CCNA 3 v3. 1 Module 2
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Transcript of CCNA 3 v3. 1 Module 2
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CCNA 3 v3.1 Module 2
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CCNA 3 Module 2
Single Area OSPF
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Distance Vector Routing
• RIPv1, RIPv2, IGRP, EIGRP
• Interior gateway routing protocol
• Sends copies of routing tables to neighbors
View network from neighbors perspective
• Updates at specified times (30 secs,90 secs)
Slow convergence
High bandwidth usage
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Link State Protocols
• Examples
OSPF (Open Shortest Path First)
IS-IS(Intermediate System to Intermediate System
• Also known as Shortest Path First (SPF)
Dijkstra algorithm calculates loop free topology
• Interior gateway protocol
Sends routing updates to all routers within the autonomous systems
• All routers have a complete view of the network topology
Routers maintain a complex database of the topology
Link state database is identical on all routers
Routers have full knowledge of distant routers and how they interconnect
Require more memory and processing
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• Triggered updates
Contain changes only
Occur when topology change occurs
Updates sent to all routers
• Triggered updates result in
Fast convergence
Efficient use of bandwidth
• OSPF configuration
OSPF routing process must be enabled
Networks must be identified
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Link State Protocol• Send triggered updates
when a network change has occurred
periodic updates known as link-state refreshes
• Each router keeps track of
the state or condition of its directly connected neighbors by multicasting hello packets
all the routers in its network using link-state advertisements (LSAs)
• Builds database about the network using
hello information and LSAs
• Calculates shortest route to each network using
shortest path first (SPF) algorithm
• Stores this route information in its routing table
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Topological Database
• Every router advertises directly connected networks
via Link State Advertisements
• Every router has it’s own view of the network
it builds a ‘topological database’
• Router A is aware of 2 paths to 192.168.157.0
this provides redundancy should one of the routers fail
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Advantages of Link State Protocols
• Use cost metrics to choose paths
Cost metric reflects the capacity of the links
• Fast convergence because
Triggers flooded updates
• No routing loops - router knows network topology
• Routing decision based on up to date of information
LSAs are sequenced and aged
• Minimised Link-state database sizes
because of Dijkstra calculations and faster convergence
• Supports
Classless interdomain routing (CIDR)
Variable-length subnet masking (VLSM)
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Disadvantages of LSP
• More memory and processing power required
In particular during initial setup
• Require strict hierarchical network design
to reduce the size of the topology tables
• Administrator must have a good understanding of link-state routing
• Flooding of LSAs during the initial discovery process
decrease network transport capability
Uses the available bandwidth
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Compare Distance Vector and Link State
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OSPF• Non-Proprietary protocol
• Preferred to RIP because it is scalable
• Open Standard - IETF RFC 2328
• Link State routing protocol
• Interior Gateway Protocol for Autonomous systems
• Metric based on bandwidth
• Supports VLSM
• OSFP can use ‘areas’ for hierarchical design
Multiple areas connect to a distribution area, area 0, also called the backbone
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OSPF Terminology• Link
an interface on the router
• Link State
Status of a link
Relationship to neighbouring router
• Flooding
sends information out all ports, except the port the information was received
• Link State Database – topological database
List of information about all other routers
Shows the internetwork topology
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• Area
A collection of networks and routers
Each router in the area has the same link-state information
A router within an area is an internal router
Area 0 - Backbone
• Cost
Based on bandwidth
Transmission speed
• Routing table
Generated when SPF algorithm is run on link-state dbase
Unique to each router
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• Adjacent database
List of all directly connected routers
• Designated Router (DR) / Backup Designated Router (BDR)
a router elected by all others to represent the network area
All LSA sent to DR/BDR instead of to every single router
Multicast address 224.0.0.6
Reduces overhead of LSA updates
Standard on multi-access networks
DR is single point of failure – solution is BDR
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OSPF Network TypesMulti-access
Number of routers is unknownInfo sent to DR and BDR
Point-to-Point2 routersDR and BDR not elected
NBMAInfo sent to DR and BDR
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OSPF Hello Protocol• Hello protocol sends hello packets at
Router starts up
To initiate new adjacency
Regular intervals
ensure neighbor still functioning
Every 10 seconds on broadcast multi-access and point-to-point networks
Every 30 seconds to NBMA such as frame relay
• Hello packets have a L3 multicast address
224.0.0.5 - This address is all OSPF routers
• Hello packet has OSPF packet header
with a type field of 1
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Configuring OSPF
• Only 1 area
Number is 0
• Multiple areas
Number between 0 to 65,535
Must connect to area 0
• Router(config)#router ospf process-id (1 to 65,535)
• Router(config-router)#network network-address wildcard-mask area-id
• Router(config)#router ospf 1
• Router(config-router)#network 10.2.1.2 0.0.0.0 area 0
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Configure OSPF loopback address
• OSPF must have an active interface at all times
Not always possible if ethernet / serial interface are down
Create a loopback interface (logical interface) to overcome this
• OSPF uses the loopback interface address as the router ID (this overrides local IP address values)
• If there is more than one loopback interface
highest loopback IP address is taken as router ID
• Loopback configuration Commands
router(config)#interface loopback number
Router(config-if)#ip address ip-address subnet-mask
Subnet mask is always 255.255.255.255
• To turn off a loopback address
Router(config)#no interface loopback number
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Setting OSPF Priority
Priorities can be set from 0 to 255
0 prevents that router from being elected
Highest OSPF priority will win the election for DR
Configuring OSPF priority
Router(config)#interface fa0/0
Router(config-if)#ip ospf priority 50
Router(config0if)#end
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OSPF Cost Metric
• OSPF uses Cost metric for path selection
• Cost is based on bandwidth
• Default bandwidth is 1.544Mbps – cost is 64
• Cost can be 1 (100Mbps) to 65535
• Cost is 108 ÷ bandwidth
• ConfigurationRouter(config)#interface serial 0/0
Router(config-if)#bandwidth 64
Router(config-if)#ip ospf cost number
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OSPF Authentication
• To guarantee trust OSPF uses authenticatication
• Authentication key shared between routers
password can be up to eight characters
Router(config-if)#ip ospf authentication-key password
• After password is configured, enable authentication
Router(config-router)#area area-number authentication
This is sent in plain text – easy to decode
• To send encrypted authentication
Router(config-if)#ip ospf message-digest-key key-id md5 encryption-type key
MD5 keyword is the message-digest hashing algorithm to use
Encryption type is the type of encryption
where 0 means none and 7 means proprietary.
key-id 1 through 255
key is an alphanumeric password up to sixteen characters
Neighbor routers must use the same key identifier with the same key value
Router(config-router)#area area-id authentication message-digest
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OSPF Default Route
• Used to reach networks outside the domain
gateway of last resort
• Command
Router(config)#ip route 0.0.0.0 0.0.0.0 [interface | next-hop address]
• referred to as the quad-zero route
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What Causes a failure to establish a neighbor relationship
• Hellos are not sent from both neighbors.
• Hello and dead interval timers are not the same.
• Interfaces are on different network types.
• Authentication passwords or keys are different.
• Other possibilities
All interfaces haven’t correct addresses or subnet mask
network area statements hasn’t correct wildcard masks
network area statements interfaces not in correct area
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Verifying OSPF• Show ip protocol
Displays, times, filters, metrics, networks
• Show ip route
Displays routes known and how they were learned
• Show ip ospf interfaces
Verifies interfaces configured in an area
• Show ip ospf
Displays how many times SPF has been executed
• Show ip ospf neighbor details
Displays a list of neighbors, priorities and states
• Show ip ospf database
Displays the contents of the topological database