Computer Networks and Internets, 5e By Douglas E. Comer

© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

1

Computer Networks and Internets, 5e


By Douglas E. Comer

Lecture PowerPoints


By Lami Kaya, LKaya@ieee.org

© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

2

Chapter 18



WAN Technologies

And

Dynamic Routing

© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

3

Topics Covered

•
18.1 Introduction

•
18.2 Large Spans And Wide Area Networks

•
18.3 Traditional WAN Architecture

•
18.4 Forming A WAN

•
18.5 Store And Forward Paradigm

•
18.6 Addressing In A WAN

•
18.7 Next
-
Hop Forwarding

•
18.8 Source Independence

•
18.9 Dynamic Routing Updates In A WAN

•
18.10 Default Routes

•
18.11 Forwarding Table Computation

•
18.12 Distributed Route Computation

•
18.13 Shortest Path Computation In A Graph

•
18.14 Routing Problems


© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

4

18.1 Introduction

•
This chapter

–
considers the structure of a network that spans an arbitrarily large
area

–
describes the basic components used to build a packet switching
system

–
explains the fundamental concept of routing

–
presents the two basic routing algorithms

–
explains the advantages of each algorithm

–
extends the discussion of routing to the Internet

–
presents routing protocols that use the algorithms described here


© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

5

18.2 Large Spans And Wide Area
Networks

•
Networking technologies can be classified according to the
distance spanned:

–

PAN spans a region near an individual

–

LAN spans a building or campus

–

MAN spans a large metropolitan area

–

WAN spans multiple cities or countries

•
Consider a company that uses a satellite bridge to connect
LANs at two sites

–
Should the network be classified as a WAN or as an extended LAN?



© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

6

18.2 Large Spans And Wide Area
Networks

•
The key issue that separates WAN technologies from LAN
technologies is
scalability

•
A WAN must be able to grow as needed to connect many
sites

–
spread across large geographic distances

•
A technology is not classified as a WAN unless it can deliver
reasonable performance for a large scale network

–
A WAN does not merely connect to many computers at many sites

•
it must provide sufficient capacity to permit all computers to communicate

•
Thus, a satellite bridge that connects a pair of PCs and
printers is merely an
extended LAN

© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

7

18.3 Traditional WAN Architecture

•
WAN designers chose to create a special
-
purpose hardware
device that could be placed at each site

•
A
packet switch

provides

–
local connections for computers at the site

–
as well as connections for data circuits that lead to other sites

•
A packet switch consists of a small computer system

–
with a processor, memory, and I / O devices used to send and
receive packets

•
Early packet switches were constructed from conventional
computers

–
the packet switches used in the highest
-
speed WANs require
special
-
purpose hardware

•
Figure 18.1 illustrates the internal architecture

© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

8

18.3 Traditional WAN Architecture

© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

9

18.3 Traditional WAN Architecture

•
Since the advent of LAN technology, most WANs separate a
packet switch into two parts:

–
a Layer 2 switch that connects local computers

–
a
router

that connects to other sites

•
Part 4 of the text

–
discusses
Internet routers
in detail

–
and explains how the concepts covered here apply to the Internet

•
Communication with local computers can be separated from
transmission across a WAN

•
Figure 18.2 illustrates the separation



© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

10

18.3 Traditional WAN Architecture

© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

11

18.4 Forming A WAN

•
A WAN can be formed by interconnecting a set of sites

•
The exact details of the interconnections depend on

–
the data rate needed

–
the distance spanned

–
and the delay

•
Many WANs use leased data circuits

–
However, other forms are also available

•
such as microwave and satellite channels

•
A network designer must choose a topology

–
For a given set of sites, many topologies are possible

•
Figure 18.3 illustrates a possible way to interconnect

–
a WAN does not need to be
symmetric

the interconnections among
packet switches

–
the capacity of each connection can be chosen to accommodate the
expected traffic and provide redundancy in case of failure

© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

12

18.4 Forming A WAN

© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

13

18.5 Store And Forward Paradigm

•
The goal of a WAN is to allow as many computers as possible to
send packets simultaneously

•
The fundamental paradigm used to achieve simultaneous
transmission is known as
store

and
forward

•
To perform store and forward processing

–
a packet switch buffers packets in memory

•
The store operation occurs when a packet arrives:

–
I / O hardware in the switch places a copy of the packet in memory

•
The forward operation occurs once a packet has arrived and
is waiting in memory. The processor


–
examines the packet

–
determines its destination

–
and sends the packet over the I / O interface that leads to the
destination

© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

14

18.6 Addressing In A WAN

•
From the view of an attached computer

–
a traditional WAN network operates similar to a LAN

–
Each WAN technology defines the exact frame format a computer
uses when sending and receiving data

–
Each computer connected to a WAN is assigned an address

•
WANs addresses follow a key concept that is used in the
Internet:
hierarchical addressing

–
Hierarchical addressing divides each address into two parts:



(site, computer at the site)

–
In practice, instead of a identifying a site, each packet switch is
assigned a unique number

•
first part of an address identifies a packet switch

•
second part identifies a specific computer

•
Figure 18.4 shows two
-
part hierarchical addresses assigned
to computers connected to a pair of packet switches

© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

15

18.6 Addressing In A WAN

© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

16

18.6 Addressing In A WAN

•
Figure 18.4 shows each address as a pair of decimal integers

–
A computer connected to
port 6

on packet
switch 2
is assigned address
[2, 6]

•
In practice, an address is represented as a single binary value
with some bits of the binary value

–
used to represent a packet switch

–
and others used to identify a computer

•
In Part 4 of the text, we’ll show that each Internet address
consists of a binary number, where

–
a prefix of the bits identify a specific network in the Internet

–
the remainder of the bits identify a computer attached to the network


© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

17

18.7 Next
-
Hop Forwarding

•
What is the importance of hierarchical addressing?

•
When a packet arrives

–
a switch must choose an
outgoing path
over which to forward it

•
If a packet is destined

–
for a local computer

•
the switch sends the packet directly to the computer

–
Otherwise

•
the packet must be forwarded over to another switch

•
To make the choice, a packet switch

–
examines the destination address in the packet

–
and extracts the packet switch number

•
If the number in the destination address is identical to the packet switch's own
ID the packet is intended for a computer on the local packet switch

•
Otherwise, the packet is intended for a computer on another switch

•
Algorithm 18.1 explains the computation

© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

18

18.7 Next
-
Hop Forwarding

© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

19

18.7 Next
-
Hop Forwarding

•
A packet switch does not need to keep complete information
about how to reach all possible computers

–
nor does a switch need to compute the entire route a packet will follow

•
Instead, a switch bases forwarding on packet switch IDs

–
which means that a switch only needs to know which outgoing link

•
A switch only needs to compute the
next hop
for a packet

•
The process is called
next
-
hop forwarding

–
and is analogous to the way airlines list flights

•
To make the computation efficient,

–
switches use
table lookup

–
That is, each packet switch contains a
forwarding table

–
such tables were originally called
routing tables

•
lists all possible packet switches and gives a next hop for each

•
Figure 18.5 illustrates next
-
hop forwarding with a trivial example

18.7 Next
-
Hop Forwarding

© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

20

© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

21

18.7 Next
-
Hop Forwarding

•
Using only one part of a
two
-
part hierarchical address

to
forward a packet has two practical consequences

–
First, the
computation time
required to forward a packet is reduced
because the forwarding table can be organized as an array that uses
indexing instead of searching

–
Second, the forwarding table contains one entry per packet switch
instead of one entry per destination computer

•
The reduction in table size can be substantial, especially for a large WAN that
has many computers attached to each packet switch

•
A two
-
part hierarchical addressing scheme allows packet
switches to use only the first part of the destination address
until the packet reaches the
final switch

–
Once the packet reaches the final switch

•
the switch uses the second part of the address to choose a specific computer

–
As Algorithm 18.1 describes


© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

22

18.8 Source Independence

•
Next
-
hop forwarding does not depend on the packet's
original source or on the path the packet has taken before it
arrives at a particular packet switch

–
Instead, the next hop to which a packet is sent depends only on the
packet's destination

–
The concept, which is known as
source independence

•
Source independence allows the forwarding mechanism in a
computer network to be

compact
and

efficient

–
Because all packets follow the same path, only one table is required

–
Because forwarding does not use source information, only the
destination address needs to be extracted from a packet

–
A single mechanism handles forwarding
uniformly

packets that
originate on directly connected computers and packets that arrive
from other packet switches use the same mechanism

© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

23

18.9 Dynamic Routing Updates In A WAN

•
Forwarding table must guarantee the following:

•

Universal

communication

–
The forwarding table in each switch must contain a valid next
-
hop
route for each possible destination address

•

Optimal

routes

–
In a switch, the next
-
hop value in the forwarding table for a given
destination must point to the shortest path to the destination

•
Network failures further complicate forwarding

–
For example, if two paths exist to a given destination and one of the
paths becomes unavailable because hardware fails, forwarding
should be changed to avoid the unavailable path

–
A network manager cannot merely configure a forwarding table to
contain
static

values that do not change

–
Instead, software running on the packet switches continually tests for
failures, and
reconfigures

the forwarding tables automatically



© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

24

18.9 Dynamic Routing Updates In A WAN

•
We use the term
routing software
to describe software that
automatically reconfigures
forwarding tables

•

Route computation
in a WAN is to think of a graph that
models the network

–
software uses the graph to compute the shortest path to all possible
destinations

•
Each
node

in the graph corresponds to a packet switch in
the network (individual computers are not part of the graph)

•
If the network contains a direct connection between a pair of
packet switches

–
the graph contains an
edge

or
link

between the corresponding nodes

•
Figure 18.6 shows an example WAN and the corresponding
graph


© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

25

18.9 Dynamic Routing Updates In A WAN

© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

26

18.9 Dynamic Routing Updates In A WAN

•
As the figure 18.6 shows, nodes in the graph are given a
label

that is the same as the number assigned to the
corresponding packet switch

•
A graph representation is useful in computing next
-
hop
forwarding

–
because graph theory has been studied and efficient algorithms have
been developed

–
a graph abstracts away details, allowing routing software to deal with
the essence of the problem

•
When it computes next
-
hop forwarding for a graph

–
a routing algorithm must identify a
link

•
Our examples will used the notation (
k, j
) to denote a link
from node
k

to node
j

–
When a routing algorithm runs on the graph in Figure 18.6b

–
The algorithm produces output as shown in Figure 18.7


© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

27

18.9 Dynamic Routing Updates In A WAN

© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

28

18.10 Default Routes

•
The forwarding table for node
1

in Figure 18.7 raises an
important point:

–
a forwarding table may contain many entries that point to the same
next hop

•
An examination of the WAN in Figure 18.6a reveals why all
remote entries contain the same next hop:

–
the packet switch has only one connection to the network

–
Therefore, all outgoing traffic must be sent across the same
connection

–
Consequently, except for the entry that corresponds to the node
itself, all entries in node
1
's forwarding table have a next hop that
points to the link from node
1

to node
3

•
In our trivial example, the list of
duplicate entries
in the
forwarding table is short

© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

29

18.10 Default Routes

•
A large WAN may contains hundreds of duplicate entries

•
Most WAN systems include a mechanism that can be used
to eliminate the common case of duplicate entries

•

Default route
is a mechanism that allows a single entry in a
forwarding table to replace a long list of entries that have the
same next
-
hop value

–
Only one default entry is allowed in a forwarding table

–
and the entry has lower priority than other entries

•
If the forwarding mechanism does not find an explicit entry
for a given destination

–
it uses the default

•
Figure 18.8 shows the forwarding tables from Figure 18.7
revised to use a default route

© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

30

18.10 Default Routes

© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

31

18.10 Default Routes

•
Default routing is optional

–
a default entry is present only if more than one destination has the
same next
-
hop value

•
For example, the forwarding table for node
3

does not
contain a default route

–
because each entry has a unique next hop

•
However, the forwarding table for node
1
benefits from a
default route

–
because all remote destinations have the same next hop


© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

32

18.11 Forwarding Table Computation

•
Basic approaches to construct forwarding tables?

•

Static routing

–
A program computes and installs routes when a packet switch
boots

–
The routes do not change

•

Dynamic routing

–
A program builds an initial forwarding table when a switch boots

–
Program then alters the table as conditions in the network
change

•
Each approach has advantages and disadvantages

–
Advantages of static routing are simplicity and low overhead

–
Disadvantage is inflexibility

•
static routes cannot be changed when communication is disrupted

•
Large networks are designed with redundant connections to
handle occasional hardware failures

–
most WANs use a form of dynamic routing

© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

33

18.12 Distributed Route Computation

•
Algorithm 18.2 shows how a forwarding table can be
computed

–
after information about a network is encoded in a graph

•
In practice, WANs need to perform
distributed route
computation

–
Instead of a centralized program computing all shortest paths

•
each packet switch must compute its own forwarding table locally

–
All packet switches must participate in distributed route computation

•
There are two general forms:

–

Link
-
State Routing
(LSR), which uses
Dijkstra's algorithm

–

Distance
-
Vector Routing
(DVR), which uses another approach

•

The next sections describe each of the two approaches

–
Chapter 27 explains how each approach is used to control routes in
the Internet

© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

34

18.12 Distributed Route Computation

18.12.1 Link
-
State Routing (LSR)

•

Link
-
state routing
or
Link
-
status routing

–
the approach also known as
Shortest Path First

(SPF) routing

–
Dijkstra algorithm used it to characterize the way it works

•
actually all routing algorithms find shortest paths

•
To use LSR, packet switches
periodically

send messages
across the network that carry the
status of a link

–
For example, packet switches
5
and
9

measure the link between
them and send a status message

•
such as ``the link between 5 and 9 is
up
''

–
Each status message is broadcast to all switches

•
Every switch collects incoming status messages

–
and uses them to build a graph of the network

•
Each switch then uses Algorithm 18.2 to produce a
forwarding table by choosing itself as the source

© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

35

Algorithm 18.2


A version of Dijkstra’s
algorithm that computes
R,
a nexthop
forwarding table,
and
D, the distance to each
node from
the specified
source node

© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

36

18.12 Distributed Route Computation

18.12.1 Link
-
State Routing (LSR)

•

An LSR algorithm can adapt to hardware failures

•
If a link between packet switches
fails

–
the attached packet switches will detect the failure

–
and broadcast a status message that specifies the link is
down

•
All packet switches receive the broadcast

–
change their copy of the graph to reflect the change in the link's
status

–
and re
-
compute shortest paths

•
Similarly, when a link becomes available again

–
the packet switches connected to the link detect that it is working

–
and start sending status messages that report its
availability


© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

37

18.12 Distributed Route Computation

18.12.2 Distance Vector Routing (DVR)

•
As with LSR, each link in the network is assigned a
weight

•
The
distance

to a destination between two packet switches
is defined to be the sum of weights along the path between
the two

•
Like LSR, DVR arranges for packet switches to exchange
messages periodically

•
In DVR, a switch send a complete list of destinations and
the current cost of reaching each

•
When it sends a DVR message

–
a switch is sending a series of individual statements, of the form:


“I can reach destination X, and its current distance from me is Y”


© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

38

18.12 Distributed Route Computation

18.12.2 Distance Vector Routing (DVR)

•
DVR messages are not broadcast

–
Each switch periodically sends a DVR message to its neighbors

•
Each message contains pairs of (
destination, distance
)

•
Each packet switch must keep a list of possible destinations

–
along with the current distance to the destination and the next hop to
use

–
The list of destinations and the next hop for each can be found in the
forwarding table

•
DVR software can be considered as maintaining an
extension to the forwarding table

–
that stores a distance for each destination


© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

39

18.12 Distributed Route Computation

18.12.2 Distance Vector Routing (DVR)

•
When a message arrives at a switch from neighbor
N

–
the switch examines each item in the message

–
and changes its forwarding table if the neighbor has a shorter path to
some destination than the path currently being used

•
Example:

–
if neighbor
N

advertises a path to destination
D

as having cost
5

and
the current path through neighbor
K

has cost
100

•
the current next hop for
D
will be replaced by
N


•
and the cost to reach
D

will be
5

plus the cost to reach
N

•
Algorithm 18.3 specifies how routes are updated when using
the distance
-
vector approach


© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

40

Algorithm 18.3


Distance
-
vector algorithm
for route computation

© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

41

18.13 Shortest Path Computation In A
Graph

•
Once a graph has been created that corresponds to a
network

–
software uses a method known as
Dijkstra's Algorithm

•
To find the shortest path from a source node to each of the
other nodes in the graph:

–
a next
-
hop forwarding table is constructed during the computation of
shortest paths

–
The algorithm must be run once for each node in the graph

–
That is, to compute the forwarding table for packet switch
P

•
the node that corresponds to
P

is designated as the source node

•
and the algorithm is run



© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

42

18.13 Shortest Path Computation In A
Graph

•

Dijkstra's algorithm is popular

–
because it can be used with various definitions of shortest path

–
In particular, the algorithm does not require edges in the graph to
represent geographic distance. Instead, the algorithm

•
allows each edge to be assigned a

nonnegative
value called a weight

•
defines the distance between two nodes to be the sum of the weights along
a path between the nodes

•
Figure 18.9 illustrates the concept of weights

–
by showing an example graph with an integer weight assigned to
each edge

–
and a least
-
weight path between two nodes in the graph



© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

43

18.13 Shortest Path Computation In A
Graph

© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

44

18.13 Shortest Path Computation In A
Graph

•
Dijkstra's algorithm maintains a
set

of nodes,
S

–
for which the minimum distance and next hop have not been computed

•
The set is initialized to all nodes except the source

•
The algorithm then iterates until set
S

is empty

•
On each iteration

–
the algorithm removes a node from
S

that has the least distance from
the source

–
As it deletes node
u
, the algorithm examines the current distance from
the source to each of the neighbors of
u

that remains in the set

–
If a path from the source through
u

to the neighbor has less weight than
the current path, the algorithm updates the distance to the neighbor

•
After all nodes have been removed from
S

–
the algorithm will have computed the minimum distance to each node
and a correct next
-
hop forwarding table for all possible paths

© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

45

18.13 Shortest Path Computation In A Graph

•
Dijkstra's algorithm needs
3

data structures to store:

–
the current distance to each node

–
the next hop for the shortest path

–
and information about the remaining set of nodes

•
Nodes can be numbered from
1
to
n

as Figure 18.9

–
which makes the implementation efficient

•
because a node number can be used as an index into a data structure

•
The algorithm can use two arrays,
D

and
R

–
each indexed by the node number

–
The
i
th

entry in array
D

stores a current value of the minimum
distance from the source to node

i

–
The
i
th

entry in array
R
stores the next hop used to reach node
i

along the path being computed

–
The set
S

can be maintained as a
doubly linked
list of node numbers

•
which facilitates searching the entire set or deleting an entry

© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

46

18.13 Shortest Path Computation In A Graph

•
Algorithm 18.2 uses weight
(i, j
) as a function

–
that returns the weight of the edge from node
i
to node
j

•
Function
weight

is assumed to return a reserved value
infinity

if no edge exists from node
i

to node
j

–
In practice, any value can be used to represent infinity provided the
value is larger than the sum of weights along any path in the graph

–
One way to generate a value infinity consists of adding one to the
sum of all weights on all edges

•
Allowing arbitrary weights to be assigned to edges of a
graph means that

–
one algorithm can be used with different measures of distance

–
For example, some WAN technologies measure distance by
counting the number of packet switches along a path


© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

47

18.13 Shortest Path Computation In A Graph

•
To use the algorithm for such technologies

–
each edge in the graph is assigned a weight of
1

•
In other WAN technologies, weights are assigned to reflect
the
capacity

of the underlying connections.

–
A network manager can assign weights to links to control routing

–
For example, consider a case where two separate paths exist
between a pair of packet switches with

•
one path designated to be the
primary path

•
and the other designated to be a
backup path

•
To enforce such a policy

–
a network manager can assign the primary link a low weight and the
other link a high weight

–
Routing software will configure forwarding tables to use the path with
low weight

•
unless the path is not available

•
in which case routing software will select the alternative path


© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

48

18.14 Routing Problems

•
In theory, either LSR or DVR routing will compute shortest
paths

•
Furthermore, each approach will eventually
converge

–
meaning that the forwarding tables in all packet switches agree

•
However, problems do occur

–
For example, if LSR messages are lost, two packet switches can
disagree

about the shortest path

•
DVR problems can be more severe

–
because a link failure can cause two or more packet switches to
create a
routing loop

•
in which each packet switch thinks the next packet switch in the set is the
shortest path to a particular destination

•
As a result, a packet can circulate among the switches indefinitely


© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

49

18.14 Routing Problems

•
One of the primary reasons DVR protocols exhibit problems comes from
backwash


–
(i.e., a packet switch receives information that it sent)

•
For example, suppose a switch tells its neighbors


“I can reach destination
D
at cost
3
''

–
If the connection leading to destination
D

fails

•
the switch will remove the entry for
D

from its forwarding table


(or mark the entry invalid)

–
But the switch has told neighbors that a route exists

•
Imagine that just after the link fails

–
one of the neighbors sends a DVR message that specifies


“I can reach destination

D
at cost
4
”

•
Unfortunately

–
the message will be believed

–
and a
routing loop

will be created

© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

50

18.14 Routing Problems

•
Most practical routing mechanisms contain constraints and
heuristics

to prevent problems like routing loops

•
For example, DVR schemes employ
split horizon

–
which specifies that a switch does not send information back to its
origin

•
Furthermore, most practical routing systems introduce
hysteresis


–
that prevents the software from making many changes in a short
time

•
However, in a large network where many links fail and
recover frequently, routing problems can occur