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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
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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
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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
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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?
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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
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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
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8
18.3 Traditional WAN Architecture
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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
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10
18.3 Traditional WAN Architecture
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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
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18.4 Forming A WAN
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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
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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
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18.6 Addressing In A WAN
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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
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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
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18
18.7 Next
-
Hop Forwarding
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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
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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
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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
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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
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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
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18.9 Dynamic Routing Updates In A WAN
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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
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18.9 Dynamic Routing Updates In A WAN
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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
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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
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18.10 Default Routes
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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
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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
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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
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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
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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
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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
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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”
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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
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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
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40
Algorithm 18.3
Distance
-
vector algorithm
for route computation
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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
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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
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43
18.13 Shortest Path Computation In A
Graph
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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
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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
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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
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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
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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
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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
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