Chapter 3
Dynamic Routing Protocols
CIS 82 Routing Protocols and Concepts
Rick Graziani
Cabrillo College
graziani@cabrillo.edu
Last Updated: 3/3/2009
Note
 This presentation will be updated prior to March. 25, 2008
 The audio of the lecture for this presentation will be available on
my web site after March. 25, 2008
 My web site is www.cabrillo.edu/~rgraziani.
 For access to these PowerPoint presentations and other
materials, please email me at graziani@cabrillo.edu.
2
For further information
 This presentation is an
overview of what is
covered in the
curriculum/book.
 For further explanation
and details, please read
the chapter/curriculum.
 Book:
 Routing Protocols
and Concepts
 By Rick Graziani and
Allan Johnson
 ISBN: 1-58713-206-0
 ISBN-13: 978-58713206-3
3
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 LinkState
 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
4
Introduction to Dynamic Routing
Protocols
 Perspective and Background
 Network Discovery and Routing Table Maintenance
 Dynamic Routing Protocol Advantages
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.
6
Perspective and
Background
 Classful (does not support
CIDR and VLSM)
 Classless (supports CIDR
and VSLM)
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
7
Perspective and
Background
Exterior Routing Protocols or Exterior Gateway Protocols (EGP)
 Border Gateway Protocol (BGP)
 Between ISPs (Internet service providers)
 Between ISPs and their larger private clients
 Path Vector routing protocol, metric – attributes (policies)
 Replaced EGP
8
Role of Dynamic Routing Protocol
Update
Update
Update
Update
Update
Update
 Dynamic Routing Protocols:
 Exchange of routing information between routers
 Dynamically learn information about remote networks
 Determines the best path to each network
 Automatically learn about new networks
 Automatically finds alternate paths if needed (link failure in current path)
9
Role of Dynamic Routing Protocol
Static Routes
Add new static routes…
Make additions, deletions, and
modifications to static routes!
 What is an advantage of dynamic routing protocols compared to
static routes?
 Less administrative overhead (change modifications)
 More work and more chances to make a mistake.
10
Role of Dynamic Routing Protocol
Dynamic Routing Protocol
No changes!
Add dynamic routing
protocol…
 Change in network using a dynamic routing protocol.
 Dynamic routing protocols automatically adjust to changes in the network:
 New networks
 Deleted networks
 Changes in topology
 Configuration is less error-prone
 Scales better with larger networks
11
Role of Dynamic Routing Protocol
 Disadvantages of using Dynamic routing protocol
 More CPU and memory requirements (not usually a problem)
 This is not that big an issue in most networks and with
modern routers.
 “Less secure” if routing updates are sent unencrypted.
 Most networks use both dynamic and static routes
12
Purpose of Dynamic Routing Protocols
My
I’m
routes
R1
Changes
I’m routes
My
Changes
R3
 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.
 Components of a routing protocol (depending upon the routing protocol):
 Data structures: Tables or databases for their operations, kept in RAM.
 Algorithm:
 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
13
Dynamic Routing Protocol Operation
I just learned
I’m running RIP and
about remote
will share with other
networks
routers
running
I no
longer
have aRIP.
from
R2 and
connection
to Update
R3.
this network!
Update
Update
Update
X
I just learned about
I’m running RIP
too.
remote
from
Gotnetworks
it, I will tell
R1 and
R3…R3.
Update
I just learned about
remote
networks
from
I’m running
RIP too.
R1 Got
and it!R2.
Update
The operations of a dynamic routing protocol vary depending on the type of
routing protocol, but in general:
1. The router shares routing messages and routing information with other
routers that are using the same routing protocol.
2. The router sends and receives routing messages on its interfaces.
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.
14
Static Routing
Usage,
Advantages, and
Disadvantages
 When would you use a static route?
 With smaller networks that are not expected to grow significantly.
 Routing to and from stub networks
 Default route
15
Classifying Dynamic Routing
Protocols
 IGP and EGP
 Distance Vector and Link-State
 Classful and Classless
 Convergence
Classifying Routing Protocols
 Routing Protocols can be classified by:
 IGP or EGP
 Distance vector or link-state
 Classful or classless
17
IGP and EGP
CENIC
BGP
PAIX
Palo Alto
Cabrillo
College
 An autonomous system (AS)—otherwise known as a routing domain—is
a collection of routers under a common administration.
 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: Within Cabrillo College & within CENIC
 Routing inside an autonomous system
 Exterior routing protocols: Between ISPs, CENIC and PAIX
 Routing between autonomous systems
18
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
19
Distance Vector Routing
Protocol Operation
 What does a street sign like this tell
you?
 How far (distance)
 Which way (direction)
 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 (shortest)
route determination
20
Distance Vector Routing
Protocol Operation
 Routing protocol
 Does not know the topology of an
internetwork.
 Only knows the routing information
received from its neighbors.
 Does not know if another path
would actually be faster.
Would another path
that is longer
actually be faster?
(speed limit)
I don’t have a map
of the network.
All I know is how far
and which direction
(to next hop router)
Distance Vector routing protocols are like signposts along the path to the final destination.
21
Distance Vector Routing
Protocol Operation
 Where might you use a distance
vector routing protocol?
 Simple and flat network
 The administrators do not have
enough knowledge about 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.
 More in Chapter 4.
22
Link-State Protocol Operation
 Link-state routing protocol can
create a “complete view,” or
topology, of the network.
 Link-state protocols are
associated with Shortest Path
First (SPF) calculations.
 A link-state router uses the linkstate information to:
 Create a topology map
 Select the best path to all
destination networks in the
topology.
 Each router makes the
decision!
OR
Link State routing protocols is like having a complete map of the network topology
23
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.
24
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
25
Classful
Routing
Protocols
172.16.0.0
192.168.1.0
192.168.2.0
172.16.0.0
192.168.1.0
192.168.2.0
 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.
 Network mask determined based on value of first octet of the
network address.
26
Classful Routing Protocols
172.16.0.0/16
Major Classful
Network
 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.
 More later!
27
Classless routing Protocols
172.16.1.0/28
192.168.1.0/24
192.168.2.4/30
172.16.0.0/28
192.168.1.0/24
192.168.2.0/30
 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 include the subnet mask with the
network address in routing updates.
28
Classless routing Protocols
172.16.0.0/16
Major Classful
Network
/27 and /30
subnets
172.16.128.0/30
172.16.132.0/30
172.16.136.0/30
 Classless routing protocols are required in most networks today
because of their support for:
 VLSM
 CIDR
 Discontiguous networks.
29
Dynamic Routing Protocols and Convergence
Got it, I will tell
R3…
I no longer have a
connection to
this network!
X
Got it!
 Convergence is when the routing tables of all routers are at a state of
consistency.
 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
 Update their routing tables
 A network is not completely operable until the network has converged;
therefore, most networks require short convergence times.
30
Dynamic Routing Protocols and Convergence
R2’s Routing
Table
R1’s Routing
Table
R3’s Routing
Table
 Generally, convergence time:
 Slow: RIP and IGRP
 Faster: EIGRP, OSPF, and IS-IS
31
Metrics
 Purpose of the Metric
 Metrics and Routing Protocols
 Load Balancing
Purpose of a Metric
?
 Metrics are a way to measure or compare.
 Determine the best path.
 Routing protocol learns multiple routes to the same destination.
 Metric is used to determine which path is most preferable
33
Purpose of a Metric
 What might be some ways (metrics) that routing protocols might use to
determine the “best path?
 Routing protocol metrics:
 RIP: Hop count
 IGRP and EIGRP: Bandwidth, delay, reliability and load
 OSPF (Cisco’s version): Bandwidth
 IS-IS: Four values (Cisco uses “default”) – Covered in CCNP
 BGP: Attributes – Covered in CCNP
 More later
34
Metric
Parameters
56 Kbps
 R1 to reach the 172.16.1.0/24 network.
 RIP: Fewest number of hops via R2.
 OSPF: Path with the highest cumulative bandwidth through R3.
 This results in faster packet delivery.
35
Metric Field in the Routing Table
R2# show ip route
C
C
C
S
R
R
172.16.0.0/24 is subnetted, 3 subnets
172.16.1.0 is directly connected, FastEthernet0/0
172.16.2.0 is directly connected, Serial0/0/0
192.168.1.0/24 is directly connected, Serial0/0/1
192.168.2.0/24 [1/0] via 192.168.1.1
192.168.7.0/24 [120/1] via 192.168.4.1, Serial0/0/1
192.168.8.0/24 [120/2] via 192.168.4.1, Serial0/0/1
 The routing table displays the metric for each dynamic and static
route.
 Static routes always have a metric of 0.
 Routing protocols install route in routing table with the lowest metric.
36
R2# show ip route
<output omitted>
Gateway of last resort is not set
R
192.168.1.0/24 [120/1] via 192.168.2.1, 00:00:24, Serial0/0/0
C
192.168.2.0/24 is directly connected, Serial0/0/0
C
192.168.3.0/24 is directly connected, FastEthernet0/0
C
192.168.4.0/24 is directly connected, Serial0/0/1
R
192.168.5.0/24 [120/1] via 192.168.4.1, 00:00:26, Serial0/0/1
R
192.168.6.0/24 [120/1] via 192.168.2.1, 00:00:24, Serial0/0/0
[120/1] via 192.168.4.1, 00:00:26, Serial0/0/1
R
192.168.7.0/24 [120/1] via 192.168.4.1, 00:00:26, Serial0/0/1
R
192.168.8.0/24 [120/2] via 192.168.4.1, 00:00:26, Serial0/0/1
 All routers running RIP
 What is the metric for R2
to reach the 192.168.8.0
network?
 2 (hops away)
37
Load
Balancing
R2# show ip route
<output omitted>
R
192.168.6.0/24 [120/1] via 192.168.2.1, 00:00:24, Serial0/0/0
[120/1] via 192.168.4.1, 00:00:26, Serial0/0/1
 What happens when two or more routes to the same destination have
identical metric values?
 The router load balances between these equal-cost paths.
 All routing protocols do equal cost load balancing.
 EIGRP also does unequal cost load balancing.
38
Administrative Distance
 Purpose of Administrative Distance
 Dynamic Routing Protocols and Administrative
Distance
 Static Routes and Administrative Distance
 Directly Connected Networks and Administrative
Distance
Purpose of Administrative Distance
 What if a router learns about a remote
network from two different routing
sources.
 What if RIP advertises the network as
10 hops away but OSPF advertises it
as a cumulative bandwidth of 100,000.
 Which is better RIP or OSPF?
 Can’t tell
 Can’t compare apples and oranges.
 Note: This is not common.
 Administrative distance (AD) is:
 Used to determine which routing
source takes precedence.
 Used when there are multiple
routing sources for the same
destination network address.
 Lower the AD the more preferred the
routing source.
?
So, which one would
be preferred RIP or
OSPF? OSPF
RIP: 1.1.1.1.
is 10 hops
OSPF: 1.1.1.1.
is 100,000 BW
Which route would be
preferred, OSPF or a Static
Route to the same
network? Static Route
40
Purpose of Administrative Distance
 Cisco uses Administrative distance
(AD) to define the preference of a
routing source.
 Routing sources:
 Directly connected networks
 Static routes
 Specific routing protocols
 AD for static and dynamic can be
modifed (in CCNP)
Note
 The term trustworthiness is
commonly used when defining
administrative distance.
 The lower the administrative distance
value, the more trustworthy the route.
41
Purpose of Administrative Distance
 AD: 0 to 255.
 The lower the value, the more
preferred the route source.
 AD of 0 is the most preferred.
 Only a directly connected
network has an administrative
distance of 0, which cannot be
changed.
 No better route to a network
than being directly connected
to that network.
 AD of 255 means the router will
not believe the source of that route
 Route will not be installed in
the routing table.
42
Verifying AD: show ip route
R2# show ip route
D
192.168.6.0/24 [90/2172416] via 192.168.2.1, 00:00:24, Serial0/0/0
What is the AD of this route? 90
43
Verifying AD: show ip protocols
R2# show ip protocols
Routing Protocol is “eigrp 100 “
Outgoing update filter list for all interfaces is not set
Incoming update filter list for all interfaces is not set
Default networks flagged in outgoing updates
Default networks accepted from incoming updates
EIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0
EIGRP maximum hopcount 100
EIGRP maximum metric variance 1
Redistributing: eigrp 100
Automatic network summarization is in effect
Automatic address summarization:
Maximum path: 4
Routing for Networks:
192.168.2.0
192.168.3.0
192.168.4.0
Routing Information Sources:
Gateway
Distance
Last Update
192.168.2.1
90
2366569
Distance: internal 90 external 170
<continued next slide?
44
show ip protocols (continued)
Routing Protocol is “rip”
Sending updates every 30 seconds, next due in 12 seconds
Invalid after 180 seconds, hold down 180, flushed after 240
Outgoing update filter list for all interfaces is not set
Incoming update filter list for all interfaces is not set
Redistributing: rip
Default version control: send version 1, receive any version
Interface
Send
Recv
Triggered RIP Key-chain
Serial0/0/1
1
2 1
FastEthernet0/0
1
2 1
Automatic network summarization is in effect
Maximum path: 4
Routing for Networks:
192.168.3.0
192.168.4.0
Passive Interface(s):
Routing Information Sources:
Gateway
Distance Last Update
192.168.4.1
120
Distance: (default is 120)
 More on show ip protocols later
45
Static Routes and Administrative Distance
 Static routes
 Default AD = 1
 After directly connected networks (AD = 0), static routes are the
most preferred route source.
46
Static Routes and Administrative Distance
Exit Interface: ip route 172.16.3.0 255.255.255.0 serial 0/0/0
R2# show ip route
C
C
S
C
S
172.16.0.0/24 is subnetted, 3 subnets
172.16.1.0 is directly connected, FastEthernet0/0
172.16.2.0 is directly connected, Serial0/0/0
172.16.3.0 is directly connected, Serial0/0/0
192.168.1.0/24 is directly connected, Serial0/0/1
192.168.2.0/24 [1/0] via 192.168.1.1
Next-hop: ip route 192.168.2.0 255.255.255.0 192.168.1.1
 What is the AD of a Static Route?
 Static route: default AD = 1 (never 0)
 Exit-interface: AD = 1
 Next-hop IP address: AD = 1
 After directly connected networks (AD = 0), static routes are the
most preferred route source.
47
Static Routes and Administrative Distance
Exit Interface: ip route 172.16.3.0 255.255.255.0 serial 0/0/0
R2# show ip route
C
C
S
C
S
172.16.0.0/24 is subnetted, 3 subnets
172.16.1.0 is directly connected, FastEthernet0/0
172.16.2.0 is directly connected, Serial0/0/0
172.16.3.0 is directly connected, Serial0/0/0
192.168.1.0/24 is directly connected, Serial0/0/1
192.168.2.0/24 [1/0] via 192.168.1.1
 The static route to 172.16.3.0 is listed as “directly connected”.
 It is common misconception to assume that the AD value of this route
must be 0 because it states “directly connected a” - false assumption.
48
Static Routes and Administrative Distance
Exit Interface: ip route 172.16.3.0 255.255.255.0 serial 0/0/0
R2# show ip route 172.16.3.0
Routing entry for 172.16.3.0/24
Known via “static”, distance 1, metric 0 (connected)
Routing Descriptor Blocks:
* directly connected, via Serial0/0/0
Route metric is 0, traffic share count is 1
 View AD value this static route with an exit-interface, use command show ip
route [route] option.
49
Directly Connected Networks and Administrative Distance
R2# show ip route 172.16.3.0
Routing entry for 172.16.1.0/24
Known via “connected”, distance 0, metric 0 (connected,
via interface)
Routing Descriptor Blocks:
* directly connected, via FastEthernet0/0
Route metric is 0, traffic share count is 1
 To see the AD value of a directly connected network, use the command
show ip route [route] option.
50
Floating Static
Route (Extra)
X
R2: ip route 192.168.8.0 255.255.255.0 192.168.4.1
R2: ip route 192.168.8.0 255.255.255.0 192.168.2.1 5
 There are situations when an administrator will configure a static route to
the same destination that is learned using a dynamic routing protocol, but
using a different path.
 The static route will be configured with an AD greater than that of the routing
protocol.
 If there is a link failure in the path used by the dynamic routing protocol, the
route entered by the routing protocol is removed from the routing table.
 The static route will then become the only source and will automatically be
added to the routing table.
 This is known as a floating static route and is discussed in CCNP courses.
51
Chapter 3
Dynamic Routing Protocols
CIS 82 Routing Protocols and Concepts
Rick Graziani
Cabrillo College
graziani@cabrillo.edu
Last Updated: 3/3/2009