Download Introduction to Dynamic Routing Protocol

Introduction to Dynamic
Routing Protocol
Routing Protocols and Concepts – Chapter 3
Modified by Tony Chen
02/19/2010
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Notes:

If you see any mistake on my PowerPoint slides or if
you have any questions about the materials, please
feel free to email me at [email protected].
Thanks!
Tony Chen
College of DuPage
Cisco Networking Academy
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Objectives

Describe the role of dynamic routing protocols and
place these protocols in the context of modern
network design.

Identify several ways to classify routing protocols.

Describe how metrics are used by routing protocols
and identify the metric types used by dynamic routing
protocols.

Determine the administrative distance of a route and
describe its importance in the routing process.

Identify the different elements of the routing table.
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Dynamic Routing Protocols
 Dynamic routing protocols are usually
used in larger networks to ease the
administrative and operational overhead
of using only static routes.
 Typically, a network uses a combination
of both a dynamic routing protocol and
static routes.
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The Evolution of Dynamic Routing Protocols
 One of the earliest routing protocols was Routing Information Protocol (RIP).
–RIP has evolved into a newer version RIPv2. However,
–The newer version of RIP still does not scale to larger network implementations.
 To address the needs of larger networks, two advanced routing protocols were
developed: Open Shortest Path First (OSPF) and Intermediate System-toIntermediate System (IS-IS).
 Cisco developed Interior Gateway Routing Protocol (IGRP) and Enhanced IGRP
(EIGRP), which also scales well in larger network implementations.
 Additionally, there was the need to interconnect different internetworks and provide
routing among them. Border Gateway Routing (BGP) protocol is now used between
ISPs as well as between ISPs and their larger private clients to exchange routing
information.
 With the advent of numerous consumer devices using IP, the IPv4 addressing space
is nearly exhausted. Thus IPv6 has emerged. To support the communication based
on IPv6, newer versions of the IP routing protocols have been developed (see the
IPv6 row in the table).
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Dynamic Routing Protocols
 Function(s) of Dynamic Routing Protocols:
-Dynamically share information between routers.
-Automatically update routing table when topology changes.
-Determine best path to a destination.
–Compared to static routing, dynamic routing protocols require less administrative
overhead.
•However, the expense of using dynamic routing protocols is dedicating part of a router's
resources for protocol operation including CPU time and network link bandwidth.
– One of the primary benefits to using a dynamic routing protocol is that routers
exchange routing information whenever there is a topology change. This exchange
allows routers to automatically learn about new networks and also to find alternate
paths when there is a link failure to a current network.
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Dynamic Routing Protocols
 Despite the benefits of dynamic routing, static routing still
has its place.
 There are times when static routing is more appropriate and
other times when dynamic routing is the better choice.
 More often than not, you will find a combination of both
types of routing in any network that has a moderate level of
complexity.
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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
 The purpose of a dynamic routing protocol is to:
-Discover remote networks
-Maintaining up-to-date routing information
-Choosing the best path to destination networks
-Ability to find a new best path if the current path is no longer available
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Dynamic Routing Protocols
 Components of a routing protocol
–Data structures
•Some routing protocols use tables and/or databases for its operations.
This information is kept in RAM
–Algorithm
•Algorithm is a finite list of steps used in accomplishing a task
•Algorithms are used for facilitating routing information and best path
determination
–Routing protocol messages
•These are messages for discovering neighbors and exchange of
routing information , and other tasks to learn and maintain accurate
information about the network.
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Dynamic Routing Protocol Operation
 All routing protocols have the same purpose - to learn about remote networks
and to quickly adapt whenever there is a change in the topology.
 The method that a routing protocol uses to accomplish this depends upon the
algorithm it uses and the operational characteristics of that protocol.
 In general, the operations of a dynamic routing protocol can be described as
follows:
–The router sends and receives routing messages on its interfaces.
–The router shares routing messages and routing information with other routers that
are using the same routing protocol.
–Routers exchange routing information to learn about remote networks.
–When a router detects a topology change the routing protocol can advertise this
change to other routers.
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Dynamic Routing Protocols
 Advantages of dynamic routing
 Advantages of static routing
-It can backup multiple
interfaces/networks on a router
-Minimal CPU processing
-Easier for administrator to
understand
-Easy to configure
-No extra resources are needed
-More secure
 Disadvantages of static routing
-Administrator has less work
maintaining the configuration when
adding or deleting networks.
-Protocols automatically react to the
topology changes.
-Configuration is less error-prone.
-More scalable, growing the network
usually does not present a problem
-Network changes require manual  Disadvantages of dynamic routing
reconfiguration
-Router resources are used (CPU
-Configuration and maintenance is
cycles, memory and link bandwidth).
time-consuming
-More administrator knowledge is
-Does not scale well in large
required for configuration,
topologies
verification, and troubleshooting.
-Configuration is error-prone,
especially in large networks
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Dynamic Routing Protocols
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Classifying Routing Protocols
 Dynamic routing protocols are grouped according to
characteristics. Examples include:
-RIP
-IGRP
-EIGRP
-OSPF
-IS-IS
-BGP
 Autonomous System is a group of routers under the control of
a single authority.
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Classifying Routing Protocols
 Dynamic routing protocols:
–RIP
•A distance vector interior routing protocol
–IGRP
•The distance vector interior routing
developed by Cisco (deprecated from 12.2
IOS and later)
–EIGRP
•The advanced distance vector interior
routing protocol developed by Cisco
–OSPF
•A link-state interior routing protocol
–IS-IS
•A link-state interior routing protocol
–BGP
•A path vector exterior routing protocol
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Classifying Routing Protocols
 An autonomous system (AS) - otherwise known as a
routing domain - is a collection of routers under a
common administration.
 Because the Internet is based on the ASs concept, two
types of routing protocols are required: interior and
exterior routing protocols.
-Interior Gateway Protocols (IGP)
•are used for intra-autonomous system routing - routing
inside an autonomous system
•IGPs are used for routing within a routing domain, those
networks within the control of a single organization.
–An autonomous system is commonly comprised of many
individual networks belonging to companies, schools, and
other institutions.
• IGPs for IP include RIP, IGRP, EIGRP, OSPF, and IS-IS
-Exterior Gateway Protocols (EGP)
•are used for inter-autonomous system routing - routing
between autonomous systems that are under the control
of different administrations
•At the ISP level, there are often more important issues
than just choosing the fastest path.
•BGP is typically used between ISPs and sometimes
between a company and an ISP
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Autonomous systems
 An autonomous system (AS) is a collection of
networks under a common administration
sharing a common routing strategy.
To the outside world, an AS is viewed as a single
entity. The AS may be run by one or more
operators while presenting a consistent view of
routing to the external world.
 The American Registry of Internet Numbers
(ARIN), a service provider, or an administrator
assigns an identifying number to each AS. This
autonomous system number is a 16 bit number.
Routing protocols, such as Cisco’s IGRP,
require assignment of a unique, autonomous
system number.
American Registry for Internet Numbers
http://www.arin.net/registration/asn/index.html
Autonomous System number (ASN) resource guide
http://www.apnic.net/services/asn_guide.html
IS-IS
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Autonomous systems
 Cisco system AS number:
 http://ws.arin.net/cgi-bin/whois.pl
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Autonomous systems
 http://arin.net/education/asn_process/index.html
RFC 1930
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
AS just like IP, it needs
to apply from ARIN or
the appropriate region
and be unique on the
internet.

The Internet Assigned
Numbers Authority
(IANA) has reserved the
following block of AS
numbers for private use
(not to be advertised on
the global Internet):
64512 through 65535
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Classifying Routing Protocols
 IGP: Comparison of Distance Vector & Link
State Routing Protocols
Distance vector
– routes are advertised as vectors of distance &
direction.
•Distance is defined in terms of a metric such as hop
count (RIP)
•Direction is simply the next-hop router or exit
interface
•Distance vector protocols typically use the BellmanFord algorithm for the best path route determination
– incomplete view of network topology.
•Distance vector protocols use routers as sign posts
along the path to the final destination.
•Distance vector routing protocols do not have an
actual map of the network topology
– Generally, periodic updates.
•Some distance vector protocols periodically send
complete routing tables to all connected neighbors.
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Classifying Routing Protocols
 IGP: Comparison of Distance Vector &
Link State Routing Protocols
Link state
– complete view of network topology is created.
•The sign posts along the way from source to
destination are not necessary, because all linkstate routers are using an identical "map" of the
network.
– updates are not periodic.
•After the network has converged, a link-state
update only sent when there is a change in the
topology.
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Classifying Routing Protocols
 Comparison of Distance Vector & Link State Routing Protocols
 Distance vector protocols work
best in situations where:
–The network is simple and flat
and does not require a special
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.
 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 linkstate routing protocol.
–Fast convergence of the network is
crucial.
–Worst-case convergence times
in a network are not a concern.
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Classifying Routing Protocols
 Classful routing protocols
–Do NOT send subnet mask in routing updates,
–Do NOT support VLSM,
–Classful routing protocols cannot be used when
a network is subnetted using more than one
subnet mask,
• Tony: This does not mean you can not
subnet the classfull network. You can still
subnet it, but can only do it once and all
network needs to have the identical mask.
– Routing protocols such as RIPv1 and IGRP.
 Classless routing protocols
–Do send subnet mask in routing updates.
–support variable length subnet masks (VLSM).
•In the figure, the classless version of the network is
using both /30 and /27 masks in the same topology.
•Tony: It means you can create the network
with all different sizes of subnets. They don’t
need to have the same mask.
•Classless routing protocols are RIPv2, EIGRP,
OSPF, IS-IS, BGP.
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Classifying Routing Protocols
 Convergence is defined as when all routers’ routing
tables are at a state of consistency
– The network has converged when all routers have complete and
accurate information about the network
 Convergence time is the time it takes routers to share
information, calculate best paths, and update their routing
tables.
 Routing protocols can be rated based on
the speed to convergence; the faster the
convergence, the better the routing
protocol.
–RIP and IGRP are slow to converge
–EIGRP and OSPF are faster to converge.
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Routing Protocols Metrics
 To select the best path, the routing
protocol must be able to evaluate and
differentiate between the available paths.
For this purpose a metric is used.
 Metric
–A value used by a routing protocol to
determine which routes are better than others.
 Each routing protocol uses its own metric.
–RIP uses hop count,
•The hop count refers to the number of routers
a packet must cross to reach the destination
network.
•For R3 in the figure, network 172.16.3.0 is two
hops, or two routers away.
–EIGRP uses a combination of bandwidth and
delay,
–OSPF uses bandwidth (cost).
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Routing Protocols Metrics
 Metrics used in IP routing protocols
–Bandwidth
•Influences path selection by preferring the path
with the highest bandwidth
–Cost
•A value determined either by the IOS or by the
network administrator to indicate preference for a
route. Cost can represent a metric, a combination
of metrics or a policy.
–Delay
•Considers the time a packet takes to traverse a
path
–Hop count
•A simple metric that counts the number of routers
a packet must traverse
–Load
•Considers the traffic utilization of a certain link
–Reliability
•Assesses the probability of a link failure,
calculated from the interface error count or
previous link failures
OSPF
RIP
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Routing Protocols Metrics
 The Metric Field in the Routing Table
 Metric used for each routing protocol
-RIP - hop count
-IGRP & EIGRP - Bandwidth (used by
default), Delay (used by default), Load,
Reliability
-IS-IS & OSPF – Cost, Bandwidth
(Cisco’s implementation)
 Refer to the example in the figure The
routers are using the RIP routing
protocol.
–The metric associated with a certain
route can be best viewed using the
show ip route command.
–The metric value is the second value in
the brackets for a routing table entry.
–In the figure, R2 has a route to the
192.168.8.0/24 network that is 2 hops
away.
•R 192.168.8.0/24 [120/2] via
192.168.4.1, 00:00:26, Serial0/0/1
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Routing Protocols Metrics
 Load balancing
–when two or more routes to the same
destination have identical metric values
–This is the ability of a router to
distribute packets among multiple same
cost paths
Load balancing does not
automatically means the interfaces
will get use equally.
??????
R2 load balances
traffic to PC5 over two
equal cost paths.
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Routing Protocols Metrics
 Load balancing can be done either
per packet or per destination.
–How a router actually load balances
packets between the equal-cost paths is
governed by the switching process.
Example
R2 load balances
traffic to PC5 over two
equal cost paths.
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Router Paths: Equal Cost Load Balancing
 To solve this dilemma, a router will use Equal Cost Load
Balancing. This means the router sends packets over the multiple
exit interfaces listed in the routing table.
–per-packet load balancing
•( Process Switching)
–per-destination load balancing.
•(Fast Switching)
Router(config-if)# ip route-cache
ping 10.0.0.2
ping 10.0.0.1
Router(config-if)#no ip route-cache
ping 10.0.0.2
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ping 10.0.0.1
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Load balancing with RIP
per-packet
load balancing
debug ip packet
IP packet debugging is on
GAD#
*Mar 1 19:10:29.646: IP: tableid=0, s=192.168.14.2 (FastEthernet0/0), d=192.168.16.2 (Serial0/1), routed via RIB
*Mar 1 19:10:29.646: IP: s=192.168.14.2 (FastEthernet0/0), d=192.168.16.2 (Serial0/1), g=192.168.13.2, len 60, forward
*Mar 1 19:10:30.654: IP: tableid=0, s=192.168.14.2 (FastEthernet0/0), d=192.168.16.2 (Serial0/0), routed via RIB
*Mar 1 19:10:30.654: IP: s=192.168.14.2 (FastEthernet0/0), d=192.168.16.2 (Serial0/0), g=192.168.15.2, len 60, forward
*Mar 1 19:10:31.654: IP: tableid=0, s=192.168.14.2 (FastEthernet0/0), d=192.168.16.2 (Serial0/1), routed via RIB
*Mar 1 19:10:31.654: IP: s=192.168.14.2 (FastEthernet0/0), d=192.168.16.2 (Serial0/1), g=192.168.13.2, len 60, forward
*Mar 1 19:10:32.218: IP: s=0.0.0.0 (FastEthernet0/0), d=255.255.255.255, len 604, rcvd 2
*Mar 1 19:10:32.654: IP: tableid=0, s=192.168.14.2 (FastEthernet0/0), d=192.168.16.2 (Serial0/0), routed via RIB
*Mar 1 19:10:32.654: IP: s=192.168.14.2 (FastEthernet0/0), d=192.168.16.2 (Serial0/0), g=192.168.15.2, len 60, forward
*Mar 1 19:10:33.654: IP: tableid=0, s=192.168.14.2 (FastEthernet0/0), d=192.168.16.2 (Serial0/1), routed via RIB
*Mar 1 19:10:33.654: IP: s=192.168.14.2 (FastEthernet0/0), d=192.168.16.2 (Serial0/1), g=192.168.13.2, len 60, forward
*Mar 1 19:10:34.654: IP: tableid=0, s=192.168.14.2 (FastEthernet0/0), d=192.168.16.2 (Serial0/0), routed via RIB
*Mar 1 19:10:34.654: IP: s=192.168.14.2 (FastEthernet0/0), d=192.168.16.2 (Serial0/0), g=192.168.15.2, len 60, forward
*Mar 1 19:10:35.654: IP: tableid=0, s=192.168.14.2 (FastEthernet0/0), d=192.168.16.2 (Serial0/1), routed via RIB
*Mar 1 19:10:35.654: IP: s=192.168.14.2 (FastEthernet0/0), d=192.168.16.2 (Serial0/1), g=192.168.13.2, len 60, forward
*Mar 1 19:10:35.974: IP: s=192.168.13.1 (local), d=255.255.255.255 (Serial0/1), len 72, sending broad/multicast
*Mar 1 19:10:36.654: IP: tableid=0, s=192.168.14.2 (FastEthernet0/0), d=192.168.16.2 (Serial0/0), routed via RIB
*Mar 1 19:10:36.654: IP: s=192.168.14.2 (FastEthernet0/0), d=192.168.16.2 (Serial0/0), g=192.168.15.2, len 60, forward
RIB:
Router(config-if)#no ip route-cache
http://www.cisco.com/en/US/products/ps5763/products_configuration_guide_chapter09186a00802a1fae.html#wp1045020
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Load balancing with RIP
per-destination load balancing
debug ip packet
IP packet debugging is on
GAD#
*Mar 1 19:14:36.006: IP: tableid=0, s=192.168.14.2 (FastEthernet0/0), d=192.168.16.2 (Serial0/0), routed via RIB
*Mar 1 19:14:36.006: IP: s=192.168.14.2 (FastEthernet0/0), d=192.168.16.2 (Serial0/0), g=192.168.15.2, len 60, forward
*Mar 1 19:14:36.026: IP: tableid=0, s=192.168.16.2 (Serial0/1), d=192.168.14.2 (FastEthernet0/0), routed via RIB
*Mar 1 19:14:36.026: IP: s=192.168.16.2 (Serial0/1), d=192.168.14.2 (FastEthernet0/0), g=192.168.14.2, len 60, forward
*Mar 1 19:14:37.978: IP: s=0.0.0.0 (FastEthernet0/0), d=255.255.255.255, len 604, rcvd 2
*Mar 1 19:14:44.122: IP: s=0.0.0.0 (FastEthernet0/0), d=255.255.255.255, len 604, rcvd 2
*Mar 1 19:14:46.562: IP: s=192.168.14.1 (local), d=255.255.255.255 (FastEthernet0/0), len 92, sending broad/multicast
*Mar 1 19:14:47.278: IP: s=192.168.15.1 (local), d=255.255.255.255 (Serial0/0), len 72, sending broad/multicast
*Mar 1 19:14:50.266: IP: s=0.0.0.0 (FastEthernet0/0), d=255.255.255.255, len 604, rcvd 2
*Mar 1 19:14:51.958: IP: s=192.168.13.2 (Serial0/1), d=255.255.255.255, len 72, rcvd 2
*Mar 1 19:14:51.962: IP: s=192.168.15.2 (Serial0/0), d=255.255.255.255
Router(config-if)# ip route-cache
RIB:
http://www.cisco.com/en/US/products/ps5763/products_configuration_guide_chapter09186a00802a1fae.html#wp1045020
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 Unequal Cost Load Balancing with EIGRP
What is unequal cost load balancing?
 EIGRP Load Balancing
Every routing protocol supports equal cost
path load balancing.
In addition to that, IGRP and EIGRP also
support unequal cost path load balancing.
Use the variance command to instruct
the router to include routes with a metric
less than n times the minimum metric
route for that destination, where n is the
number specified by the variance
command.
Example: E-C-A: 20 * 2 = 40. Therefore,
E-C-A and E-B-A will be used for load
balancing.
router eigrp 1
network x.x.x.x
variance 2
http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a008009437d.shtml
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Administrative Distance of a Route
 In fact, a router might learn of a
route to the same network from
more than one source.
– For example, a static route might have
been configured for the same
network/subnet mask that was learned
dynamically by a dynamic routing
protocol, such as RIP. The router must
choose which route to install.
 Purpose of a metric
–It’s a calculated value used to determine
the best path to a destination
 Purpose of Administrative Distance
–It’s a numeric value that specifies the
preference of a particular route source.
For equal cost routes to be
installed they both must be static
routes or they both must be RIP
routes.
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Administrative Distance of a Route
 Administrative distance is an integer value from 0 to 255.
 The lower the value the more preferred the route source.
–An administrative distance of 0 is the most preferred.
–Only a directly connected network has an administrative distance
of 0, which cannot be changed
–An administrative distance of 255 means the router will not believe
the source of that route and it will not be installed in the routing
table.
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Administrative Distance of a Route
 Identifying the Administrative Distance (AD) in a routing
table
It is the first number in the brackets in the routing table
•R2 is running both RIP and EIGRP routing
protocols.
•R2 has learned of the 192.168.6.0/24 route from
R1 through EIGRP updates and from R3 through
RIP updates.
•RIP has an administrative distance of 120, but
EIGRP has a lower administrative distance of 90.
•So, R2 adds the route learned using EIGRP to
the routing table and forwards all packets for the
192.168.6.0/24 network to router R1.
This show ip rip database command
shows all RIP routes learned by R2,
whether or not the RIP route is installed in
the routing table.
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Administrative Distance of a Route
 The AD value can also
be verified with the
show ip protocols
command.
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Administrative Distance of a Route
 Directly connected routes
-Immediately appear in the routing table as soon as the
interface is configured
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Administrative Distance of a Route
 Directly connected routes
Have a default AD of 0
 Static Routes
Administrative distance of a static route has a default value of 1
 A static route using either a next-hop IP address or an exit
interface has a default AD value of 1.
–However, the AD value is not listed in show ip route when you
configure a static route with the exit interface specified. When a static
route is configured with an exit interface, the output shows the network
as directly connected via that interface.
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Administrative Distance of a Route
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Summary
 Dynamic routing protocols fulfill the following functions
-Dynamically share information between routers
-Automatically update routing table when topology changes
-Determine best path to a destination
 Routing protocols are grouped as either
-Interior gateway protocols (IGP)Or
-Exterior gateway protocols(EGP)
 Types of IGPs include
-Classless routing protocols - these protocols include subnet mask
in routing updates
-Classful routing protocols - these protocols do not include subnet
mask in routing update
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Summary
 Metrics are used by dynamic routing protocols to calculate the
best path to a destination.
 Administrative distance is an integer value that is used to
indicate a router’s “trustworthiness”
 Components of a routing table include:
-Route source
-Administrative distance (The smaller the better)
-Metric (The smaller the better)
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