PWR
    1
    Demo
    Demo
    SNMP
    Demo
    SNMP
    Demo
    Demo
    Status
    Filter%
    SNMP
    Forward%
    Filter%
    Forward%
    Filter%
    Filter%
    SNMP
    Status
    Filter%
    SNMP
    Forward%
    Forward%
    Config
    Link
    Config
    Link
    Config
    Link
    Config
    Link
    Forward%
    Config
    Link
    Status
    Status
    Status
    Select/Link
    Select/Link
    Select/Link
    Select/Link
    Select/Link
    Diag
    Diag
    Diag
    Diag
    Diag
    Full/Half
    Full/Half
    Full/Half
    Full/Half
    Full/Half
    10
    M/ 100 M
    10
    M/ 100 M
    10
    M/ 100 M
    10
    M/ 100 M
    10
    M/ 100 M
    Example of Link-State Algorithm
    Area 2
    Area 1
    N9
    N10
    ASBR
    N1
    N3
    AS 200
    Router
    Router
    AS 100
    Area Border Router
    (ABR)
    N6
    N7
    N8
    N4
    N2
    Router
    ASBR
    N5
    AS 300
  • 53. 49
    Basic Operation of Link-State Algorithm
    Exchange of Routing Information
    - Each router periodically sends out a description of its connections to
    its neighbors.
    - Routers are neighbors if they are directly connected via a common network.
    - A router sends the LSA to each of its neighbors. The LSA includes a listing
    of all interfaces & configured “cost” of each link and each configured cost-
    TOS pairing.
    2) Routing Area
    - The LSA is flooded throughout the router’s domain. The router’s domain
    may be entire AS, or limited area within the AS.
    - Areas are configured by assigning an area_ID for each router interface.
    If the area_ID is identical for all ports on a router, then the router is
    contained in a single area.
    3) Link-State Database
    - Each router in the domain maintains an identical, synchronized copy of a
    database composed of this link-state information.
    - Router belonging to multiple areas maintain a separate Link_State database
    for each area.
  • 54. 50
    Basic Operation of Link-State Algorithm(Con’t)
    4) Shortest Path Tree
    - Each router runs an algorithm on the database used to create a shortest-path
    tree. A different shortest-path tree is constructed for each TOS support.
    - The shortest-path tree contains the shortest path to every router and
    every network that other routers can reach. The router performing the
    calculation places itself at the root of each tree.
    5) Routing Table
    - The resulting shortest-path trees determine total cost to the destination
    network and next hop router. The shortest-path trees are used as the
    basis of creating the routing table. A different routing table is created
    for each TOS.
    6) Optional TOS Support
    - OSPF allows the network administrator to configure OSPF routers to
    calculate/use only single routing table(TOS 0 table).
    - A router desiring to calculate/use single table informs its peers by resetting
    the TOS-capable bit in the option field of the router’s links advertisement.
    If a route cannot be found that uses a non-zero TOS value, the traffic is
    forwarded along the TOS 0 route.
  • 55. 51
    How to Forward datagrams
    The forwarding process uses routing tables to route datagrams.
    The destination network number is extracted from an incoming
    datagram.
    The TOS field is examined for information pointing to
    The datagram is forwarded towards its final destination.
    Datagram having same final destination may be routed along
    different paths based on the TOS requested by the source station.
  • 56. 52
    Initial Link-State Database Synchronization
    A pair of routers attempting to become adjacent send a summary of their Link-
    State databases to one another. This summary is called a
    “Database Description Packet”.
    The Database Description Packet consists of a list of abbreviated link-state
    Advertisements(LSA).
    Based on the Database Description Packet received from its neighbor, each
    router builds a list of requests for LSAs, required to update its own database.
    A router builds this list by checking its link-state database for a copy of each
    LSA received in the summary. If the router doesn’t have a particular LSA
    in its link-state database, or determines that its neighbor has a more recent
    version of A LSA, that LSA is added to the request list.
    Each router sends this list in a Link-State Request packet to its neighbor.
    Each router responds to a received Link-State Request packet with a Link-
    State Update packet containing the requested LSAs. The neighbors become
    Fully adjacent when they have received all requested LSAs.
    Once the routers become fully adjacent, they run the SPF algorithm on the data-
    Base, add OSPF routes to their routing tables, and periodically exchange LSAs.
  • 57. 53
    Maintain Link-State Database Synchronization
    Flooding Procedure
    When an LSA is flooded, it is passed from a router to an adjacent router until
    It has been distributed throughout the routing domain.
    Each router determines, individually, whether the LSA should be passed to
    An adjacent neighbor. More details are described in Section 13, 13.3 & 13.4 of
    RFC 2328.
    Reliable Updates
    Reliability is accomplished by requiring that both the receipt and transfer of
    an LSA be acknowledged by adjacent router. In the absence of such an
    acknowledgment, the source router retransmits the LSA until it is acknowledged.
    - Each router originates a router-LSA.
    - Area Border routers originate a single summary-LSA for each known inter-
    area destination.
    - AS Boundary routers originate a single As-external-LSA for each known
    AS external destination.
    (Ten events can cause new instance of an LSA to be originated)
  • 58. 54
    Maintain LS Database Synchronization(Con’t)
    Link-State Age
    An LSA’s age field is periodically incremented while residing in a router’s link-
    state database. It is possible for an LSA to reach an age where it is no longer used
    in the flooding procedure, and must be flushed from the link-state database. If
    There’s a change in the link-state database, a new shortest-path tree is constructed
    and the routing table is updated.
    Link-State Sequence Numbers
    A common event is for an LSA to be replaced by the receipt of more recent LSA
    from its adjacent neighbor. Each LSA contains 32-bit sequence number used
    by OSPF routers to detect old, or duplicate LSAs. A linearly ordered
    sequence number is used for LSA identification. All routers keep their link-state
    databases synchronized by ageing LSAs in their database, and updating with
    new incoming LSAs.