1. Module 4 OSPF Overview and Configuration
LE STUDENT GUIDE Rev1.2_Oct99

2. Module Objectives Define OSPF, features, advantages over RIP
Describe OSPF LSA, format and Types
Define LSDB, Initial synchronization, database exchange & reliable flooding
Describe Routing Calculations, supported network type & Database Synchronization
Describe how to build OSPF Networks
Define the OSPF routing, areas, router types and virtual Links
Differentiate DR, BDR, Router Election
OSPF Configuration and Examples
Lab Exercises
Summary
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3. Define OSPF Dynamic Routing Protocol Link State Protocol
Employ a distributed database model
More efficient than RIP
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4. The OSPF Advantage
OSPF is an interior gateway protocol (IGP) that is more efficient than RIP.
Consumes fewer network resources
Highly scalable
Faster convergence
A more descriptive routing metric
Route load sharing
Greater security
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5. Link-state Protocol Employ a distributed database model.
Each router advertises a description of its local environment
interfaces
costs
neighbor information
Uses a single synchronized database for collecting advertisements (LSDB)
Routing table is derived from this database
Utilize a shortest-path first algorithm
OSPF is a Link-state routing protocol
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6. Basic Features of OSPF Hello Packets Link State Advertisements (LSA)
Link State Database
Reliable Flooding
Shortest Path First Routing Calculations
Areas and Inter-area Routing
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7. OSPF Hello Packet & Neighbor Discover
OSPF Hello packets are sent out all of a router’s interfaces to advertise itself to neighbor routers
A router learns about its neighbors when it receives neighbor router’s Hello packet
Hello packets are sent out every 10 seconds by default
If subsequent Hello packet is not received within 40 seconds, neighbor relationship is terminated
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8. OSPF Hello Packets
Will only be recognized by routers attached to the same subnet with same subnet mask
Contains information on parameters for
Hello Interval and
Router Dead Interval
This information is used by neighbor routers to agree on the communication variables
This allows an occasional lost Hello packet not to be interpreted as a link down condition.
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9. OSPF Hello Packets (cont.)
In a broadcast environment, it contains the OSPF router IDs of all routers the sender has heard up to the point of transmission
This reduces overhead of sending multiple Hellos
Ensure that the OSPF link is bi-directional
NOTE: An OSPF router will not forward data packets over a unidirectional link.
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10. Link State Advertisement (LSA)
Each OSPF router is responsible for describing its local piece of the routing topology through the transmission of link-state advertisements.
Every thirty minutes a router will -- even in the absence of any change, retransmit this self-originating data in the event it may have been lost or corrupted in a neighbor router’s tables.
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11. OSPF LSA Format All OSPF LSAs start with a 20-byte common header
This provides orderly updating and removal of LSAs and organization to the LSDB
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12. LSA Format - LS Age
Number of seconds since the LSA was originated normally mins.
If LSA reaches 30 minutes, originating router will refresh the LSA by flooding a new instance.
If LSA reaches 1 hour, it is deleted from the database.
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13. LSA Format - LS Type Classifies the LSA according to function Type 1
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14. LSA Format - Link State ID
A unique identification
Used to describe a router in the OSPF routing domain
Depends on the LS Type
Type 1, 2, 3, 4, 5 or 7
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15. LSA Format - Advertising Router
The originating router’s OSPF router ID
In practice, this is one of the router’s IP address
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16. LSA Format - LS Sequence Number
A linear sequence number
Used to compare a new LSA with an old LSA
The LSA instance having the larger LS Sequence Number is considered to be more recent.
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17. LSA Format - LS Checksum
Used to detect data corruption.
Does not include LS Age field
Derived using Fletcher checksum algorithm
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18. LSA Types Type Number Description 1 Router-LSAs 2 Network-LSAs
3 Summary-LSAs (IP network)
4 Summary-LSAs (ASBR)
5 AS external-LSAs
7 NSSA external-LSAs
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19. LS Type 1 - Router-LSAs Generated by each OSPF router
It describes the router’s set of active interfaces, its associated cost and any neighbor information
Link State ID is set to the router’s OSPF Router ID
Flooded throughout a single area only
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20. LS Type 2 - Network-LSAs Generated by OSPF Designated Routers (DRs)
Describes a network segment - i.e., broadcast domain along with the IDs of all currently attached routers.
Link State ID field lists the IP interface address of the DR
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21. LS Type 3 - Summary-LSAs (IP Network)
This originate from Area Border Routers (ABRs)
Supports hierarchical routing through the use of OSPF areas
Link State ID field is an IP network number
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22. LS Type 4 - Summary-LSAs (ASBR)
This originate from Area Border Routers (ABRs)
Similar to LS Type 3
Used when destination is an Autonomous System Boundary Router (ASBR)
The Link State ID is the AS boundary router’s OSPF Router ID
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23. LS Type 5 - AS-external-LSAs
Originated by AS boundary routers and describes destinations ex-ternal to the AS.
Link State ID field specify an IP network number
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24. LS Type 7 - NSSA external-LSAs
Allows the import of external routes that will not be advertised out of the NSSA
NSSA - Not So Stubby Area
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25. Router LSA Format - Link ID
Originating router’s link information follows the LSA header.
There are four Link IDs determined by Link Type.
Type 1 Neighboring router’s Router ID
Three of this Link ID are relevant in a broadcast network
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26. Router LSA Format - Link Data
For transit and Virtual Links
specifies the IP address of associated router interface.
For stub networks
Specifies the stub network mask
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27. Router LSA Format - Metric
The cost of using this router link.
A user-configurable value from ,535
The larger the metric, the less likely (more expensive) data will be routed over that particular link.
Connections to STUB networks are allowed to advertise a metric of zero.
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28. Link-State Database (LSDB)
The collection of all OSPF LSAs received
Each OSPF router has an identical LSDB
Gives complete description of the network:
routers
network segments
interconnectivity (how it is interconnected)
LSDBs are exchanged between neighboring routers soon after routers discover each other
Maintained through a procedure called reliable flooding
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29. LSDB Initial Synchronization
When two neighbors first start communicating, they must synchronize their databases before forwarding traffic over their shared link to prevent routing loops from occurring.
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30. OSPF-specified Database Exchange
Procedure used by the routers to synchronize their databases once the hello protocol determines a bi-directional connection between router neighbors.
During synchronization, the neighbor routers do three things:
Forward current LSA headers
Compares the header received to the LSDB
Request the full LSA for new or newer headers
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31. Example LSDB Initial Synchronization
Switches A thru F are in a stable OSPF network and have fully synchronized databases
OSPF is restarted on Switch F, forcing database synchronization with switch A.
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32. Example LSDB Initial Synchronization
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33. OSPF Database - Reliable Flooding
LSA Updates are periodically generated by a router wishing to update a self-originated LSA because:
The router’s local state may have changed
The router wants to delete one of its self-originated LSAs
Used to propagate LSA Updates throughout the routing domain
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34. Reliable Flooding - What Happens
A router will generate a Link-state Update packet containing one or more LSAs
Update is forwarded out all interfaces.
Neighbor router receives the Update and compares the LSAs with the LSDB
More recent LSAs are installed in LSDB
Acknowledgement is sent back to originating route
New Link-state Update containing the LSA is sent out all interfaces except receiving one.
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35. OSPF Routing Calculations
With router LSDBs synchronized for all routers in routing domain
The router will use Dijkstra’s Shortest Path First algorithm
This allows calculation of shortest paths to all destinations
Routing table is constructed from the calculations and includes
network destinations
associated costs
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36. OSPF Routing Calculations
Every link carries an associated cost.
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37. OSPF Routing Calculations
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38. OSPF Routing Calculations
Applying Dijkstra’s SPF algorithm, Switch C’s routing table would incorporate the highlighted links
Note that Switch A will never talk directly to Switch B as long as the links thru Switch C remain stable.
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39. OSPF Routing Calculations
Note how changing a link cost affects the route calculation for the shortest path
With this configuration, Switch C now has two paths of equal cost to communicate with Switch J.
Communication with Switch B is no longer direct, but must routed thru Switch A.
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40. OSPF Network Types Point-to-Point networks
Serial lines
Non-broadcast Multi-access (NBMA) networks
X.25, ATM
Point-to-Multipoint networks
Frame Relay
Broadcast networks
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41. OSPF Network Type - Broadcast Networks
A network with more than two attached devices
Has the ability to address a single physical message to all of the attached devices (broadcast)
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42. OSPF Network Type - Broadcast Networks
Only network type supported by Extreme switches
Other Network Types are for WAN use
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43. Broadcast Networks Terminology
DR - Designated Router
BDR - Backup DR
DR and BDR Election
Network LSAs
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44. Broadcast Networks - Designated Router
Every broadcast network has a Designated Router (DR) and a Backup Designated Router (BDR)
Each router on the network exchanges link state information only with the DR and BDR.
This information is used to maintain database synchronization between the DR and neighbor routers
This reduces the amount of traffic otherwise consumed by routing protocol traffic
Only a DR generates a LS Type 2 - Network-LSAs
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45. DR and BDR Election
First OSPF router on an IP subnet always becomes the DR
Second OSPF router always becomes BDR
If DR or BDR fail, the OSPF router with the highest Router Priority will replace the BDR
If two OSPF routers have same Router Priority, then the OSPF Router ID will break the tie
A Router Priority of 0 will prevent an OSPF router from ever being elected as DR or BDR
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46. Database Synchronization
An OSPF router will send its Link State Update (LSU) to the DR and BDR
The destination IP address for the LSU will be multicast address (All DRouters).
The DR will then flood the update to all OSPF routers
The destination IP address for the LSU will be multicast address (All OSPFRouters).
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47. Representing Broadcast Subnet in LSDB
If an OSPF router included all known routers on a common subnet in its router-LSA, there would be n*(n-1) links in the OSPF database.
By using a new LSA type, the Network-LSA, to represent the broadcast subnet, the number of links is reduced from n*(n-1) to n*2.
Each network LSA has a link to every router-LSA, and every router-LSA has a link to the broadcast subnet’s network-LSA.
DR maintains the network-LSA
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48. Type 2: Network LSAs
Created in order to reduce the number of links in each router’s resulting LSDB
Describes the subnet, all routers on that network DR identity
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49. Type 2: Network LSAs
The network-LSA helps in database synchronization, since a router having a router-LSA with a link to the network-LSA and vice-versa is known to have a database synchronized with the Designated Router.
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50. Building OSPF Networks
Hierarchical Routing
OSPF Routing Hierarchy
OSPF Areas
OSPF Types of Routers
Virtual Links
CLI Commands for OSPF Configuration
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51. Hierarchical Routing
OSPF implements a two-level hierarchical routing scheme
Saves router memory consumed by the routing table
Saves router resources when computing the routing table
Saves link bandwidth when distributing routing data
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52. OSPF Areas Defined
The OSPF hierarchy is maintained through the deployment of OSPF areas
Each OSPF area is identified by a 32-bit Area ID
Each area consists of a collection of one or more network segments interconnected by routers
An area has its own LSDB consisting of router-LSAs and network-LSAs
These LSAs describe area’s topology
Routing with an area is flat.
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53. Area’s Router-LSA and Network-LSA
Not flooded beyond the area’s borders
Detailed knowledge of area’s topology is hidden from all other areas in the parent Autonomous System.
Area “A” does not know the internal topology of Area “B” and vice versa.
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54. Area Border Router (ABR)
Manages inter-area communication
Attached to two or more areas, running multiple copies of the basic algorithm
Maintain LSDBs for each attached area and an additional copy for the backbone
Condense the topological information of their attached areas into Type 3: Summary-LSAs for distribution to the backbone.
The backbone in turn distributes the information to the other areas.
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55. Area Border Router (ABR) cont.
ABRs can be configured to aggregate some or all of the networks within its dependent area into a single, summary network address with a less discreet network mask.
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56. Types of OSPF Areas Normal Area Stub Areas Not-So-Stubby-Areas (NSSA)
Area 0 (Backbone)
Stub Areas
Not-So-Stubby-Areas (NSSA)
Virtual Links
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57. Normal Area An area which is not a:
Stub area
NSSA
Can be placed anywhere within the OSPF routing domain
Support ASBRs
Support Virtual Links
Can distribute external routes
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58. Area 0 (Backbone) Is a Normal Area
Responsible for distributing routing information between areas
Consist of
all area border routers
networks not contained in any other area
and their attached routers
Has an Area ID of
Only one backbone area per AS
All areas are required to attach directly to the OSPF backbone area
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59. Stub Area For routers with limited resources
reduce memory and computation requirements
The LSDB is kept as small as possible
External route information is not distributed
Will use default routes to ABRs instead
Does not support ASBRs
Does not support virtual links
Appears to lie on the edge of an OSPF domain (AS) in configuration
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60. Not-So-Stubby-Area (NSSA)
Similar to existing OSPF Stub Area with two additional capabilities
External routes originating from ASBR connect- ed to NSSA can be advertised within
External routes originating from NSSA can be propagated to other areas, including the backbone.
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61. OSPF Defines Three types of Routers
Internal Router (IR)
Has all of its interfaces within the same area
Area Border Router (ABR)
Autonomous System Boundary Router (ASBR)
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62. OSPF Hierarchical Components OSPF Router Types Displayed
Internet
ABR
ABR
ASBR
ABR
*All other routers that are not labeled ASBR or ABR are Internal Routers
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63. Virtual Links
Used when a new area is introduced that does not have a direct physical attachment to the backbone
Provides a logical path between the ABR of the disconnected area and the backbone
A virtual link is established between two ABRs that have a common area, with one ABR connected to the backbone
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64. OSPF Configuration Example
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65. Configuring OSPF Must have a unique router ID for each switch
Recommend manually setting the router ID of the switches participating in OSPF
Simplifies router management
Prevents corruption of virtual links
Do not use as a router ID
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66. OSPF CLI Commands enable/disable ospf config ospf add/delete vlan
create/delete ospf area
config ospf vlan area
show ospf area
show ospf interfaces
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67. CLI Command - enable/disable ospf
enable ospf
disable ospf
Enable or disable OSPF for the whole router. Default is disabled.
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68. CLI Command - config ospf add/del vlan
config ospf add vlan [<name> | all]
config ospf delete vlan [<name> | all]
Enables or disables OSPF on one or all VLANs (router interfaces).
The default setting is disabled.
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69. CLI Command - create/del ospf area
create ospf area <areaid>
delete ospf area [<areaid> | all]
Create or delete an OSPF area. Area does not need to be created. It exists by default. It also cannot be deleted.
Once an OSPF area is removed, the associated OSPF area and OSPF interface information will also be removed.
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70. CLI Command - config ospf vlan area
config ospf vlan <name> area <areaid>
Associates a VLAN (router interface) with/from an OSPF area. The area must already be defined. All router interfaces must have an associated OSPF area.
By default, all router interfaces are associated with area (backbone).
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71. show ospf area {<areaid>}
CLI Command - show ospf
show ospf
Displays global OSPF information.
show ospf area {<areaid>}
Displays information about a particular OSPF area, or all OSPF areas.
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72. Example: show ospf area
OSPF_Switch4: show ospf area
Area: Stub: FALSE Rtr Id:
Spf Runs: 48 Num ABR: 6 Num ASBR: 0 Num LSA: 43 LSA Chksum:0x170863
Interfaces:
IP addr Ospf State DR IP addr BDR IP addr
/24 E DOWN
/24 E BDR
Inter-Area route Filter: None
External route Filter: None
Configured Address Ranges:
Area: Stub: FALSE Rtr Id:
Spf Runs: 21 Num ABR: 2 Num ASBR: 0 Num LSA: 35 LSA Chksum:0x16c3de
/24 E DR
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73. CLI Command - show ospf interfaces
show ospf interfaces {vlan <name> | area <areaid>|all}
Displays information about one or all OSPF area, or all OSPF areas.
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74. Example: show ospf interface
Summit4: sh ospf interface
Interface(rif4): /24 Vlan: norm151 Ospf: ENABLED Router: ENABLED
AreaId: RtId: Cost: 1 Pri: 1 Transit Delay: 1
Hello Interval: 10s Rtr Dead Time: 40s Retransmit Interval: 5s
Authentication: NONE
State: BDR Number of events: 1
DR RtId: DR IP addr: BDR IP addr:
Neighbours:
RtrId: IpAddr: Pri: 1
State: FULL Dr: BDR: Dead Time: 7
RtrId: IpAddr: Pri: 1
State: FULL Dr: BDR: Dead Time: 6
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75. Laboratory Exercise Lab4 - OSPF Configuration I
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76. Lab4 - Network Topology
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77. More OSPF Configuration
config ospf add virtual-link
config ospf delete virtual-link
show ospf virtual-link
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78. CLI Command - config ospf add virtual-link
config ospf add virtual-link <routerid> <areaid>
config ospf delete virtual-link <routerid> <areaid>
Adds or deletes a virtual link connected to another area border router (ABR).
Specify the following:
routerid -- Far-end router ID.
Areaid -- Transit area used for connecting the two end-points. The transit area cannot be the backbone ( ).
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79. CLI Command - show ospf virtual-link
show ospf virtual-link {<areaid><routerid> | all}
Displays virtual link information about a particular router or all routers. Default is all. Contains:
Area ID and neighbor router ID
Receive interval, transit delay, Hello interval, and dead interval.
Authentication configuration
Virtual link type and number of events
Neighbor router ID, IP address, and priority
Link state, DR ID, BDR ID, and dead time
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80. Example: show ospf virtual-link
OSPF_LAB_2:19 # show ospf virtual-link
AreaId: NbrRouterId:
Rxmit Interval: 5 Transit Delay: 1 Hello Interval: 10 Dead Interval: 40
Auth Type: NONE
State: P2P Number of Events: 3
RtrId: IpAddr: Pri: 1
State: FULL Dr: BDR: Dead Time: 9
AreaId: NbrRouterId:
State: P2P Number of Events: 1
RtrId: IpAddr: Pri: 1
State: FULL Dr: BDR: Dead Time: 7
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81. Laboratory Exercise Lab5 - OSPF Configuration II
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82. Lab5 - Network Topology 1
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83. Lab5 - Network Topology 2
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84. More OSPF Configuration - Areas
config ospf area normal
config ospf area stub
config ospf area nssa
show ospf
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85. CLI Command - config ospf area normal
config ospf area <areaid> normal
Configure an OSPF area as a normal area
Default area type is normal
Area can only be normal
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86. CLI Command - config ospf area stub
config ospf area <areaid> stub [summary|nosummary] stub-default-cost <cost>
Configures an OSPF area as a stub area.
Stub-default-cost is the the cost of the default summary-LSA that the router should advertise into the area
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87. CLI Command - config ospf area nssa
config ospf area <areaid> nssa [summary|nosummary] stub-default-cost <cost> {translate}
Configures an OSPF area as a NSSA.
Stub-default-cost is the the cost of the default summary-LSA that the router should advertise into the area
Translate option: determines whether type 7 LSAs are translated into type 5 LSAs.
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88. CLI Command - show ospf show ospf Display global OSPF information:
Router ID
OSPF state
Is router an autonomous system boundary router (Y/N)?
Is router an area border router (Y/N)?
Number of external LSAs processed
External LSA checksum
Number of originating new LSAs
Number of received new LSAs
Shortest Path First hold time
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89. Example: show ospf Summit4: show ospf
Router Id OSPF ASBR ABR ExtLSA ExtLSACsum OrigNewLSA RxNewLSA SpfHoldTime
E NO YES x
RouterId Selection: User Configured Export Static: Disabled
Export Static: Disabled
Export Rip: Disabled
ASBR route Filter: None
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90. Laboratory Exercise OSPF Lab6 Utilizing the CLI Commands:
config ospf area normal
config ospf area stub
show ospf
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91. Lab6 - Network Topology
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92. More OSPF Configuration Commands
config ospf routeid
config ospf area add range
config ospf vlan area cost
config ospf vlan area priority
*enable iproute sharing
enable/disable ospf exportstatic type
config ospf vlan area virtual link authentication
config ospf vlan area virtual link timer
config ospf spf-hold-time
show ospf lsdb
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93. CLI Command - config ospf area add range
config ospf area <areaid> add range <ipaddress> <mask> [advertise | noadvertise] {type 3|type 7}
config ospf area <areaid> delete range <ipaddress> <mask>
Configures or deletes a range of IP addresses in an OSPF area.
On add, and if advertised, the range is exported as a single LSA by the ABR.
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94. CLI Command - config ospf … cost
config ospf [vlan <name> | area <areaid> | all] cost <number>
Configures the cost (metric) of one or all interface(s).
The default cost of an interface is 1.
The maximum cost is 65,535.
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95. CLI Command - config ospf … priority
config ospf [vlan <name> | area <areaid> | all] priority <number>
Configures the priority used in the designated router-election algorithm for one or all IP interface(s) or for all the interfaces within the area.
The range is
The Default priority setting is 1
A value of 0 disqualifies the router from election
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96. CLI Command - enable iproute sharing
Enables load sharing if multiple routes to the same destination are available (equal cost multipath)
Only paths with the same lowest cost are shared.
The default setting is disabled.
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97. CLI Command - enable/disable ospf exportstatic
enable ospf export static cost {<metric>} [ase-type-1|ase-type-2] {tag <number>}
disable ospf export static
Enable/disables the distribution of static routes into the OSPF domain.
The default tag number is 0.
The default setting is disabled.
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98. CLI Command - config ospf … authentication
config ospf [vlan <name> | area <areaid> | virtual-link <routerid> <areaid>] authentication
[simple-password <password> | md5 <md5_key_id> <md5_key> | none]
Configure OSPF authentication information for one interface or all the interfaces in an AREA.
When the OSPF AREA ID is specified, then the authentication information is applied to all the OSPF interfaces within the area.
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99. CLI Command - config ospf … timer
config ospf [vlan <name> | area <areaid> | virtual-link <routerid>] timer <retransmission_interval> <transmission_delay> <hello_interval> <dead_interval>
Configures the timers for one interface or all the interfaces in the same OSPF area.
The following default, min, and max values (in seconds) are used:
Variable Default Min Max
RETRANSMISSION
DELAY
HELLO
DEAD INTERVAL
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100. CLI Command - config ospf spf-hold-time
config ospf spf-hold-time {<seconds>}
Configures the minimum number of seconds between Shortest Path First (SPF) recalculations.
The default setting is 3 seconds.
LE STUDENT GUIDE Rev1.2_Oct99

101. CLI Command - show ospf lsdb
show ospf lsdb {detail} area [<areaid> | all] [router | network | summary-net |
summary-asb | as_external | external-type 7 | all}
Displays a table of the current LSDB.
The user can filter the display using either area ID, the remote router's router ID, or the link state ID.
Default is all with no detail.
If detail is specified, each entry includes complete LSA information
LE STUDENT GUIDE Rev1.2_Oct99

102. Example: show ospf lsdb
OSPF_LAB_2:18 # show ospf lsdb area all all
Router LSAs for area
Link State ID Adv Router Seq# Age Chksum #Links
x d xafc6 1
x x835f 3
x800040a xf018 1
x x6e4c 1
x800001ab x7f83 3
x c x
Network LSAs for area
Link State ID Adv Router Seq# Age Chksum
x xb14f
Summary LSAs for area
x x dfc
x b xd109
x x22f2
x xc698
LE STUDENT GUIDE Rev1.2_Oct99

103. Laboratory Exercise
OSPF Lab7
LE STUDENT GUIDE Rev1.2_Oct99

104. Lab7 - Network Topology
LE STUDENT GUIDE Rev1.2_Oct99

105. Summary Define OSPF, features, advantages over RIP
Describe OSPF LSA, format and Types
Define LSDB, Initial synchronization, database exchange & reliable flooding
Describe Routing Calculations, supported network type & Database Synchronization
Describe how to build OSPF Networks
Define the OSPF routing, areas, router types and virtual Links
Differentiate DR, BDR, Router Election
OSPF Configuration and Examples
Lab Exercises
LE STUDENT GUIDE Rev1.2_Oct99