Link state routing protocols like OSPF calculate routes based on the full network topology and converge faster than distance vector protocols. OSPF uses the shortest path first algorithm to determine the best routes and shares link state advertisements between all routers to maintain a common view. It supports features like VLSM and CIDR and generally outperforms RIP. OSPF uses the designated router concept and hello packets to efficiently flood link state updates and detect neighbors.

1. CCNA 3 Week 2
Link State Protocols
OSPF

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Distance Vector vs Link State
Distance Vector
– Copies Routing Table to
neighbours
– Updates frequently
– Distance metrics e.g. hop count
– Views network from prospective
of neighbours
– Slow to converge
– Loops a problem
– Easy to configure
– Consumes bandwidth
Link State
– Uses shortest path
– Updates event triggered
– Sends link-state packets to all
– Common view of network
– Fast convergence
– Less vulnerable to loops
– Harder to Configure
– Requires more memory and
CPU power

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Link State Features
Router learn network info from ALL routers in a net
Routers multicast frequent hello packets
containing details of directly connected networks
Link-State Advertisments keep track of other
routers
Information used by router to calculate Shortest
Path to each network

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Example Topology

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Maintaining Routing Information
When a failure occurs, LSAs are multicast
– Routers forward multicasts on all ports except receiver
Each Link-State router updates its topology database with
new network state and recalculates
A link = an interface
Link-State info includes: IP Addr & subnet, net type,
connected routers
Best routes calculated from topology database and placed
in routing table

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Network View

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Advantages & Disadvantages
Advantages
– Fast convergence: changes
reported at once
– Robust against loops
– Routers all know topology
– Link-state packets
sequenced and aged
– Careful design minimises
link-state table size
Disadvantages
– Significant RAM and CPU
– Network design follows strict
rules
– Network admin must be
knowledgable (you guys!)
– Initial flooding impedes
performance

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Comparison
After initial flood, routing table only changes if net
conditions change
– The partial update only contains information about links that have
changed.
– Reduces network load, faster propagation of changes
Link-state protocols support CIDR and VLSM.
In fact, link-state protocols generally outperform distance
vector protocols on any size network.
– Not always implemented due to hardware requirements and
admin expertise

9. OSPF
Open Shortest Path First

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Introduction
Open non-proprietary standard protocol for TCP/IP
Widely used in industry
Preferable to RIP v1 or v2 because metrics are
better than simple hop count
Uses a hierarchical approach, with different areas
defined within a network

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OSPF Terminology
Link: an interface on a routerLink-state: The status of the link between two routersTopological Database: The status of all routers on the
network
Area: each router within an area has the same link-state
information
Cost: assigned to each link, based on bandwidthRouting Table: generated when routing algorithm runs on
topological database
Adjacency Database: List of routers communicating directlyDesignated Router (DR): Elected to represent all routers
on the network

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OSPF vs Distance Vector
Speed of convergence
Support for Variable Length Subnet Mask (VLSM)
Network size
Path selection
Grouping of members

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SPF Algorithm

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SPF Algorithm

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OSPF Network Types
OSPF interfaces automatically recognize 3 types
of networks:
– Broadcast multi-access, such as Ethernet
– Point-to-point networks
– Nonbroadcast multi-access (NBMA), e.g. Frame Relay
Not desirable for every router to be adjacent to
every other in a broadcast network – overhead
Pick Designated Router (and Backup)

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Use of Designated Router
Routers send their updates to the designated
router (Multicast to 224.0.0.6)
Designated Router updates all routers on network
using 224.0.0.5
As Designated Router is single point of failure,
Backup is also established

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Hello Packets
Small layer 3 packets
Sent every 10 seconds to multicast 224.0.0.5
Used to ensure neighbours still running
Protocol used in election of DR

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Protocol Steps
Use hello packets to announce selves
Develop adjacency relationships with neighbours
Use LSA and LSU packets to learn about topology
Calculate routing tables from topology
If changes occur, recalculate

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Configure
R1(config)# router ospf <proc_id>
R1(config-router)# network <addr> <wildcard>
area <area>

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Need to add loopback address
No active interfaces means router drops out of
OSPF
Logical loopback keeps process available
If present, loopback used as router ID
R1(config)# interface loopback 1
R1(config-if)# ip address <addr> <subnet>

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Link costs
OSPF uses 108
/bandwidth as link cost
Set bandwidth of interface if needed:
R1(config-if)# bandwidth 56
May be necessary if mixed network uses other
calculations
R1(config-if)# ip ospf cost <number>

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Authentication
Can require common password for updates
May be encrypted using MD5
See lab for details

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Timers
OSPF routers must use the same timer values
Router(config-if)#ip ospf hello-intervalseconds
Router(config-if)#ip ospf dead-intervalseconds

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Default Route
On the router connected to default connection:
Router(config)#ip route 0.0.0.0 0.0.0.0 [interface |
next-hop address ]
Must then instruct router to propagate default route
Router(config-router)#default-information
originate