CCNA 2 v3.1 Module 6
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CCNA 2
Module 6
Routing & Routed Protocols
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Introducing
Routing
• Routers must learn the direction to remote
networks in order to forward packets.
• There are 2 ways to learn this information:
Dynamic Routing
Static Routing
• Routers use the routing process to
Forward packets toward the destination network
Decisions based upon the destination IP address
• Dynamic routing
Routers learn information from other routers
Scalable – each change learned from another router
• Static routing
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• Configured
manually by the network administrator
Static
Routing
Add and remove static routes when topology changes
• Static Route Operations are divided into three parts:
Network administrator configures the route
Router installs the route in the routing table
Packets are routed using the static route
• Command
Router#config terminal
Router(config)#ip route 172.16.1.0 255.255.255.0 s0
Router(config)#ip route destination subnet
network
interface
outgoing
mask
or
next
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Static Routes using Outgoing Interface
Specifies
Outgoing
Interface
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Static Route Using Next Hop
Specifies
Next Hop
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• The Difference between the 2 options is
Administrative distance assigned to the route
• Administrative distance
Optional
Measures the reliability of the route (0 – 255)
The lower the number the more reliable the route
Set to 1 for next hop
Set to 0 for outgoing interface
• Routers choose the route with the lowest
administrative distance
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• To set a static route that is not 1 or 0
ip route 192.21.121.0 255.255.255.0 192.21.122.1 130
• Static routes
Can be used as a backup if dynamic route fails
Must have higher admin no. to dynamic route
Try the interactive media lab CCNA 2 Module 6 Page 6.1.2
Try the interactive media lab CCNA 2 Module 6 Page 6.1.3
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Configuring a Default Route
• Default routes
Route packets with destinations that do not match
any of the other routes in the routing table
Often used for
internet-bound traffic
Non-directly connected networks
• Special form of a static route
ip route 0.0.0.0 0.0.0.0 [next-hop-address or outgoing if]
Example
ip route
0.0.0.0
0.0.0.02s0
Try the
interactive
media
lab CCNA
Module 6 Page 6.1.4
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Verifying static route configuration
• show running-config
Views the active configuration in RAM to verify that the
static route was entered correctly
• show ip route
Make sure that the static route is present in the routing
table
• Ping <ip address>
check a connection
• Traceroute <ip address>
Shows the path to the ip address
Can be used to identify where the connection fails
Try Interactive media labs CCNA 2 Module 6 Page 6.1.5
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Dynamic Routing
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Routing Protocols
• Allow routers to share information with other
routers regarding
the networks it knows about
its proximity to other routers
• Used to build and maintain a routing table
• Examples:
Routing Information Protocol - RIP
Interior Gateway Routing Protocol - IGRP
Enhanced Interior Gateway Routing Protocol -EIGRP
Open Shortest Path First - OSPF
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Routed Protocol
• Directs user traffic
• Provides enough information in its network layer
address to allow a packet to be forwarded from
one host to another based on the addressing
scheme
Internet Protocol (IP)
Internetwork Packet Exchange (IPX)
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Autonomous Systems
• A collection of networks under
a common administration – e.g., MBNA_Europe
sharing a common routing strategy
• American Registry of Internet Numbers -ARIN
Assigns an identifying number to each AS
It is a 16 bit number
• IGRP and EIGRP have autonomous system
numbers
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Purpose of a routing protocol and autonomous
systems
• Routing protocol
Learns all available routes
Places the best routes into the routing table
Removes routes that are no longer valid
• Router use information in the routing table
To forward routed protocol packet
• Routing protocols change/update routing tables
When the network topology changes
• Internetwork is converged when
All routers in an internetwork are operating with the
same knowledge
Fast convergence is desirable
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Classes of routing protocols
1. Distance vector
Uses distance and direction
2. Link-state
Shortest path first
Recreates the exact topology of the entire
internetwork
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Distance vector routing protocol features
• Router passes periodic copies of a routing table
to directly connected router
These updates communicate topology changes
Updates do not allow router to know exact internetwork
topology
it only sees its neighbours routers
• Also called Bellman-Ford algorithms
• When a router receives an update where there is
an information change
It uses this information to alter the distance and update
the routing table
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Distance Vector Concept
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Link-state routing protocol features
• Link-state algorithms are also known as
Dijkstras algorithm
or SPF (shortest path first) algorithms
• A link-state routing algorithm
Maintains a complete database of the topology
A full knowledge of distant routers and how they
interconnect
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Link State Routing use
• Link-state advertisements (LSAs)
A small packet of routing information sent
between routers
• Topological database
A collection of information gathered from LSAs.
• SPF algorithm (shortest path first)
A calculation performed on the database resulting
in the SPF tree
• Routing tables
A list of the known paths and interfaces
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Link State Routing Protocol Features
• Network discovery processes for link state routing
LSAs are exchanged between directly connected routers with
information about directly connected networks
These LSAs are accumulated on each router and a topological
database is constructed
The SPF algorithm uses this database to calculate shortest
path
It then builds a tree, with itself as the root, consisting of all
possible paths to each network
It sorts these paths Shortest Path First (SPF)
Lists the best paths and ports to these destination networks in
the routing table
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• To achieve convergence
Each router keeps track of its neighbor routers
Router, Name, Interface status, Cost of the link
The first router to become aware of a topology change
Forwards the information to all other routers
Using a Link State Advertisement (LSA)
When a router receives a LSA it
It adds the information to the routers database and the
SPF algorithm is run again
• Link-state concerns:
Processor overhead
Memory requirements
Bandwidth Consumption
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Path determination
• A router determines the path using:
A path determination function
A switching function
• Path determination
Occurs at network layer
Router uses the routing table to determine the best path
• Switching
Router accepts a packet on one interface and forward it to a
second interface on the same router
Router encapsulates the packet in the appropriate frame type
for the next data link
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Routing Configuration
• To enable IP routing protocol on a router
Setting global parameters
and routing parameters
• Global tasks
Selecting a routing protocol (RIP, IGRP, EIGRP or
OSPF)
Specify the network
• Routing metric
Help routers find the best path to each network
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• Command Layout
Router(config)#router <protocol> <option>
Router(config-router)#network <network no>
• <protocol> e.g., rip, igrp, ospf…
• <option> e.g., autonomous system no
• <network no> directly connected network
• Commands
Router(config)# router rip
Router(config-router)#network 192.101.1.0
Try lab activity CCNA 2 Module 6 Page 6.3.2
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Routing Protocols
• RIP – Distance vector interior routing protocol
• IGRP – Cisco's distance vector interior routing protocol
• OSPF – A link-state interior routing protocol
• EIGRP – Cisco’s advanced distance vector interior routing
protocol
• BGP – A distance vector exterior routing protocol
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• Routing Information Protocol (RIP)
Originally specified in RFC 1058
Interior gateway protocol
Distance vector routing protocol
Metric for path selection is hop count
If hop count is greater than 15, packet discarded
Routing updates broadcast every 30 seconds
• Interior Gateway Routing Protocol (IGRP)
Proprietary protocol developed by Cisco
Distance vector routing protocol
Metrics used are
Bandwidth, load, delay and reliability
Routing updates broadcast every 90 seconds
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• Open Shortest Path First (OSPF)
Nonproprietary CISCO protocol
Link-state routing protocol
Originally described in RFC 2328.
Uses SPF algorithm to calculate the lowest
cost to a destination
Routing updates are flooded as topology
changes occur
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• EIGRP - Enhanced Interior Gateway
Routing Protocol
Cisco proprietary protocol
Enhanced distance vector routing protocol
Uses load balancing
Combination of distance vector and link-state
Calculates shorted path first using Diffused
Update Algorithm (DUAL)
Routing updates
Broadcast every 90 seconds
or as triggered by topology changes
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• Border Gateway Protocol (BGP)
Exterior Gateway Protocol
Distance vector routing protocol
Used between ISPs or ISPs and clients
Used to route Internet traffic between
autonomous systems
Try Interactive Lab CCNA 2 Module 6 Page 6.3.3
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Exterior Gateway Protocols
• EGP designed for use between
two different networks that are under the control of different
organizations, ISPs.
• In order for routing to begin you need:
A list of neighbor routers with which to exchange routing
information
A list of networks to advertise as directly reachable
The autonomous system number of the local router
Try interactive lab CCNA 2 Module 6 Page 6.3.4
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Distance vector (RIP or IGRP)
• Known as Bellman-Ford algorithm
• Routing decisions based upon information provided
by its neighboring routers
• Inform neighbors of your routing table on periodic
basis
• Advantages
use few resources
• Disadvantages
Slow convergence
Metrics don’t scale well
• Decisions made based on
Finding the distance (number of hops)
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Link-state (OSPF or IS-IS)
• Shortest Path First algorithm
• Flood routing information to all routers
• Each router gets a complete view of internetwork
• Routing updates sent on topology change (LSA)
They are event triggered
Resulting in quick convergence
• Quick convergence
Prevents routing loops
Prevents routing errors
Use more system resources
• Expensive to implement but scalable
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