Download Control Message

WXES2106
Network Technology
Semester 1 2004/2005
Chapter 7
TCP/IP Suite Error and
Control Messages
CCNA2: Module 8, 9
Contents
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Introduction
ICMP Message
Control Message
Examining the Routing Table
Network Testing
Troubleshooting Router
Introduction
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IP is an unreliable method for delivery of network data
It is known as a best effort delivery mechanism.
Internet Control Message Protocol (ICMP) is an error
reporting protocol for IP.
ICMP reports on the status of the delivered packet only
to the source device.
ICMP messages are encapsulated into datagrams in the
same way any other data is delivered using IP.
Introduction
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If a router receiving a packet that it is unable to deliver to
its ultimate destination, the router sends an ICMP host
unreachable message to the source.
Introduction
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If the destination device receives the ICMP echo
request, it formulates an echo reply message to send
back to the source of the echo request.
If the sender receives the echo reply, this confirms that
the destination device can be reached via the IP
protocol.
Introduction
Introduction
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Excessively long route
 The packet that exceed the maximum hop count,
which is also known as reaching its time-to-live (TTL).
The hop limit of RIP is 15. When the TTL of the
datagram value reaches zero, the packet is
discarded.
 Two routers continually route a datagram back and
forth between them never reaching its destination.
ICMP Message
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Each ICMP message start with these fields:
 Type
 Indicates the type of ICMP message being sent
 Code
 Includes further information specific to the message
type
 Checksum
 Verify the integrity of the data
ICMP Message
ICMP Message
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Destination unreachable message
 Hardware failures, improper protocol configuration,
down interfaces and incorrect routing information
 May also be sent when packet fragmentation is
required in order to forward a packet.
 May also be generated if IP related services such as
FTP or Web services are unavailable.
ICMP Message
Code Values for Destination Unreachable Message
Control Message
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Control messages are used to inform hosts of
conditions such as network congestion or the
existence of a better gateway to a remote network.
A common ICMP control message is the ICMP
redirect/change request.
This type of message can only be initiated by a router.
When a host connects to a segment that has two or
more directly connected routers. The default gateway of
the host may need to use a redirect/change request to
inform the host of the best path to a certain network.
Control Message
Redirect/change request Message and Code
Control Message
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Default gateways send ICMP redirect/change request
messages when
 The interface on which the packet comes into the
router is the same interface on which the packet
gets routed out.
 The subnet/network of the source IP address is
the same subnet/network of the next-hop IP
address of the routed packet.
 The datagram is not source-routed.
 The route for the redirect is not another ICMP redirect
or a default route.
 The router is configured to send redirects.
Control Message
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The ICMP timestamp request message allows a host to
ask for the current time according to the remote host.
The remote host uses an ICMP timestamp reply
message to respond to the request.
Originate timestamp
 The time on the requesting host just before the
timestamp request is sent
Receive timestamp
 The time that the destination host receives the ICMP
timestamp request.
Transmit timestamp
 Filled in before the ICMP timestamp reply is returned.
Control Message
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Address mask
 If a host does not know the subnet mask, it may send
an address mask request to the local router or
broadcast.
 When the router receives the request, it will respond
with an address mask reply.
Router Discovery
 The host sending a router solicitation message to
all routers, using the multicast address 224.0.0.2
as the destination address.
 When a router that supports the discovery process
receives the router discovery message, a router
advertisement is sent in return.
Control Message
Router Discovery Message
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A host generates an ICMP router solicitation
message in response to a missing default gateway.
Control Message
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ICMP source-quench messages asks senders to
reduce the rate at which they are transmitting
packets during congestion
However, most Cisco routers do not send source-quench
messages by default, because the source-quench
message may itself add to the network congestion.
It is used effectively in SOHO
Examine Routing Table
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Routers store the routing information in routing tables
using on-board dynamic random access memory
(DRAM).
The show ip route command displays the contents of
the IP routing table.
Examine Routing Table
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Routers keep a default route, or a gateway of last
resort. Default routes are used when the router is unable
to match a destination network with a more specific entry
in the routing table.
Before routers can dynamically exchange information, an
administrator must configure at least one router with a
default route.
 ip default-network or
 ip route 0.0.0.0 0.0.0.0
Examine Routing Table
Examine Routing Table
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The administrative distance of the route is the key
information that the router uses in deciding which is the
best path to a particular destination.
The lower the administrative distance, the more
trustworthy the source.
Examine Routing Table
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Routing protocols use metrics to determine the best
route to a destination.
The smaller the metric number, the better the path.
Static metric: bandwidth and delay
Dynamic metric: Load and reliability
Examine Routing Table
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IGRP Metric
 Metric=K1 *Bandwidth + (K2 * Bandwidth)/256-load) +
K3 * Delay
 The default constant values are K1=K3=1 and
K2=K4=K5=0 so:
 Metric=Bandwidth + Delay
Command to determine the last routing update
 Router#show ip route
 Router#show ip route network
 Router#show ip protocols
 Router#show ip rip database
Network Testing
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Troubleshooting is a process that allows a user to find
problems on a network.
Basic testing of a network should proceed in sequence
begin with Layer 1 and work to Layer 7.
Layer 1 errors can include:
 Broken
cables, disconnected cables, cables
connected to the wrong ports
 Intermittent cable connection, Wrong cables used
 Transceiver problems
 DCE cable problems, DTE cable problems
 Devices turned off
Network Testing
Troubleshooting
Network Testing
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Layer 2 errors can include:
 Improperly configured serial interfaces or Ethernet
interfaces
 Improper encapsulation set
 Improper clockrate settings on serial interfaces
 Network interface card (NIC) problems
Layer 3 errors can include:
 Routing protocol not enabled
 Wrong routing protocol enabled
 Incorrect IP addresses or subnet masks
Network Testing
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Indicator lights are a useful tool for troubleshooting
The ping utility is used to test network connectivity.
The telnet utility is a virtual terminal protocol that is part
of the TCP/IP protocol suite. It allows verification of the
application layer software between source and
destination stations.
Troubleshooting Router
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The Cisco IOS contains a rich set of commands for
troubleshooting.
Router#show interfaces
 check the status and statistics of the interfaces
Router#show interfaces FastEthernet
 view the status of the FastEthernet interfaces
Router#show interfaces serial0/0
 view the status of Serial 0/0
Troubleshooting Router
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Layer 1 Problem
 Cable
 Connectors
 Interfaces
Layer 2 Problem
 No keepalives
 No clock rate
 Mismatch in encapsulation type
Router#show cdp neighbors
 Displays
information about
neighbors.
directly
connected
Troubleshooting Router
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Traceroute
 It is used to discover the routes that packets take
when traveling to their destination
 Test the network layer (Layer 3) on a hop-by-hop
basis and provide performance benchmarks.
 The output of the traceroute command generates a
list of hops that were successfully reached.
 If an asterisk (*) appears, the packet failed.
 Sends out a sequence of User Datagram Protocol
(UDP) datagrams from the router to an invalid port
address on the remote host.
 This router then responds with an ICMP Time
Exceeded Message (TEM)
Troubleshooting Router
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Router#show ip protocols
 display information about routing protocols and the
routing table
Router#show ip route
 displays the contents of the IP routing table.
Router#show controllers serial
 determine the type of cable connected
The debug command is used to display dynamic data
and events.
By default, the router sends the debug output and
system messages to the console.
Troubleshooting Router
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Router#terminal monitor
 redirected debug output to the remote terminal
Router#service timestamps debug uptime
 provides the time when the debug event occurred and
the duration of time between events.
Router#no debug all
 turn off all diagnostic output.
Router#undebug all
 turn off all diagnostic output.