Routing Protocols and Concepts: Chapter 3 pptx

Chapter 3
Dynamic Routing Protocols
Routing Protocols and Concepts

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Topics
 Introduction to Dynamic Routing
Protocols
 Perspective and Background
 Network Discovery and
Routing Table Maintenance
 Dynamic Routing Protocol
Advantages
 Classifying Dynamic Routing
Protocols
 IGP and EGP
 Distance Vector and Link-
State
 Classful and Classless
 Convergence
 Metrics
 Purpose of the Metric
 Metrics and Routing Protocols
 Load Balancing
 Administrative Distance
 Purpose of Administrative
Distance
 Dynamic Routing Protocols
and Administrative Distance
 Static Routes and
Administrative Distance

 Directly Connected Networks
and Administrative Distance
Introduction to Dynamic Routing
Protocols
 Perspective and Background
 Network Discovery and Routing Table Maintenance
 Dynamic Routing Protocol Advantages
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Perspective and Background
 Dynamic routing protocols have evolved over several years
 As networks have evolved and become more complex, new routing
protocols have emerged.
 Most institutions have migrated to new protocols, others are still in use.
 The first version of RIP was released in 1982, but some of the basic
algorithms within the protocol were used on the ARPANET as early as
1969.
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Perspective and
Background
Interior Routing Protocols or Interior Gateway Protocols (IGP)
 Distance Vector
 RIPv1 – Simple, Classful, limited metrics (hop count)
 RIPv2 – Simple, Classless, limited metrics (hop count)
Cisco Proprietary
 IGRP – Simple, Classful, better metric (BW, delay, reliab., load)
 EIGRP – Simple, Classless, same metric, DUAL (backup routes)
 Link State
 OSPF – Perceived complex, classless, Cisco metric BW, IETF
 IS-IS - Perceived complex, classless, metric “default”, ISO
 Classful (does not support

CIDR and VLSM)
 Classless (supports CIDR
and VSLM)
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Perspective and
Background
Exterior Routing Protocols or Exterior Gateway Protocols (EGP)
 Border Gateway Protocol (BGP) is now used between Internet service
providers (ISP) as well as between ISPs and their larger private clients to
exchange routing information.
 Path Vector routing protocol, metric – attributes (policies)
 Replaced EGP
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Role of Dynamic Routing Protocol
 Dynamic Routing Protocols:
 Exchange of routing information between routers
 Dynamically learn information about remote networks
 Determines the best path to each network
 Adds routes to routing tables
 Automatically learn about new networks
 Automatically finds alternate paths if needed (link failure in current path)
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Role of Dynamic Routing Protocol
 Compared to Static Routes:
 Advantages of Dynamic Routing Protocols:
 Less administrative overhead (change modifications)
 Disadvantage of Dynamic Routing Protocols
 More CPU and memory requirements
 This is not that big an issue in most networks and with modern
routers.

 Configuration is less error-prone
 Scales better with larger networks
 “Less secure” if routing updates are sent unencrypted.
 Most networks use both dynamic and static routes
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Purpose of Dynamic Routing Protocols
 A routing protocol is a set of processes, algorithms, and messages that
are used to exchange routing information and populate the routing table
with the routing protocol’s choice of best paths.
 Purpose:
 Discovering remote networks
 Maintaining up-to-date routing information
 Choosing the best path to destination networks
 Having the ability to find a new best path if the current path is no longer
available
 Components of a routing protocol (depending upon the routing protocol):
 Data structures: Tables or databases for their operations, kept in RAM.
 Algorithm:
 An algorithm is a finite list of steps used in accomplishing a task.
 Routing protocols use algorithms for processing routing information
and for best-path determination.
 Routing protocol messages:
 Discover neighboring routers
 Exchange routing information
 Learn and maintain accurate information about the network
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Dynamic Routing Protocol Operation
The operations of a dynamic routing protocol vary depending on the type of
routing protocol, but in general:
1. The router sends and receives routing messages on its interfaces.

2. The router shares routing messages and routing information with other
routers that are using the same routing protocol.
3. Routers exchange routing information to learn about remote networks.
4. When a router detects a topology change, the routing protocol can advertise
this change to other routers.
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Static Routing
Usage,
Advantages, and
Disadvantages
 Primary uses:
 Smaller networks that are not expected to grow significantly.
 Routing to and from stub networks
 Default route
Classifying Dynamic Routing
Protocols
 IGP and EGP
 Distance Vector and Link-State
 Classful and Classless
 Convergence
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Classifying Routing Protocols
 Routing Protocols can be classified by:
 IGP or EGP
 Distance vector or link-state
 Classful or classless
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IGP and EGP
 An autonomous system (AS)—otherwise known as a routing domain—is
a collection of routers under a common administration. sharing a common

routing strategy. Each AS has a 16 bit autonomous system number
 Company’s internal network
 An ISP’s network.
 Because the Internet is based on the autonomous system concept, two
types of routing protocols are required:
 Interior routing protocols
 Exterior routing protocols
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IGP and EGP
 Interior gateway protocols (IGP):
 Used for intra-autonomous system routing
 Routing inside an autonomous system
 Exterior gateway protocols (EGP):
 Used for inter-autonomous system routing
 Routing between autonomous systems
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Distance Vector and Link-State Routing Protocols
 Interior gateway protocols (IGP) can be classified as two types:
 Distance vector routing protocols
 Link-state routing protocols
 They work in different ways but they have the same purposes
 Discover routes and put the best ones in the routing table
 Remove routes that are no longer available
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Distance Vector Routing
Protocol Operation
 Distance vector
 Routes are advertised as vectors of
distance and direction.
 Distance is defined in terms of a metric

 Such as hop count,
 Direction is simply the:
 nexthop router or
 exit interface.
 Typically use the Bellman-Ford algorithm
for the best-path route determination
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Distance Vector Routing
Protocol Operation
 Routing protocol
 Does not know the topology of an
internetwork.
 Only knows the routing information
received from its neighbors.
 Like signposts along the path to the final
destination.
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Distance Vector Routing
Protocol Operation
 Distance vector protocols work
best in situations where:
 The network is simple and flat and
does not require a hierarchical
design.
 The administrators do not have
enough knowledge to configure
and troubleshoot link-state
protocols.
 Specific types of networks, such as
hub-and-spoke networks, are

being implemented.
 Worst-case convergence times in
a network are not a concern.
 Easier to configure and troubleshoot
than link-state protocols

Slower to converge than link state
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Link-State Protocol Operation
 Link-state routing protocol can
create a “complete view,” or
topology, of the network.
 Like having a complete map of the
network topology
 Link-state protocols are
associated with Shortest Path
First (SPF) calculations.
 A link-state router uses the link-
state information to:
 Create a topology map
 Select the best path to all
destination networks in the
topology.
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Link-State Protocol Operation
 Link-state protocols work best
in situations where
 The network design is
hierarchical, usually occurring
in large networks.

 The administrators have a
good knowledge of the
implemented link-state routing
protocol.
 Fast convergence of the
network is crucial.
 More in later chapters.
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Classful and Classless Routing Protocols
 All routing protocols can also be classified as either
 Classful routing protocols
 Classless routing protocols
 IPv6 routing protocols are classless
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Classful
Routing
Protocols
 Classful routing protocols do not send subnet mask information in routing
updates.
 The first routing protocols, such as RIP
 When network addresses were allocated based on classes.
 Class A, B, or C.
 Routing protocol did not need to include the subnet mask in the routing
update.
 Network mask determined based on value of first octet of the
network address.
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Classful Routing Protocols
 Classful routing protocols do not include the subnet mask
 Therefore do not support VLSM and CIDR.

 All subnets within the same “major classful network address” must have the
same mask.
 Other limitations to classful routing protocols, including:
 Inability to support discontiguous networks (later)
 More later!
172.16.0.0/16
Major Classful
Network
All /24 subnets
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Classless routing Protocols
 Classless routing protocols include the subnet mask with the network
address in routing updates.
 Today’s networks are no longer allocated based on classes
 Subnet mask cannot be determined by the value of the first octet.
 Classless routing protocols are required in most networks today because of
their support for:
 VLSM
 CIDR
 Discontiguous networks.
172.16.128.0/30 172.16.132.0/30
172.16.136.0/30
172.16.0.0/16
Major Classful
Network
/27 and /30
subnets