1. OSPF

2. OSPF Topics OSPF features OSPF areas OSPF operation
OSPF on multi-access broadcast networks
OSPF LSAs
OSPF metric
OSPF load balancing
OSPF configuration
OSPF authentication

3. OSPF Features Fast convergence Granular metric Supports VLSM
Hierarchical design/route summarization
Uses multicasts
Supports authentication

4. OSPF Overview Link State Routing Protocol
Router advertises the state of it’s links (connections to networks)
Forms neighbor relationships (adjacencies) before exchanging routing information
Neighbor information maintained in neighbor table
Advertisements flooded within area

5. OSPF Areas
Areas restrict certain advertisements so that in a large environment not all advertisements flooded everywhere
Every AS should have an area 0 - Backbone
All other areas connect to Backbone
Summarization can be configured at area borders – further reduces advertisements
Router with interfaces in two or more areas is Area Border Router (ABR)

6. OSPF Operation First forms adjacencies Floods LSAs
requires that certain criteria be met
Floods LSAs
sent to
collected in LSA database
Calculates shortest routes - Dykstra
Maintains adjacencies with hellos
Sends updates only when changes occur

7. OSPF Operation Multi-access network
Designated Router first determined
First router active becomes DR
Second router active becomes BDR
All others become ODR
If DR goes down, BDR becomes DR and new BDR elected
Router with highest priority wins. If tied,
Router with highest OSPF Router ID

8. OSPF Priority and RID OSPF priority set on per-interface basis
default priority is 1
command is “ip ospf priority <number>”
priority of 0 means can’t become DR
OSPF Router ID – set 3 possible ways
Router ID command. If not set then -
Highest loopback address. If not set then –
Highest active IP address

9. OSPF LSAs LSA Type 1 – LSA Type 2 – LSA Type 3 – sent by every router
advertises it’s OSPF interfaces
LSA Type 2 –
sent by DR
contains a list of all routers on subnet
LSA Type 3 –
sent by ABR into other areas
advertises area networks and metrics

10. OSPF Metric 10 to the 8th/Bandwidth Referred to as cost 19.2 kbps
Source
Destination
Purpose: This figure presents the IGRP metric with its five possible components.
Emphasize : Bandwidth and delay are the two metrics that are most commonly used. They also comprise the default metric.
Note:
Changing IGRP metrics can have great impact on network performance.
Describe the IGRP 24-bit metric field:
Bandwidth—Minimum bandwidth on the route in kilobits per second.
Delay—Route delay in tens of microseconds.
Reliability—Likelihood of successful packet transmission expressed as an integer from 0 to 255.
Loading—Effective bandwidth of path.
MTU—Minimum MTU in path, expressed in bytes.
The following equation calculates the metric. It is presented for instructors and is not required to be taught.
metric = [k1 * bandwidth + (k2 * bandwidth) / (256 - load) + k3 * delay]
If k5 does not equal 0, an additional operation is done:
metric = metric * (k5/(reliability + k4))
The default constant values are k1=k3=1 and k2=k4=k5=0.
Again, if default values are set, metric = bandwidth + delay.
The constants (k1, k2, k3) can be changed using the metric weights command. Changes to the IGRP constant values should be made with great care.
10 to the 8th/Bandwidth
Referred to as cost

11. OSPF Load Balancing Maximum six equal paths – default 4 equal paths
New Route
Source
Initial Route
Purpose: The figure presents how IGRP load sharing improves throughput and increases reliability.
Emphasize: Only feasible paths can be used for IGRP load sharing.
Load-balancing methods vary according to the switching mode because the data structures for process switching, fast switching, and autonomous switching are all different. When process switching, the processor load balances packet by packet. When fast, autonomous, or silicon switching, load balancing is done destination by destination.
By default, the amount of variance is set to one, which results in equal-cost load balancing.
You can use the default-metric command to change the default metric.
Transition: The following pages describe how to configure the IGRP routing protocol.
Destination
Maximum six equal paths – default 4 equal paths

12. Configuring OSPF Router(config)#router ospf process-id
Defines OSPF as the IP routing protocol
Purpose: This figure explains how to use the router igrp and network commands to configure an IGRP process.
Emphasize: Note that the AS keyword is required for IGRP.
You can use multiple network commands to specify all networks that are to participate in the IGRP process. Only those networks specified will be published to other routers.
network network-number wild-card-mask area-id
Selects participating attached networks

13. OSPF Configuration Example A C
router ospf 50
network area 0
network area 0
Purpose: The figure shows how the IGRP commands operate on the example network.
Emphasize:
An administrator only specifies directly connected networks that he wishes to publish to other routers.
Without the network command, nothing is advertised. With a network command, the router will advertise every subnet within the Class A, B, or C network specified in the configuration.
router ospf 75
network area 0
network area 0

14. Verifying the Routing Protocol—OSPF A B C
Routing Protocol is "ospf 1"
Outgoing update filter list for all interfaces is not set
Incoming update filter list for all interfaces is not set
Router ID
Number of areas in this router is 1. 1 normal 0 stub 0 nssa
Maximum path: 4
Routing for Networks:
area 0
area 0
Routing Information Sources:
Gateway Distance Last Update
Distance: (default is 110)
Purpose: This figure shows how the show ip protocol command is used to monitor IGRP operation.
Emphasize: The command displays the routing protocols that are active on the router for IP. It also gives network and timer information. In this example, IGRP is displayed.
Point out the timing information.
Point out the list of networks for which the router is injecting routes.
Point out the administrative distance metric.

15. OSPF Show Commands show ip route ospf show ip ospf interfaces
show ip ospf neighbors
show ip ospf database