Download Class Power Points for Chapter #6

Course ILT
Bridges, routers, & brouters
Unit objectives
 Discuss basic internetworking concepts
 Describe the functions of bridges,
switches and routers, describe routing
protocols
 Discuss Windows 2000/Server 2003
routing configuration
Course ILT
Topic A
 Topic A: Introduction to internetworking
 Topic B: Introducing bridges, routers and
switches
Course ILT
Internetworking
 Can be defined as the technology
and devices by which computers can
communicate across differing types
of networks
 Depends on:
– The number of computers on a cable
segment
– The route data has to take to get to its
destination
Course ILT
Internetworking devices
 At the Data Link level, “Switches” are more
appropriate than “Bridges”, which are all but obsolete.
 Also note that “Gateways” exist at all seven OSI
layers.
Course ILT
Internetworking devices
 Repeaters (and Hubs! – no one uses
repeaters any longer. A hub is really a
“multiport repeater”.
 Bridges (and Switches, &%$@!!!)
(as above, a switch is really a
“multiport bridge”.)
 Routers
 Gateways
Course ILT
Segments and backbones
 A segment is the portion of the
network on either side of two
network transmission devices –
normally, this will be a router.
 A backbone is a high-speed network
link connecting only segments
Course ILT
Segments connected to a backbone
Course ILT
The role of the MAC address
 Is used to keep track of where the data packet
is going next on its way to a destination.
 In a “frame” that exists at the data link layer, the
frame header has a source and destination
MAC address.
 It also encapsulates a “packet” from the
Network layer that contains, in its header, a
source and destination IP address.
 The destination IP address is always the final
address of the frame, but the destination MAC
address is the address of the “next hop.”
– So, the MAC address changes every time it passes
through a router, but an IP address never does.
Course ILT
Repeaters (and Hubs)
 Repeaters (and hubs) operate at the
Physical layer of the OSI Model
 Repeaters connect network segments of
similar media
 Problem areas that are not addressed by
repeaters
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Signal quality
Time delays
Network traffic
Node limitations
- Page 6-6
Course ILT
Activity A-1
Discussing internetworking basics
Course ILT
Topic B
 Topic A: Internetworking
 Topic B: Introducing bridges and
routers
Course ILT
Bridges and routers
 Provide a way of segmenting network traffic and
connecting different LAN types
 Careful planning and proper implementation of
routers help you to build an efficient
communications environment
 The book discusses “brouters”, which combine
the functions of bridges and routers. These are
obsolete, and were never very popular in the
first place. “Layer 3 switches”, on the other
hand, do the work of switches – which are layer
2 devices, just as bridges are, as well as
routers – which are layer 3 devices. They are
very popular – Cisco 3550, 3560, etc. You find
them in wiring closets, replacing routers, and
sitting on top of several switches, which provide
all the connections to the various devices such
as workstations, printers, etc.
Course ILT
Bridges
 Read the physical (MAC) address of
devices on a network and filter
information before passing it to another
network segment. A bridge divides a
network into 2 “segments”, and
“filtering” is a decision process in
which the bridge looks at a MAC
address of a packet and decides
whether to send it to the other segment
or to simply drop the packet because it
is destined for the same segment from
which it came and will therefore be
picked up by the proper device without
any action by the bridge whatsoever.
 Bridges are obsolete and have been
replaced by switches, which do the
same things as bridges except they use
hardware rather than software and
have multiple ports instead of just two.
Course ILT
Collision Domains with Bridge
Heterogeneous (translating) bridges
Course ILT
 Interconnects different types of networks, such
as Ethernet and Token Ring.
– The reason they can do this is because bridges (i.e.,
and switches) operate by dealing with the physical, or
MAC addresses, found in the MAC sublayer of the
Data Link layer.
– A MAC address is the same, whether it exists on a
token ring or on an ethernet network. These 2
topologies also share the LLC sublayer of the Data
Link layer. 802.3 is the IEEE’s ethernet
implementation, and 802.5 is it’s Token Ring. But, the
DIX version of ethernet is normally not compatible
with the IEEE’s token ring, because DIX uses its own
version of ethernet which never broke up the MAC and
LLC sublayers! Good diagram on this.
Course ILT
Encapsulating bridge
 Packages (encapsulates) frames of one
format into the format of another.
 This way, the frame is not read until it
reaches its final destination, so only the
format of the encapsulating technology
must be compatible.
Course ILT
Routing management for bridges
 Eliminates the possibility of duplicate
frames that might be generated by
having segments with multiple links
that form loops in a bridged network.
 Right idea, wrong choice of words. It
is “switching” or “looping”
management, but not “routing”.
Routing is a layer 3 function and has
its own rules governing traffic. See ff.
Course ILT
Flow control in a bridge
 Is necessary to know the relative capacity of
each of the various bridge segments.
 Once you know the capacity, you can create
rules to govern the rate at which data can be sent
and the mechanism for adjusting that rate.
 Is necessary to make sure that segments with
multiple links do not reproduce and distribute
the same information.
 The problem to be avoided is “Loops” – bridging
loops and routing loops.
 With bridges and switches, the “Spanning Tree
Algorithm” prevents loops by ensuring that only
one path exists between any two points.
Course ILT
Flow control in a bridge
 The book describes the “Spanning Tree
Routing Algorithm.” This is a very bad
choice of words, since there are indeed
routing algorithms, but they exist at layer
3, with routers, not layer 2 with switches
with which we are concerned with in this
section – very misleading.
 Also, the books continues to speak of
bridges implementing this algorithm.
Switches implement Spanning Tree. A
bridge cannot shut down redundant
ports because it only has two ports in the
first place! See the Spanning Tree ff.
Solution: Spanning Trees
Course ILT
 Ensure the topology has no loops
– Avoid using some of the links when flooding
– … to avoid forming a loop
 Spanning tree
– Sub-graph that covers all vertices but contains
no cycles
– Links not in the spanning tree do not forward
frames
20
Course ILT
Constructing a Spanning Tree
 Elect a root
– The switch with the smallest
identifier
 Each switch identifies if its interface
is on the shortest path from the root
– And it exclude from the tree if not
– Also exclude from tree if same
One hop
distance,
but with a higher identifier
 Message Format: (Y, d, X) i.e., (RootDistance-Node)
– From node X
– Claiming Y as root
– Distance is d
root
Three hops
21
Steps in Spanning Tree Algorithm
 Initially, every switch announces itself as the root
Course ILT
– Example: switch X announces (X, 0, X)
 Switches update their view of the root
– Upon receiving a message, check the root id
– If the new id is smaller, start viewing that switch as root
 Switches compute their distance from the root
– Add 1 to the distance received from a neighbor
– Identify interfaces not on a shortest path to the root and
exclude those ports from the spanning tree
22
Switch # 4’s Viewpoint
 Switch #4 thinks it is the root
– Sends (4, 0, 4) message to 2 and 7
1
Course ILT
 Switch #4 hears from #2
– Receives (2, 0, 2) message from 2
3
– … and thinks that #2 is the root (lower id)
– And realizes it is just one hop away
 Switch #4 hears from #7
5
2
4
– Receives (2, 1, 7) from 7
7
6
 (i.e., 2 is the root, it’s 1 hop from “me”, I’m node
7.)
– And realizes this is a longer path
 because it adds 1 to the path from 7 – 2, wh/is
already 1 hop, so 4-7-2 = 2 hops
– So, prefers its own one-hop path (4-2
=1hop)
– And removes 4-7 link from the tree.
23
Robust Spanning Tree Algorithm
 Algorithm must react to failures
Course ILT
– Failure of the root node
 Need to elect a new root, with the next lowest
identifier
– Failure of other switches and links
 Need to recompute the spanning tree
 Root switch continues sending messages
– Periodically reannouncing itself as the root (1, 0,
1)
– Other switches continue forwarding messages
 Detecting failures through timeout
 Switch waits to hear from others
– Eventually times out and claims to be the root
24
Course ILT
A learning (transparent) bridge
 Automatically identifies devices on the segments
it connects.
 Listens to each of the attached cable
segments and creates a table of addresses
originating on each segment.
 Both bridges and switches create tables. When
a switch receives a frame, it makes a note of the
port on which it entered and the MAC address
of the device with the frame. Then, when the
switch receives a frame destined for the device
with the MAC address that came in on the port
in question, it knows which port it should exit
on, to get to that device.
Learning bridge
Course ILT
 For a more detailed view of how a
switching table is created, and how a
switch or bridge learns whether to drop or
forward a frame, and where to forwards it if
it is not dropped, see the Cisco Press pdf
document, pages 8 and 9.
Course ILT
Simple Switch/Router Table
Course ILT
Local and remote bridges
 Local bridge has a LAN link directly
attached on each side
 A “remote bridge link” is a local
network across a wide area segment
 This means you have one network, but it
exists across a wide, geographic area.
This is a fairly recent phenomenon. In the
past, if you had a geographic gulf, you put
in a router and created two networks. But
a single network works faster and is more
efficient. With the newer solutions for
cabling and data transfer, wide area
switching is becoming more prevalent.
Course ILT
Layer 2 switches
 Is a more modern term for multiport
bridge (i.e., a switch. More modern,
like since 1983!)
 Operate at the data link layer of the
OSI model
 Implement advanced filtering
techniques to optimize performance
known as Virtual LAN (VLAN)
features
-
page 6-13
Course ILT
Activity B-1
Identifying types of bridges and switches
Course ILT
Routers
 Are used to segment an extended
internetwork into manageable, logical
subnets
About routers
Course ILT
 Early routers supported a single protocol.
 Today, multiple protocol routers might
support 15 to 20 protocols simultaneously.
 A router has significantly greater overhead
than a switch, so they are slower; i.e., they
must not only wait while a switch examines
the MAC address, but then must examine
the logical, IP address as well. And both
the switching table – MAC address to
port, as well as the routing table – IP
address to router interface, must be
populated.
Course ILT
Router features
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Processor/memory/storage
Physical interfaces (ports) supported
Protocols supported
Configuration/management
(open/proprietary)
Course ILT
Key points about routers
 Connect two or more subnetworks, which are
defined by the router interfaces at each end.
 Might be configured to support one or more
protocols
 Only process packets specifically addressed
to them as a destination, i.e., ip address.
 Packets destined for a locally connected
subnetwork are passed to that network
 Packets destined for a remote subnetwork are
passed to the next router in the path
 A router that exists in the same subnet as a
host can be configured as a default gateway.
Key points about routers - cont
Course ILT
 A routing table is normally populated
dynamically, when the routing protocol takes an
incoming packet and places the source ip address
of the packet into a routing table row headed by
the router interface on which the packet entered
the router.
 An administrator can manually enter routes into
the router, which ensures that packets will take
that route to a destination.
 When choosing between alternative routes, a
router relies on various factors. The reliability of
a route is the key in choosing a route. A static
route is the most reliable route there is, next to
being an interface that actually exists on the
router.
Course ILT
Routers with static routes
 Note that the book reads “static or dynamic
“routers”. A router is neither static nor dynamic
– only routes are static or dynamic, and the
routing protocols used to route traffic create
dynamic routes, that change when a current route
goes down, or when a better route is found.
 A route that is manually configured and that the
router must follow when sending out a packet, is
a static route.
 Static routes are more difficult to manage and
less efficient than their dynamic counterparts for
several reasons
– Manual configuration
– Manual updates
– Changing environments
Course ILT
Routers with dynamic routes
 Dynamic routes use an Interior Gateway
Protocol (IGP) to communicate with each
other
 The two most common Interior Gateway
protocols are:
– Routing Information Protocol (RIP) – a
“distance vector” routing protocol that is now
obsolete. RIPv.2 often replaces RIP, as does
EIGRP or IGRP, both proprietary Cisco
protocols. Uses the Bellman-Ford algorithm.
– Open Shortest Path First (OSPF) – a “link
state” routing protocol, based on the Dykstra,
or the “Open Shortest Path First” protocol.
Routing tables
Course ILT
 Routers using static and dynamic routes use
routing tables to pass packets to subnetworks.
 A routing table matches an incoming packet’s
source ip address with the router’s interface on
which it entered, in a spreadsheet layout of
column and row.
 A routing protocol will populate this table
dynamically, as soon as the router is turned on.
This is “convergence”. Updates are made
dynamically, at intervals, depending on the metric
a protocol uses to measure the value of a route.
 An administrator will create, and later update
routes by manually entering the source and
destination IP address as well as other factors,
both when the routes are created, as well as
when changes occur in the internetwork that
require a change of route.
Course ILT
Sample routing table
Course ILT
Routing examples
 Some specific situations are handled
as follows
– Local destination
 The packet will be addressed to the
destination host and other systems,
including routers, will ignore the packet
– Remote destination, next hop known
 The source host will place the IP address
for the next router as the immediate
destination
Routing examples
Course ILT
– Remote destination, next hop unknown
 The source host will place the IP address for
the default gateway as the immediate
destination
Course ILT
Brouters
 Operate at both the network layer for
routable protocols and at the Data
Link layer for non-routable protocols
 Handle both routable and nonroutable features by acting as routers
for routable protocols and bridges for
non-routable protocols
Course ILT
Bridges vs. routers
 Routers should be given preference
over bridges when designing and
configuring WANs
 Bridges, by design, can escalate a
transient reliability problem into a
serious network failure
Course ILT
Physical & Data Link Layers with
Ethernet and other Layer 2 topos
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Old and New(er) Ethernet
Packet Types.
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IP Header
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IP Header Fields Explained
1. Version - The version is a binary number that is four bits long. It indicates which version of IP is
being used. Currently we are using IP version four, although IP version six will soon make an
impact on the networking world.
2. IHL (Internet Header Length) - The IHL simply measures the length of the IP header in 32-bit words. The
minimum header length is five 32-bit words.
3. Type of Service - This field is for specifying special routing information. This field in particular relates to
Quality of Service technologies quite well. Essentially, the purpose of this 8-bit field is to prioritize
datagrams that are waiting to pass through a router.
4. Total Length - This 16-bit field includes the length of the IP datagram. This length includes the IP header
and also the data itself.
5. Identification - This is a 16-bit field that acts as a means of organizing chunks of data. If a message is too
large to fit in one data packet, it is split up and all of its child packets are given the same identification
number. This is handy to ensure data is rebuilt on the receiving end properly.
6. Flags - This field signifies fragmentation options- such as whether or not fragments are allowed. The
Flags field also has capability to tell the receiving source that more fragments are on the way, if enabled.
This is done with the MF flag, also known as the more fragments flag.
7. Fragment Offset - This is a 13-bit field that assigns a number value to each fragment. The receiving
computer will then use these numbers to reassemble the data correctly. Obviously this is only applicable if
fragments are allowed.
8. Time to Live - This is often known as TTL. It is a field that indicates how many hops a data packet should
go through before it is discarded. Every successful pass through a router, known as a hop, decrements this
field by one. When it reaches zero, it is discarded.
9. Protocol - This 8-bit field indicates which protocol should be used to receive the data. Some of the more
popular protocols such as TCP and UDP are identified by the numbers 6 and 17 respectively.
10. Header Checksum - This 16-bit field holds a calculated value that is used to verify that the header is still
valid. Each time a packet travels through a router this value is recalculated to ensure the header is still
indeed valid.
11. Destination IP Address - This 32-bit field holds the IP address of the receiving computer. It is used to
route the packet and to make sure that only the computer with the IP address in this field obtains the packets.
12. Source IP Address - This 32-bit field holds the IP address of the sending computer. It is used to verify
correct delivery, and will also be the return address in case an error occurs.
13. IP Options - This field can hold a fair number of optional settings. These settings are primarily used for
testing and security purposes. Although clever settings such as keeping timestamp data from each router
hop may seem handy, it will actually degrade speed more often than not.
14. Padding - Since the IP options field varies in length depending on the configuration, we need to have this
field set to occupy left over bits. This is because the header needs to be ended after a 32-bit word: no more,
no less.
15. Data - This is fairly self explanatory- it is simply the data that is being sent.
Course ILT
 Right: TCP header model
 Left: Capture of IP and
TCP headers using a
Packet Sniffer.
Explanation of TCP header fields:
–
Course ILT
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Source and destination port :These fields identify the local endpoint of the
connection. Each host may decide for itself how to allocate its own ports starting at 1024. The
source and destination socket numbers together identify the connection.
Sequence and ACK number : This field is used to give a sequence number to each
and every byte transferred. This has an advantage over giving the sequence numbers to
every packet because data of many small packets can be combined into one at the time
of retransmission, if needed. The ACK signifies the next byte expected from the source
and not the last byte received. The ACKs are cumulative instead of selective.Sequence
number space is as large as 32-bit although 17 bits would have been enough if the
packets were delivered in order. If packets reach in order, then according to the
following formula:
(sender's window size) + (receiver's window size) < (sequence number space)
the sequence number space should be 17-bits. But packets may take different routes
and reach out of order. So, we need a larger sequence number space. And for
optimisation, this is 32-bits.
Header length :This field tells how many 32-bit words are contained in the TCP header.
This is needed because the options field is of variable length.
Flags : There are six one-bit flags.
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URG : This bit indicates whether the urgent pointer field in this packet is being used.
ACK :This bit is set to indicate the ACK number field in this packet is valid.
PSH : This bit indicates PUSHed data. The receiver is requested to deliver the data to the
application upon arrival and not buffer it until a full buffer has been received.
RST : This flag is used to reset a connection that has become confused due to a host crash or
some other reason.It is also used to reject an invalid segment or refuse an attempt to open a
connection. This causes an abrupt end to the connection, if it existed.
SYN : This bit is used to establish connections. The connection request(1st packet in 3-way
handshake) has SYN=1 and ACK=0. The connection reply (2nd packet in 3-way handshake)
has SYN=1 and ACK=1.
FIN : This bit is used to release a connection. It specifies that the sender has no more fresh
data to transmit. However, it will retransmit any lost or delayed packet. Also, it will continue to
receive data from other side. Since SYN and FIN packets have to be acknowledged, they must
have a sequence number even if they do not contain any data.
Explanation of TCP header fields:
Course ILT
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Window Size : Flow control in TCP is handled using a variable-size sliding
window. The Window Size field tells how many bytes may be sent starting at the
byte acknowledged. Sender can send the bytes with sequence number between
(ACK#) to (ACK# + window size - 1) A window size of zero is legal and says that
the bytes up to and including ACK# -1 have been received, but the receiver would
like no more data for the moment. Permission to send can be granted later by
sending a segment with the same ACK number and a nonzero Window Size field.
Checksum : This is provided for extreme reliability. It checksums the header, the
data, and the conceptual pseudoheader. The pseudoheader contains the 32-bit IP
address of the source and destination machines, the protocol number for TCP(6),
and the byte count for the TCP segment (including the header).Including the
pseudoheader in TCP checksum computation helps detect misdelivered packets,
but doing so violates the protocol hierarchy since the IP addresses in it belong to
the IP layer, not the TCP layer.
Urgent Pointer : Indicates a byte offset from the current sequence number at
which urgent data are to be found. Urgent data continues till the end of the
segment. This is not used in practice. The same effect can be had by using two
TCP connections, one for transferring urgent data.
Options : Provides a way to add extra facilities not covered by the regular header.
eg,
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Maximum TCP payload that sender is willing to handle. The maximum size of segment is
called MSS (Maximum Segment Size). At the time of handshake, both parties inform
each other about their capacity. Minimum of the two is honoured. This information is sent
in the options of the SYN packets of the three way handshake.
Window scale option can be used to increase the window size. It can be specified by
telling the receiver that the window size should be interpreted by shifting it left by
specified number of bits. This header option allows window size up to 230.
Data : This can be of variable size. TCP knows its size by looking at the IP size
header.
Course ILT
IP “Packet” Encapsulated in a
Data Link Layer “Frame”
Course ILT
UDP Header
TCP port numbers
Table 1 - Frequently used TCP port numbers
Course ILT
Port Number
Process Name
Description
1
TCPMUX
TCP Port Service Multiplexer
5
RJE
Remote Job Entry
7
ECHO
Echo
9
DISCARD
Discard
11
USERS
Active Users
13
DAYTIME
Daytime
17
Quote
Quotation of the Day
19
CHARGEN
Character generator
20
FTP-DATA
File Transfer Protocol - Data
21
FTP
File Transfer Protocol - Control
23
TELNET
Telnet
25
SMTP
Simple Mail Transfer Protocol
27
NSW-FE
NSW User System Front End
29
MSG-ICP
MSG-ICP
31
MSG-AUTH
MSG Authentication
33
DSP
Display Support Protocol
35
Private Print Servers
37
TIME
Time
39
RLP
Resource Location Protocol
41
GRAPHICS
Graphics
- page 6-20
Course ILT
Activity B-2
Discussing routers and brouters
Understanding the routing protocols
Course ILT
 Two basic types of routing algorithms
– Distance vector algorithms
– Link state algorithms
Course ILT
Distance vector algorithms
Course ILT
Routing Protocols
 Dynamic routing using routing
protocols
 Purpose of routing protocols is to build
a “routing table” with the best routes
 Routing protocols are categorized into
two types:
– Distance Vector
– Link State
Course ILT
Routing Protocols
 Distance vector routing protocols are simple
 Generally they are easy to configure
 They use simple logic to determine the best
path to a given destination
 The term metric refers to the method or
measurement used by the routing protocol logic
to determine the “best path” to a given
network
Course ILT
Routing Protocols
 A distance vector routing protocol usually uses
hop count as its metric
 A distance vector routing protocol is
characterized by how it communicates with other
routing devices
 Distance vector routing protocols use
broadcasts to advertise their entire routing table
to “directly connected” peer routers
 A router is “directly connected” if it is at the end
of a cable or some other connecting device, the
other end of which is plugged into the router in
question, i.e., the “directly-connected” router.
 So, if I have a router with 3 interfaces, it can
have 3 directly connected “neighbors.”
Course ILT
Routing Protocols
 “Convergence” is the time it takes for a given
set of routers to learn routes to all the other
routers in the “internetwork”.
 Convergence describes the time it takes a set
of routers to learn of a change in the network
 Distance vector routing protocols generally
take longer to converge than link state
protocols because they use a periodic route
advertisement schedule.
 RIP, for example, sends it’s entire routing
table to its directly connected neighbors every
30 seconds.
 The next 3 slides are from another Power Point
Course ILT
Routing Protocols (other PPt)
 Dynamic routing uses routing
protocols
 Purpose of routing protocols is to
build a routing table with the best
routes
 Routing protocols are categorized into
two types:
– Distance Vector
– Link State
Course ILT
Routing Protocols (other PPt)
 Distance vector routing protocols are
simple
 Generally they are easy to configure
 They use simple logic (algorithms) to
determine the “best path” to a given
destination
 The term “metric” refers to the method
or measurement used by the routing
protocol logic to determine the best
path to a given network – e.g., hops,
bandwidth, latency, etc.
Course ILT
Routing Protocols (other PPt)
 A distance vector routing protocol usually uses hop
count as its metric (RIP and RIPv.2). [IGRP – Cisco
proprietary – on the other hand, uses 4 metrics and
MTU, Maximum Transmission Unit, as a tie-breaker.
The four metrics are Bandwidth, Distance, Latency
and Reliability]. Only 2 are used at any one time, with
bandwidth and delay the default metrics. The hop count
is 256 max, with 100 hops the default.
 A distance vector routing protocol is characterized by
how it communicates with other routing devices
 Distance vector routing protocols use broadcasts to
advertise their entire routing table to directly
connected peer routers. (With RIP, the broadcasts are
every 30 seconds; with IGRP it’s every 90 seconds.
This is very bandwidth-intensive and one reason that
link-state routing protocols are preferred in large
networks with many devices. The more devices there
are, the more broadcasts will be clogging the network.)
Course ILT
Routing Protocols
 A routing loop occurs when routers get
confused during update operations, causing
frames to bounce back and forth between a
set of interfaces
 Two easy methods to identify routing loops:
– Tracert or traceroute (TCP/IP utilities)
– View the routing table and the metric
associated with the network
Course ILT
Routing Protocols
 Prevent routing loops by using the following
software-based methods:
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Split horizon
Hold-down timers
Triggered updates
Hop count limits
Poisoning
 Note: the Network+ objectives don’t mention
“loops” at all, nor do they require a knowledge
of any of the loop-avoidance methods above.
The CCNA exam requires a fairly detailed
knowledge, however, so I think a brief
discussion here is appropriate.
Course ILT
Routing Loops Prevention
 First, I should note that the reason that loops
occur in the slow convergence of distance
vector protocols. Loops occur when every
router is not updated at close to the same time.
Link State protocols almost never have loops
because they converge in a very few seconds.
 Split Horizon: Information cannot be sent back
in the direction from which it was received.
 Hold-down Timers: Prevent regular update
messages from too rapidly reinstating a route
that has gone down. It allows time for the down
route to either come back up, or for the network
to stabilize before turning to the next best route.
It enforces a waiting time before changing a
route that has recently changed.
Course ILT
Routing Loops Prevention
 Triggered Updates: These go with the holddown timers, which start when a router gets a
message that route is down. A triggered update
will reset the timer under certain conditions, such
as when the hold-down timer expires or when
another update is received indicating a change
in the status of the network. The triggered
update will create a new routing table that
includes the change reflected in the new update.
 Maximum Hop Count: This is the classic
technique, built into all distance vector protocols.
It’s called “counting to infinity”. With RIP, any
more than 15 hops is considered an infinite
distance and the packet is dropped. With
IGRP/EIGRP the max hop count is 256, although
by default 100 hops is the limit.
Course ILT
Routing Loops Prevention
 Route Poisoning: (or “poison reverse”).
This technique enters a routing table entry
when a route goes down. It describes the
down route as having an infinite distance
from the network, thus preventing it from
being advertised, at least for a time. It is
used with a hold-down timer in order to
limit the duration of the poisoning.
Course ILT
Routing Protocols
 Link state routing protocols are more
intelligent than distance vector protocols
 The metric used by most link state protocols is
“cost”, based in turn on bandwidth allowing
more complex routing configurations
 Routing protocols capable of making complex
decisions use a mathematical formula or
algorithm for deriving the best path or route to
a given network
Course ILT
Routing Protocols
 Some link state protocols are
capable of determining the best
route to a destination network
based on the following:
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Delay
Bandwidth
Load
Reliability
MTU
 Distance vector routing protocols, other
than RIP, also use these same metrics;
e.g., IGRP and EIGRP.
Course ILT
Routing Protocols
 When more than one metric is used it is
referred to as a “composite metric”
 Link state protocols only send updates
when changes occur, and they only send
the changes, not the entire route table
– In fact, they do send the entire table, but only at
very long intervals, from one to several hours.
 Link state protocols use multicast and
unicast traffic instead of broadcast traffic
 Link state routers also develop an overall
picture of the networks available by
establishing “neighbor” relationships
Course ILT
RIP (v.1 and 2) (distance vector)
 Broadcasts a request for routing table
information from all other routers it can “see” –
(it can “see” any router directly connected to
one of its interfaces)
 The information received is used by the router to
determine the shortest path to each destination
 The route information is then entered into the
local routing table
 The router sends a RIP broadcast every 30
seconds
 The broadcast contains its known destinations
and the cost (in hops) to get to each
Course ILT
RIP
 RIP v.1 is almost never used any longer,
except in the smallest networks.
 First, it doesn’t “scale” well, i.e., it has a
“hop” limit of 15 hops. If it takes more than
15 hops to get to a destination, the
destination is considered to be an infinite
distance.
 Second, RIP v.2 is a vast improvement
over v.1; it’s still 15 hops max, but it can
understand “variable length subnet
masks” and supports “discontiguous”
networks (more on these later).
Course ILT
OSPF (link state)
 OSPF packets are carried within IP
datagrams
 Link State algorithm provides several
enhancements over RIP
 Hierarchical topology configuration
 Support for large internetworks
 Adaptation to changing conditions
 Traffic or “load” balancing over
multiple paths
 Authentication of router table
information exchange
Course ILT
ICMP
 Is a module of IP that provides error
reporting during datagram processing
 A common use is passing error
information between host and router
 This error data provides dynamic
routing table updates
 The “Ping” utility uses ICMP, as do
several other useful utilities.
 This doesn’t belong with RIP and OSPF.
Don’t be misled by their proximity in the
PPt.
Course ILT
Routing support in Windows
 Windows 2000 Server and Windows
Server 2003 support both RIP and
OSPF
 You have to configure routing in the
Routing and Remote Access
Service (RRAS)
 RRAS is installed by default, but not
enabled or configured
Course ILT
The General tab for a configured router
Course ILT
Activity B-3
Understanding routing protocols
Course ILT
Unit summary
 Discussed internetworking
 Described the functions of bridges,
routers, switches, routing protocols
and Windows 2000/Server 2003
routing configuration