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Switching Basics and Intermediate
Routing CCNA 3
Chapter 2
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Link-State Routing Overview
Maintaining Routing Information Via Link States
• Link-state routing algorithms, also known
as shortest path first (SPF) algorithms,
build a complex database of topology
information
– The algorithms compute the shortest path
between nodes
– Maintains full knowledge of distant routers
and how they interconnect
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Link-State Routing Overview
Maintaining Routing Information Via Link States
• Link-state routing uses link-state advertisements
(LSAs)
– A basic building block that describes a router’s local
topology and is distributed to all other routers in the
area
• Link-state routing uses a topological database
(or link-state database)
– The set of all links learned from the flooding of LSAs
– Synchronized with all other routers in the area
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Link-State Routing Overview
Maintaining Routing Information Via Link States
• OSPF and Intermediate System-to-Intermediate
System (IS-IS) are link-state routing protocols
– Collect routing information from all other routers in the
area
– Each router calculates all the best paths to all
destinations in the network
– Because each router calculates best paths, they are
less likely to propagate incorrect information learned
from a neighboring router
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Link-State Routing Overview
Maintaining Routing Information Via Link States
• Link-state routing protocols were designed
to overcome the limitations of distance
vector routing protocols
– Respond quickly to network changes
– Send only triggered updates
– Send periodic updates at long intervals, such
as every 30 minutes
– A hello mechanism determines reachability of
neighbors
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Link-State Routing Overview
Maintaining Routing Information Via Link States
Link-State Routing Relies on Complex Mechanisms to
Permit Stable, Synchronous and High-Speed Routing
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Link-State Routing Overview
Maintaining Routing Information Via Link States
• When a failure occurs in a network:
– Link-state protocols flood LSAs; use a special
multicast address
– Each link-state router takes a copy of the LSA,
updates its topological database, and forwards the
LSA to neighboring routers
– All link-state routers in the area recalculate their
routing tables using the Dijkstra SPF algorithm
• A link is similar to an interface on a router
– The state of the link is a description of the interface
and its relation to its neighboring routers
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Link-State Routing Overview
Maintaining Routing Information Via Link States
OSPF Uses a Two-Layer Hierarchy
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Link-State Routing Overview
Maintaining Routing Information Via Link States
Two primary elements exist in the two-layer hierarchy
1. Area: A grouping of contiguous networks
•
•
Areas are logical subdivisions of the autonomous system
Each area must be connected directly to the backbone area
(known as area 0)
2. Autonomous System (AS): A collection of networks
under a common administration
•
•
Share a common routing strategy
Can be logically subdivided into multiple areas
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Link-State Routing Overview
Maintaining Routing Information Via Link States
– The backbone area is the transition area
• All other areas communicate through it
• All non-backbone areas are connected to it
– These can be configured as a stub area, a
totally stubby area, or a not-so-stubby area
(NSSA) (not covered in this curriculum) to
reduce the sizes of the link-state database and
the routing table
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Link-State Routing Overview
Link-State Routing Protocol Algorithms
• Link-State Routing Protocol Algorithms:
– Rely on SPF protocols to maintain a complex
database of the network topology
– Develop and maintain a full knowledge of the network
routers and how they interconnect
• Use LSAs to exchange information with other routers
– Each router that has exchanged LSAs constructs a
topological database
• The SPF algorithm is used to compute reachability to
destination networks
• A routing table is built from this information, containing only
lowest-cost routes
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Link-State Routing Overview
Link-State Routing Protocol Algorithms
• (continued):
– LSA exchanges are triggered events
• Greatly speed up convergence process
• No need to wait for a series of timers to expire before the
networked routers can begin to converge
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Link-State Routing Overview
Link-State Routing Protocol Algorithms
Cost Metric
Determines
Shortest Path
for Link-State
Routing
Protocols
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Link-State Routing Overview
Link-State Routing Protocol Algorithms
Next Hops and
Costs for
Destination
Routes
(Previous Slide)
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Link-State Routing
Benefits of Link-State Routing
• Link-state protocols use cost metrics to choose
paths
– Cost metric reflects the capacity of the links
• Routing updates are less frequent
• Network can be segmented into area hierarchies
– Limits the scope of route changes
• Link-state protocols send only updates of a
topology change
– Use triggered, flooded updates which lead to faster
convergence times
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Link-State Routing
Benefits of Link-State Routing
• Each router has a complete and synchronized
picture of the network
– Difficult for routing loops to occur
• LSAs are sequenced and aged
– Routers always base their routing information on the
most recent set of information
• With careful design work, size of link-state
databases can be minimized
– Smaller Dijkstra calculations and faster convergence
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Link-State Routing
Limitations of Link-State Routing
• In addition to a routing table, link-state
protocols require:
– A topological database
– An adjacency database
• Lists all the relationships formed between
neighboring routers for the purpose of exchanging
routing information
– A forwarding table
• A data structure of a stripped down association
between network prefixes and next hops
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Link-State Routing
Limitations of Link-State Routing
• Dijkstra’s algorithm requires CPU cycles to
calculate best paths through the network
– If the network is large or unstable, this can require a
significant amount of CPU time
• Not a problem for most modern routers
• A strict hierarchical network design is required to
divide the network into smaller areas
– Reduces the excessive use of memory and CPU
cycles
– Reduces size of topology tables and Dijkstra
calculations
– Areas must be contiguous at all times
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Link-State Routing
Limitations of Link-State Routing
• Although configuration of link-state networks is
usually simple, configuring a large network can
be challenging
• Trouble-shooting is usually easier, as every
router has a copy of the topology
– However, interpreting the information requires a good
understanding of link-state routing concepts
• Link-state protocols usually scale to bigger
networks than distance vector protocols
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Link-State Routing
Limitations of Link-State Routing
• Link-state routing raises two concerns:
– During the initial discovery process, link-state
routing protocols flood the network with LSAs
• Significantly decreases the network’s capability to
transport data
• This is temporary, but noticeable
– Link-state routing is both memory- and
processor-intensive
• Greater demand requires higher-end routers that
cost more
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Single-Area OSPF Concepts
• OSPF was developed by the Interior
Gateway Protocol (IGP) group of the
Internet Engineering Task Force (IETF)
– Created in mid 1990s because RIP was
unable to serve large, heterogeneous
networks
• OSPF has two primary characteristics:
– Protocol is an open standard, not proprietary
– Based on the SPF algorithm
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Single-Area OSPF Concepts
Comparing OSPF with Distance Vector Routing
Protocols
• OSPF is a link-state protocol, RIP and IGRP are
distance vector protocols
– Distance vector protocols send all, or a portion of,
their routing table in updates to their neighbors
• A link is an interface on a router
– The state of the link describes the interface and its
relationship to neighboring routers
• Can include IP address, subnet mask, type of network
• The collection of link states forms a link-state
database
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Single-Area OSPF Concepts
Comparing OSPF with Distance Vector Routing
Protocols
• An OSPF router sends LSA packets to
periodically advertise its link states instead
of sending routing table updates
– Information about attached interfaces and
metrics are included
– LSAs are flooded to all routers in the area
– As OSPF routers accumulate link-state
information, they use the SPF algorithm to
calculate the shortest path to each destination
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Single-Area OSPF Concepts
Comparing OSPF with Distance Vector Routing
Protocols
• A topological (link-state) database is an
overall picture of networks in relationship
to routers
– Contains the collection of LSAs received from
all routers in the same area
– Database is pieced together from the LSAs
– Routers in the same area have identical
topological databases
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Single-Area OSPF Concepts
Comparing OSPF with Distance Vector Routing
Protocols
• OSPF can operate within a hierarchy
– The largest entity is the Autonomous System
(AS):
• A collection of networks under a common
administration that share a common routing
strategy
• An AS can be divided into several areas, which are
groups of contiguous networks and attached hosts
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Single-Area OSPF Concepts
OSPF Hierarchical Routing
• OSPF’s capability to separate a large
network into multiple areas is known as
hierarchical routing
– Hierarchical routing enables you to separate a
large internetwork (AS) into smaller
internetworks called areas
– Routing still occurs between areas
• Many of the minute internal routing operations,
such as recalculating the database, are kept within
an area
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Single-Area OSPF Concepts
OSPF Hierarchical Routing
OSPF Uses
Areas to
Provide
Hierarchy
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Single-Area OSPF Concepts
OSPF Hierarchical Routing
• OSPF’s hierarchical topology possibilities
have the following advantages:
– Reduced frequency of SPF calculations
– Smaller routing tables
– Reduced link-state update overhead
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Single-Area OSPF Concepts
Dijkstra’s Algorithm
• In Dijkstra’s algorithm, the best path is the
lowest cost path
– Named for Edsger Wybe Dijkstra, a Dutch
computer scientist
– Each link has a cost
– Each node has a name
– Each node has a complete topological
database
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Single-Area OSPF Concepts
Dijkstra’s Algorithm
Dijkstra’s Algorithm Uses Cost Metric
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Single-Area OSPF Concepts
Dijkstra’s Algorithm
• Dijkstra’s algorithm places each router at the
root of a tree
– Calculates the shortest path to each node based on
the cumulative cost to reach the destination
– Each router has its own view of the topology
– Each router uses the information in its topological
database to calculate a shortest-path tree, with itself
as the root
– The router uses this tree to route network traffic
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Single-Area OSPF Concepts
Dijkstra’s Algorithm
• The cost, or metric, of an interface
indicates the overhead that is required to
send packets across that interface
– The OSPF cost of an interface is inversely
proportional to that interface’s bandwidth
• Higher bandwidth equals lower cost
• Cost = 100,000,000 / bandwidth in bps
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Single-Area OSPF Concepts
Dijkstra’s Algorithm
Shortest Path is Measured from Each Root Node
to Build a Shortest Path Tree
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Single-Area OSPF Configuration
Basic OSPF Configuration
• The router ospf command takes a
process identifier as an argument:
– Router (config)# router ospf process-id
– The process ID is a locally significant number
between 1 and 65,535 that you select to
identify the routing process
• It does not need to match the OSPF process ID on
other OSPF routers
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Single-Area OSPF Configuration
Basic OSPF Configuration
• The network command identifies which IP
networks on the router are part of the OSPF
network:
– Router(config-router)#network address wildcardmask area area-id (all on one command line)
Parameters of a network Command
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Single-Area OSPF Configuration
Basic OSPF Configuration
• The wildcard mask is sometimes called an
inverse mask because it is the inverse of the
subnet mask for the network
– This is not required; many network administrators use the
0.0.0.0 option to match the interface
Basis OSPF Network with Each Router in Area 0
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Single-Area OSPF Configuration
Basic OSPF Configuration
Using the
network
statement in
OSPF
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Single-Area OSPF Configuration
Basic OSPF Configuration
• A router uses the OSPF hello protocol to
establish neighbor relationships
– Hello packets let other routers know they are still functional
• On networks supporting more than two routers
(multiaccess networks), such as Ethernet
networks, the hello protocol elects:
– A designated router (DR)
• Generates LSAs
• Manages link-state synchronization
– A backup designated router (BDR)
• Becomes the DR if the existing DR fails
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Single-Area OSPF Configuration
Loopback Interfaces
• The OSPF router ID is the number by which the
router is known to OSPF
• To modify the OSPF router ID to a loopback
address use this command:
– Router(config)#interface loopback number
• The highest IP address on an active interface of
a router at startup can be overridden by using a
loopback address
– OSPF is more reliable if a loopback interface is configured
because a loopback interface is always active
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Single-Area OSPF Configuration
Modifying the OSPF Cost Metric
• OSPF uses cost as the metric to
determine the best route
– Cost is associated with the output side of an
interface
– It is calculated with the formula
cost = 100,000,000/bandwidth in bps
– The lower the cost, the more likely the route is to be
used
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Single-Area OSPF Configuration
Modifying the OSPF Cost Metric
OSPF Cost Values
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Single-Area OSPF Configuration
Modifying the OSPF Cost Metric
• It is essential for proper OSPF operation that
the correct interface bandwidth is set:
– Router(config)#interface serial 0
– Router(config-if)#bandwidth 56
• Cost can be changed to influence the outcome of OSPF
cost calculation
– When costs are from different vendors are unequal, might want
to make change to match costs
– Might need to change cost to account for Gigabit Ethernet
• Use this command to change cost:
– Router(config-if)#ip ospf cost number
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Single-Area OSPF Configuration
OSPF Authentication
• A router trusts the information that is coming from a
router that should be sending it the information
• To guarantee this trust, routers in a specific area can be
configured to authenticate each other with OSPF
authentication
– Each interface can present an authentication key that the router
uses to send OSPF information to other routers on the segment
– The key, known as a password, is a shared secret between the
routers
– The key can be up to eight characters long
– The key generates the authentication data in the OSPF header
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Single-Area OSPF Configuration
OSPF Authentication
• Use the following syntax to configure OSPF
authentication:
– Router(config-if)#ip ospf authentication-key password
• After the password is configured, authentication
must be enabled:
– Router(config-router)#area area-number authentication
• With simple authentication, the password is sent as
plain text (security risk)
• Configure encryption of the password
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Single-Area OSPF Configuration
OSPF Authentication
• Authentication password encryption syntax:
– Router(config-if)#ip ospf message-digest-key key-id encryptiontype md5 key (all on one line!)
– The key-id is an identifier with a value of between 1 and 255
– The encryption-type refers to the type of encryption, where 0 means
none and 7 means proprietary
• The following is configured in router configuration mode on
a router with an interface in the area area-id
– Router(config-router)#area area-id authentication message-digest
• MD5 creates a message digest, which is scrambled data
based on the password and the message contents
– If the digests match, the receiving router trusts the data
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Single-Area OSPF Configuration
OSPF Network Types and OSPF Timers
• OSPF interfaces automatically recognize three OSPF
network types:
–
–
–
–
Broadcast multiaccess, such as Ethernet
Point-to-point networks
Nonbroadcast multiaccess networks (NBMA), such as Frame Relay
An administrator can manually configure a fourth OSPF network
type: point-to-multipoint
• In a multiaccess network, it is not known in advance how
many routers will be connected
• In point-to-point networks, only two routers will be
connected
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Single-Area OSPF Configuration
OSPF Network Types and OSPF Timers
• In a broadcast multiaccess network segment, many
routers can be connected
– If every router has to establish adjacency with every
other router, [n * (n-1) / 2] adjacencies need to be formed
• For 5 routers the formula would be 5*(5-1) / 2 = 5*4 / 2 = 20 / 2 =
10 adjacencies
• Routers hold an election for a DR router
– This router becomes adjacent to all other routers in the
broadcast segment
• All other routers send their link-state information to the DR
• The DR sends link-state information to all other routers on the
segment by using the 224.0.0.5 multicast address
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Single-Area OSPF Configuration
OSPF Network Types and OSPF Timers
• Despite the gain in efficiency that electing a DR
provides, a disadvantage exists:
– The DR is a single point of failure
• A second router is elected the BDR to take over in
case the DR fails
• To make sure that both the DR and BDR see the
link states that all routers send on the segment, the
224.0.0.6 multicast address is used
• On point-to-point networks, no DR or BDR is
elected; both routers become fully adjacent
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Single-Area OSPF Configuration
OSPF Network Types and OSPF Timers
OSPF Network Type, Characteristics, and DR Election
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Single-Area OSPF Configuration
OSPF Network Types and OSPF Timers
• OSPF uses:
– Hello intervals
• Default of 10 seconds on broadcast networks
• Default of 30 seconds on nonbroadcast networks
– Dead intervals (4 times the hellow interval by default)
• Default of 40 seconds on broadcast networks
• Default of 120 seconds on nonbroadcast networks
• To change the default times:
– Router(config-if)#ip ospf hello-interval seconds
– Router(config-if)#ip ospf dead-interval seconds
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Single-Area OSPF Configuration
Propagating a Default Route
• OSPF routing ensures loop-free paths to every
network in the routing domain
– To reach networks outside the domain, either OSPF must
know about the network or OSPF must have a default
route
• To have an entry for every network in the world would require
enormous resources for each router
• A practical alternative is to add a default route to the OSPF router
connected to the outside network
• This default route can be redistributed to each router in the AS
through normal OSPF updates
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Single-Area OSPF Configuration
Propagating a Default Route
• To configure a static default route:
– Router(config)#ip route 0.0.0.0 0.0.0.0 [interface | next
hop address]
• This is referred to as the quad-zero route
• Any destination network address is matched
– To propagate this route to all the routers in a normal
OSPF area:
• Router(config-router)#default-information originate
• All routers in the OSPF area learn a default route provided that
the interface of the border router to the gateway router is active
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Single-Area OSPF Configuration
Verifying OSPF Configuration
• Several show commands display information about
OSPF configuration:
– Display parameters about timers, filters, metrics and
networks: show ip protocols
– Display the routes that are known to the router: show ip
route
– Verify that interfaces have been configured in the
intended areas: show ip ospf interface
– Display OSPF neighbor information on a per-interface
basis: show ip ospf neighbor
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Single-Area OSPF Configuration
Troubleshooting OSPF
Output from the debug ip ospf events Command
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Single-Area OSPF Configuration
Troubleshooting OSPF
• The debug ip ospf events output might appear if:
– The IP subnet masks for routers on the same network do not match
– The OSPF hello interval does not match that configured for a
neighbor
– The OSPF dead interval does not match that configured for a
neighbor
• If a router configured for OSPF does not see a router on an
attached network
– Make sure both routers are configured with the same subnet mask,
OSPF hello and dead intervals
– Make sure both neighbors are part of the same area type
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Single-Area OSPF Configuration
Troubleshooting OSPF
Sample Output from the debug ip ospf packet Command
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Single-Area OSPF Configuration
Troubleshooting OSPF
Fields in debug ip ospf packet Output
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Single-Area OSPF Configuration
Troubleshooting OSPF
Fields in debug ip ospf packet Output (continued)
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Summary
• Link-state routing protocols such as OSPF and IS-IS quickly
and reliably propagate routing information within an AS
• Link-state routing protocols build link-state databases, which
are synchronized with link-state advertisements (LSAs)
– The link-state protocol then applies Dijkstra’s algorithm (SPF) to
determine the best path(s) to each destination, which are then
installed in the routing table
• OSPF is the most commonly deployed link-state protocol
– Employs DRs and BDRs on broadcast segments to optimize
propagation of link-state information
– Each link uses hello and dead interval timers depending on OSPF
network type: broadcast multiaccess, NBMA, point-to-point, point-tomultipoint
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Summary
• OSPF is configured by:
– Defining which interfaces will participate in a given OSPF process for
a specific area
• Use the network statements coupled with inverse masks
• Inverse masks are often created to exactly match the subnet mask of the
network associated with the given link, or they can be defined simply
with a 0.0.0.0 mask to exactly match their interface ID
• Verifying OSPF configurations is done with these
commands: show ip protocol, show ip route, show ip
ospf interface, show ip ospf neighbor
• Troubleshooting OSPF is done with these commands:
debug ip ospf events, debug ip ospf packets
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