WXES2106
Network Technology
Semester 1 2004/2005
Chapter 6
Routing Protocol
CCNA1: 10.1, 10.2, CCNA2: Module 6 and 7
Contents
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Introduction
Routed Protocol
Routing Protocol
Static Route
Dynamic Route
RIP
IGRP
Introduction
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A protocol is a set of rules that determines how
computers communicate with each other across
networks.
A protocol describes the following:
 The format that a message must conform to
 The way in which computers must exchange a
message within the context of a particular activity
Routed Protocol
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A routed protocol allows the router to forward data
between nodes on different networks.
Internet Protocol (IP) is the routed protocol of the
Internet. IPX/SPX, Appletalk are types of routed
protocol.
IP is a connectionless, unreliable, best-effort delivery
protocol. It determines the most efficient route for data
based on the routing protocol.
As a packet travels through an internetwork to its final
destination, the Layer 2 frame headers and trailers are
removed and replaced at every Layer 3 device.
This is because layer 2 data units, frames, are for local
addressing while layer 3 data units, packets, are for
end-to-end addressing.
Routed Protocol
Routed Protocol
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As a frame is received at a router interface, the
destination MAC address is extracted.
The address is checked to see if the frame is directly
addressed to the router interface, or if it is a broadcast.
In either of these two cases, the frame is accepted.
Otherwise, the frame is discarded
The packet is then checked to see if it is actually
destined for the router, or if it is to be routed to another
device in the internetwork.
If the destination IP address matches one of the router
ports, the Layer 3 header is removed and the data is
passed up to the Layer 4.
Routed Protocol
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If the packet is to be routed, the destination IP address
will be compared to the routing table.
If a match is found or there is a default route, the packet
will be sent to the interface specified in the matched
routing table statement.
When the packet is switched to the outgoing interface, a
new CRC value is added as a frame trailer, and the
proper frame header is added to the packet.
The frame is then transmitted to the final destination
Routed Protocol
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Two types of delivery services are connectionless and
connection-oriented.
In a connectionless system, the destination is not
contacted before a packet is sent.
Connectionless network processes are often referred to
as packet switched processes
In connection-oriented systems, a connection is
established between the sender and the recipient
before any data is transferred.
Connection-oriented network processes are often
referred to as circuit switched processes.
Routing Protocol
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Routing is an OSI Layer 3 function.
Routing is a hierarchical organizational scheme that
allows individual addresses to be grouped together.
Routing is the process of finding the most efficient
path from one device to another.
The primary device that performs the routing process is
the router.
Two key functions of a router:
 Maintain routing tables and make sure other routers
know of changes in the network topology.
 The router switches the packets to the appropriate
interface, adds the necessary framing information for
the interface, and then transmits the frame.
Routing Protocol
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Routing protocols use various combinations of metrics
for determining the best path for data.
Routed protocols transport data across a network.
Routing protocols allow routers to choose the best path
for data from source to destination.
Routers use routing protocols to exchange routing
tables and share routing information. It enable routers
to route routed protocols.
Main functions:
 Provides processes for sharing route information
 Allows routers to communicate with other routers to
update and maintain the routing tables
Routing Protocol
Routing Protocol
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Example, Routing Information Protocol (RIP), Interior
Gateway Routing Protocol (IGRP), Open Shortest Path
First (OSPF), Border Gateway Protocol (BGP), and
Enhanced IGRP (EIGRP).
Path Determination
 enables a router to compare the destination
address to the available routes in its routing table,
and to select the best path
 decide which port an incoming packet should be sent
out of to travel on to its destination.
 It lets the router to decide which outbound port the
packet should be sent.
Routing Protocol
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Path Determination Process
 The destination address is obtained from the packet.
 The mask of the first entry in the routing table is
applied to the destination address.
 The masked destination and the routing table entry
are compared.
 If there is a match, the packet is forwarded to the
port that is associated with that table entry.
 If there is not a match, the next entry in the table is
checked.
Routing Protocol
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If the packet does not match any entries in the table,
the router checks to see if a default route has been
set.
If a default route has been set, the packet is
forwarded to the associated port. A default route is
the route to use if there are no matches in the routing
table.
If there is no default route, the packet is discarded.
Usually a message is sent back to the sending device
indicating that the destination was unreachable.
Routing Protocol
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Routing Table
 Routing tables contain the information necessary to
forward data packets across connected networks.
 Protocol type
 The type of routing protocol that created the routing
table entry
 Destination/next-hop associations
 Inform a router that a particular destination is either
directly connected to the router, or that it can be
reached using another router called the “next-hop”
Routing Protocol
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Routing metric
 Used to determine the desirability of a route. The
Routing Information Protocol (RIP) uses hop count
as its only routing metric. Interior Gateway Routing
Protocol (IGRP) uses a combination of bandwidth,
load, delay, and reliability metrics
Outbound interfaces
 The interface that the data must be sent out on, in
order to reach the final destination.
Routing Protocol
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Routing algorithms
 Different routing protocols use different algorithms to
decide which port an incoming packet should be sent
to.
Routing algorithms Design Goal
 Optimization
 Describes the capability of the routing algorithm to
select the best route. The route will depend on the
metrics and metric weightings used in the
calculation.
 Simplicity and low overhead
 The simpler the algorithm, the more efficiently it will
be processed by the CPU and memory in the router.
Routing Protocol
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Robustness and stability
 A routing algorithm should perform correctly
Flexibility
 Should quickly adapt to a variety of network
changes. These changes include router
availability, router memory, changes in
bandwidth, and network delay.
Rapid convergence
 Convergence is the process of agreement by all
routers on available routes. When a network
event causes changes in router availability, updates
are needed to reestablish network connectivity.
Routing Protocol
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Routing Metric
 Bandwidth
 The data capacity of a link.
 Delay
 The length of time required to move a packet along
each link from source to destination.
 Load
 The amount of activity on a network resource such
as a router or a link.
 Reliability
 Usually a reference to the error rate of each network
link.
Routing Protocol
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Hop count
 The number of routers that a packet must travel
through before reaching its destination.
Ticks
 The delay on a data link using IBM PC clock ticks.
One tick is approximately 1/18 second.
Cost
 An arbitrary value, usually based on bandwidth
Routing Protocol
Routing Protocol
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An autonomous system is a network or set of
networks under common administrative control, such
as the cisco.com domain.
Two families of routing protocols are Interior Gateway
Protocols (IGPs) and Exterior Gateway Protocols
(EGPs).
EGPs route data between autonomous systems.
An example of an EGP is Border Gateway Protocol
(BGP).
Routing Protocol
Routing Protocol
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IGPs route data within an autonomous system.
 Routing Information Protocol (RIP) and (RIPv2)
 Interior Gateway Routing Protocol (IGRP)
 Enhanced Interior Gateway Routing Protocol (EIGRP)
 Open Shortest Path First (OSPF)
 Intermediate System-to-Intermediate System protocol
(IS-IS)
IGPs can be further categorized as either distancevector or link-state protocols.
Routing Protocol
Routing Protocol
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Distance Vector
 Determines the distance and direction, vector, to
any link in the internetwork.
 The distance may be the hop count to the link.
 Routers using distance-vector algorithms send all or
part of their routing table entries to adjacent
routers on a periodic basis.
 By receiving a routing update, a router can verify all
the known routes and make changes to its routing
table.
 Also known as the Bellman-Ford algorithm
 Example, RIP, IGRP and EIGRP
Routing Protocol
Routing Protocol
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Link State
 To overcome limitations of distance vector routing
protocols.
 Also known as Dijkstra's algorithm or as the
shortest path first (SPF) algorithm.
 protocols respond quickly to network changes
sending trigger updates only when a network
change has occurred.
 send periodic updates, known as link-state
refreshes, at longer time intervals, such as every 30
minutes.
 When a route or link changes, the device that
detected the change creates a link-state
advertisement (LSA) concerning that link.
Routing Protocol
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The LSA is then transmitted to all neighboring
devices. Each routing device takes a copy of the
LSA, updates its link-state database, and forwards
the LSA to all neighboring devices.
Topological database
 A collection of information gathered from LSAs
SPF algorithm
 A calculation performed on the database that results
in the SPF tree
 It computes network reachability.
Routing table
 A list of the known paths and interfaces
Routing Protocol
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Three main concerns related to link-state protocols:
 Processor overhead
 Memory requirements
 Bandwidth consumption
Examples of link-state protocols include Open
Shortest Path First (OSPF) and Intermediate Systemto-Intermediate System (IS-IS).
Routing Protocol
Routing Protocol
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RIP
 Distance vector routing protocol
 Uses hop count as its metric to determine the
direction and distance to any link in the internetwork.
 Cannot route a packet beyond 15 hops.
 Routing updates broadcast every 30 seconds
RIPV1
 All devices in the network use the same subnet mask
 Classful routing
Routing Protocol
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RIPV2
 Provides prefix routing, and does send subnet mask
information in routing updates.
 Classless routing
 Different subnets within the same network can have
different subnet masks., variable-length subnet
masking (VLSM).
IGRP
 Distance-vector routing protocol
 Based on delay, bandwidth, load, and reliability
 Classful routing
 Maximum hop 255
 Routing updates broadcast every 90 seconds
Routing Protocol
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OSPF
 Link-state routing protocol
 Address the needs of large, scalable internetworks
 Open standard routing protocol
 The SPF algorithm is used to calculate the lowest cost
to a destination.
 Routing updates are flooded as topology changes
occur.
IS-IS
 Link-state routing protocol
 Supports multiple routed protocols including IP
Routing Protocol
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EIGRP
 Proprietary Cisco protocol
 Provides superior operating efficiency such as fast
convergence and low overhead bandwidth
 It uses load balancing.
 It uses a combination of distance vector and link-state
features. Hybrid routing protocol
 It uses Diffused Update Algorithm (DUAL) to
calculate the shortest path.
 Routing updates are multicast using 224.0.0.10
every 30 seconds or as triggered by topology
changes.
Routing Protocol
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BGP
 An example of an External Gateway Protocol (EGP)
 Exchanges routing information between autonomous
systems while guaranteeing loop-free path selection.
 BGP4 is the first version of BGP
 Supports classless interdomain routing (CIDR) and
route aggregation.
 Makes routing decisions based on network policies,
or rules using various BGP path attributes
Static Route
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A network administrator configures information about
remote networks manually.
Static routing is not as scalable as dynamic routing
because of the extra administrative requirements.
Static route operations can be divided into these three
parts:
 Network administrator configures the route
 Router installs the route in the routing table
 The static route is used to route packets.
Static Route
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configure a static route
 enter global configuration mode
 Router(config)#ip route destination-network
subnet-mask outgoing-interface
 Router(config)#ip route destination-network
subnet-mask next-hop-ip-address
 Router(config)#copy running-config startup-config
 save the active configuration to NVRAM.
configure default route
 Router(config)#ip route 0.0.0.0 0.0.0.0 [next-hopaddress | outgoing interface]
Static Route
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Router#show running-config
 Verify that the static route has been correctly entered.
Router#show ip route
 Verify that the route that was configured is in the
routing table.
Use ping and tracert to troubleshoot the static route
configuration
Dynamic Routing
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The routing protocol learns all available routes, places
the best routes into the routing table, and removes
routes when they are no longer valid.
The router uses the information in the routing table to
forward routed protocol packets.
Whenever the topology of a network changes because of
growth, reconfiguration, or failure, the network
knowledgebase must also change.
The network knowledgebase needs to reflect an
accurate view of the new topology.
Dynamic Routing
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Configure Routing Protocol
 Router(config)#Router { rip | igrp | eigrp | ospf }
option
 Router(config-router)# Network network-number
 Network-number specifies the directly connected
network
Dynamic Routing
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Routing loops can occur when inconsistent routing tables
are not updated due to slow convergence in a changing
network.
To reduce routing loops and counting to infinity, RIP
uses the following techniques:
 Split horizon
 Poison reverse
 Holddown counters
 Triggered updates
Dynamic Routing
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Example
 When Network 1 fails, Router E sends an update to
Router A.
 When Router A sends out its update, Routers B and D
stop routing to Network 1. However, Router C has not
received an update. For Router C, Network 1 can still
be reached through Router B.
 Router C keep sending periodic update to Router D,
which indicates a path to Network 1 by way of Router
B. Router D changes its routing table to reflect this
incorrect information, and sends it to Router A.
Dynamic Routing
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Router A sends the information to Routers B and E, and
the process continues. Any packet destined for Network
1 will now loop from Router C to B to A to D and back to
again to C.
Dynamic Routing
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Split horizon can be used to avoid routing loops
If a routing update about Network 1 arrives from Router
A, Router B or Router D cannot send information about
Network 1 back to Router A.
Split horizon reduces incorrect routing information
and routing overhead.
Dynamic Routing
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Route poisoning is used by various distance vector
protocols to overcome large routing loops and offer
detailed information when a subnet or network is not
accessible.
The hop count is usually set to one more than the
maximum.
When Network 1 goes down, Router E will set a distance
of 16 for Network 1 to poison the route.
This indicates that the network is unreachable.
After Router B receives a route poisoning from Router E,
it sends an update, which is called a poison reverse,
back to Router E. This makes sure all routers on the
segment have received the poisoned route information.
Dynamic Routing
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Triggered updates
 A triggered update is sent immediately in response to
some change in the routing table.
 The router that detects a topology change
immediately sends an update message to adjacent
routers.
 Ensure that all routers know of failed routes before
any holddown timers can expire.
Dynamic Routing
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Holddown timers
 When a router receives an update from a neighbor,
which indicates a network fail, the router marks the
route as inaccessible and starts a holddown timer.
 Before the holddown timer expires, if an update is
received from the same neighbor, which indicates that
the network is accessible, the router marks the
network as accessible and removes the holddown
timer.
 if an update arrives from a different neighbor router
with a better metric for the network, the router marks
the network as accessible and removes the holddown
timer.
Dynamic Routing
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If an update is received from a different router with a
higher metric before the holddown timer expires, the
update is ignored
This update is ignored to allow more time for the
knowledge of a disruptive change to propagate
through the entire network.
RIP
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RIP has evolved Classful Routing Protocol, RIP Version
1 (RIP v1) to a Classless Routing Protocol, RIP Version
2 (RIP v2).
RIP v2 enhancements include:
 Ability to carry additional packet routing information
 Authentication mechanism to secure table updates
 Support for variable-length subnet mask (VLSM)
Configuring RIP
RIP
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Optional task:
 Apply offsets to routing metrics
 Adjust timers
 Specify a RIP version
 Enable RIP authentication
 Configure route summarization on an interface
 Verify IP route summarization
 Disable automatic route summarization
 Run IGRP and RIP concurrently
 Disable the validation of source IP addresses
 Enable or disable split horizon
 Connect RIP to a WAN
RIP
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Router(config)#ip classless
 forward these packets to the best supernet route.
 Example, if an enterprise uses the entire subnet
10.10.0.0 /16, then a supernet route for 10.10.10.0 /24
would be 10.10.0.0 /16
Router(config-if)#ip split-horizon
 enable split horizon (default)
Router(config-router)#timers basic update invalid
holddown flush [sleeptime]
 change holddown timer
RIP
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Router(config-router)#update-timer seconds
 Change update interval
Router(config-router)#passive-interface Fa0/0
 disable routing updates on specified interfaces
Router(config-router)#neighbor ip-address
 exchange routing information with neighboring router
Router(config-router)#version { 1|2 }
 receive and send Version 1 and 2 packets
 by default send version 1 and 2 packets receive
Version 1 packets
RIP
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Router(config-if)#ip rip {receive | send} version { 1|2 }
 receive or send Version 1 or 2 packets
Router#show ip protocols
 RIP routing is configured.
 The correct interfaces send and receive RIP updates.
 The router advertises the correct networks.
Router#debug ip rip
 displays RIP routing updates as they are sent and
received.
RIP is capable of load balancing over as many as six
equal-cost paths. The default is four paths. RIP
performs what is referred to as "round robin" load
balancing.
RIP
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Router(config-router)#maximum-paths [number]
 change the maximum number of parallel paths
Router(config)#ip route destination-network subnetmask next-hop Administrative-Distance
 Create a static floating route if AD is higher than the
normal RIP route
 floating route take the place of the RIP route in the
event that the RIP routing process fails.
IGRP
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Key design of IGRP
 Versatility
 Automatically handle indefinite, complex topologies
 Flexibility
 Segment with different bandwidth and delay
characteristics
 Scalability
 Functioning in very large networks
IGRP Metrics
 Bandwidth (K1)
 Delay (K3)
 Load
 Reliability
IGRP
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IGRP Route
 Interior Route
 Routes between subnets of a network attached to a
router interface
 System routes
 Routes to networks within an autonomous system. .
 Do not include subnet information.
 Exterior routes
 Routes to networks outside the autonomous system
that are considered when a gateway of last resort is
identified.
IGRP
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IGRP maintains many timers such as update timer, an
invalid timer, a holddown timer, and a flush timer.
Invalid timer
 Specifies how long a router should wait in the
absence of routing-update messages about a route
before it declares that route invalid.
Flush timer
 Indicates how much time should pass before a route
is flushed from the routing table
Configuring IGRP
 Router(config)#router igrp as-number
 Router(config-router)#network network-number
IGRP