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Ch. 9 – Basic Router Troubleshooting

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Title: Ch. 9 – Basic Router Troubleshooting


1
Ch. 9 Basic Router Troubleshooting
  • CCNA 2 version 3.0

2
Note
  • Most of the information in the module is a review
    of previous modules.
  • We will add some troubleshooting information to
    this presentation.

3
Overview
  • Students completing this module should be able
    to
  • Use the show ip route command to gather detailed
    information about the routes installed on the
    router
  • Configure a default route or default network
  • Understand how a router uses both Layer 2 and
    Layer 3 addressing to move data through the
    network
  • Use the ping command to perform basic network
    connectivity tests
  • Use the telnet command to verify the application
    layer software between source and destination
    stations
  • Troubleshoot by sequential testing of OSI layers
  • Use the show interfaces command to confirm Layer
    1 and Layer 2 problems
  • Use the show ip route and show ip protocol
    commands to identify routing issues
  • Use the show cdp command to verify Layer 2
    connectivity
  • Use the traceroute command to identify the path
    packets take between networks
  • Use the show controllers serial command to ensure
    the proper cable is attached
  • Use basic debug commands to monitor router
    activity

4
9.1 Examining the Routing Table
  • We have covered these and others in more depth in
    previous modules and the presentation on the
    Structure and Lookup Process of the Routing
    Table.
  • 9.1.1 The show ip route Command
  • 9.1.2 Determining the gateway of last resort
  • 9.1.3 Determining route source and destination
  • 9.1.4 Determining L2 and L3 addresses
  • 9.1.5 Determining the route administrative
    distance
  • 9.1.6 Determining the route metric
  • 9.1.7 Determining the route next hop
  • 9.1.8 Determining the last routing update
  • 9.1.9 Observing multiple paths to destination

5
Static Routing
6
Dynamic Routing
7
Default Routes
  • There a couple of items of misinformation in this
    section that we need to address.

8
Default Routes ip default-network command
  • The ip default-network command
  • Must be used with IGRP
  • Can be used with EIGRP and RIP, but not
    recommended (use ip route 0.0.0.0 0.0.0.0)
  • On router that uses ip default-network command,
    it must either have a specific route to that
    network or a 0.0.0.0/0 default route!

9
Default Routes - IGRP
ip route 0.0.0.0 0.0.0.0 s0 router igrp 10
network 172.16.0.0 network 192.168.17.0 ip
default-network 192.168.17.0
  • With IGRP
  • Use ip default-network
  • Need specific or default route, so once packets
    arrive at Cisco A it can forward those packets
    toward public network.

10
Default Routes - RIP
ip route 0.0.0.0 0.0.0.0 s0 router rip network
172.16.0.0 network 192.168.17.0
default-information originate
  • With RIP
  • Use 0.0.0.0/0 static route
  • Use default-information originate (IOS 12.0 and
    later)

11
Determining route source and destination
12
Path Switching and Packet Forwarding
Y
X
Data Link Header
IP (Network layer) Packet
Data Link Frame Data Link Header IP Packet
  • Path Switching
  • Host X has a packet(s) to send to Host Y
  • A router generally relays a packet from one data
    link to another, using two basic functions
  • 1. a path determination function - Routing
  • 2. a switching function Packet Forwarding
  • Lets go through all of the stages these routers
    use to route and switch this packet.
  • See if you can identify these two functions at
    each router.
  • Note Data link addresses have been abbreviated.

13
00-10 0A-10
192.168.4.10 192.168.1.10
  • From Host X to Router RTA
  • Host X begins by encapsulating the IP packet into
    a data link frame (in this case Ethernet) with
    RTAs Ethernet 0 interfaces MAC address as the
    data link destination address.
  • How does Host X know to forward to packet to RTA
    and not directly to Host Y? How does Host X know
    or get RTAs Ethernet address?
  • Remember, it looks at the packets destination ip
    address does an AND operation and compares it to
    its own ip address and subnet mask.
  • It determines if the two ip addresses are on the
    same subnet or not.
  • If the are on the same subnet, it looks for the
    destination ip address of the packet in its ARP
    cache. sending out an ARP request if it is not
    there.
  • If they are on different subnets, it looks for
    the ip address of the default gateway in its ARP
    cache sending out an ARP request if it is not
    there.

14
0B-31 00-20
192.168.4.10 192.168.1.10
1
3
2
  • RTA to RTB
  • 1. RTA looks up the IP destination address in
    its routing table.
  • 192.168.4.0/24 has next-hop-ip address of
    192.168.2.2 and an exit-interface of e1.
  • Since the exit interface is on an Ethernet
    network, RTA must resolve the next-hop-ip
    address with a destination MAC address.
  • 2. RTA looks up the next-hop-ip address of
    192.168.2.2 in its ARP cache.
  • If the entry was not in the ARP cache, the RTA
    would need to send an ARP request out e1. RTB
    would send back an ARP reply, so RTA can update
    its ARP cache with an entry for 192.168.2.2.

15
0B-31 00-20
192.168.4.10 192.168.1.10
1
3
2
  • RTA to RTB (continued)
  • 3. Data link destination address and frame
    encapsulation
  • After finding the entry for the next-hop-ip
    address 192.168.2.2 in its ARP cache, RTA uses
    the MAC address for the destination MAC address
    in the re-encapsulated Ethernet frame.
  • The frame is now forwarded out Ethernet 1 (as
    specified in RTAs routing table.
  • Notice, that the IP Addresses did not change.
  • Also notice that the Routing table was used to
    find the next-hop ip address, used for the data
    link address and exit interface, to forward the
    packet in a new data link frame.

16
FFFF
192.168.4.10 192.168.1.10
1
2
  • RTB to RTC
  • 1. RTB looks up the IP destination address in
    its routing table.
  • 192.168.4.0/24 has next-hop-ip address of
    192.168.3.2 and an exit-interface of s0 (serial
    0).
  • Since the exit interface not on an Ethernet
    network, RTA does not need to resolve the
    next-hop-ip address with a destination MAC
    address.
  • Remember, serial interfaces do not have MAC
    addresses.

17
FFFF
192.168.4.10 192.168.1.10
1
2
  • RTB to RTC
  • 2. Data link destination address and frame
    encapsulation.
  • When the interface is a point-to-point serial
    connection, the Routing Table process does not
    even look at the next-hop IP address.
  • Remember, a serial link is like a pipe - only
    one way in and only one way out.
  • RTA now encapsulates the IP packet into the
    proper data link frame, using the proper serial
    encapsulation (HDLC, PPP, etc.).
  • The data link destination address is set to a
    broadcast, since there is only one other end of
    the pipe and the frame is now forwarded out
    serial 0.

18
0B-20 0C-22
192.168.4.10 192.168.1.10
1
3
2
  • RTC to Host Y
  • 1. RTC looks up the IP destination address in
    its routing table.
  • 192.168.4.0/24 is a directly connected network
    with an exit-interface of e0.
  • RTC realizes that this destination ip address is
    on the same network as one of its interfaces and
    it can sent the packet directly to the
    destination and not another router.
  • Since the exit interface is on an directly
    connected Ethernet network, RTC must resolve the
    destination ip address with a destination MAC
    address.
  • 2. RTC looks up the destination ip address of
    192.168.4.10 in its ARP cache.
  • If the entry was not in the ARP cache, the RTC
    would need to send an ARP request out e0. Host Y
    would send back an ARP reply, so RTC can update
    its ARP cache with an entry for 192.168.4.10.

19
0B-20 0C-22
192.168.4.10 192.168.1.10
1
3
2
  • RTC to Host Y (continued)
  • 3. Data link destination address and frame
    encapsulation
  • After finding the entry for the destination ip
    address 192.168.4.10 in its ARP cache, RTC uses
    the MAC address for the destination MAC address
    in the re-encapsulated Ethernet frame.
  • The frame is now forwarded out Ethernet 0 (as
    specified in RTAs routing table.

20
Determining the route administrative distance
  • Not the best path, but the best source of routing
    information.
  • The administrative distance of the route is the
    key information that the router uses in deciding
    what is the best source of routing information to
    a particular destination.

21
Routing Metrics - Corrections
  • MTU is not and has never been used as a routing
    metric with RIP, IGRP, EIGRP, OSPF, IS-IS, or BGP.

22
Observing multiple paths to destination
  • Cisco routers will choose up to six equal cost
    paths to the same destination network, four by
    default.
  • Router(config-router)maximum-paths 6
  • Fast Switching vs. Process Switching (see
    presentation Ch. 7 Distance Vector Routing
    Protocols, Part 1 of 2 Distance Vector Routing
    and RIP)
  • This assumes the same routing protocols or the
    use of static routes, as you cannot compare RIP
    metrics with IGRP metrics.
  • Administrative distance will always play a role
    in choosing one routing source over another,
    static routes over dynamic, IGRP over RIP, etc.
  • The variance command and IGRP/EIGRP is never
    explained in this curriculum.
  • For more information about the variance command
    see
  • How Does Unequal Cost Path Load Balancing
    (Variance) Work in IGRP and EIGRP?
  • http//www.cisco.com/en/US/tech/tk365/tk207/techno
    logies_tech_note09186a008009437d.shtml

23
Network Testing
24
Network Testing and Troubleshooting
  • You most likely do troubleshooting already
  • Cars, cooking, computer, home repairs, etc.
  • Approach might vary slightly depending upon the
    scenario
  • Lab
  • New implementation
  • Existing network
  • Change made
  • No changes made
  • Use all possible resources
  • Fellow technical professionals
  • Search engines, web sites and newsgroups
  • Books
  • Support contracts

25
Different Models
26
Layer 1 testing
  • Media, telecom equipment, CSU\DSU etc

27
Testing using the OSI Model
  • Layer 1 errors can include
  • Broken cables
  • Disconnected cables
  • Cables connected to the wrong ports
  • Intermittent cable connection
  • Wrong cables used for the task at hand (must use
    rollovers, crossover cables, and straight-through
    cables correctly)
  • Transceiver problems
  • DCE cable problems
  • DTE cable problems
  • Devices turned off

28
Layer I
  • Carrier transitions are normally telecom
    equipment failures, CSU/DSU problems or telephone
    service problems.

29
Layer 2 testing
30
Testing using the OSI Model
  • Layer 2 errors can include
  • Improperly configured serial interfaces
  • Improperly configured Ethernet interfaces
  • Improper encapsulation set (HDLC is default for
    serial interfaces)
  • Improper clockrate settings on serial interfaces
  • Network interface card (NIC) problems

31
Layer 3 testing
  • Path determination

32
Testing using the OSI Model
  • Layer 3 errors can include
  • Routing protocol not enabled
  • Wrong routing protocol enabled
  • Typos are the major reason for the following
    problems
  • Incorrect IP addresses
  • Incorrect subnet masks

33
show ip route
  • View of routing tables

34
Understand show ip route
  • The show ip route command displays the contents
    of the IP routing table.
  • This table contains entries for all known
    networks and subnetworks, as well as a code that
    indicates how that information was learned.

35
Arguments to sh ip route
  • show ip route connected
  • show ip route network
  • show ip route rip
  • show ip route igrp
  • show ip route static

36
(No Transcript)
37
Show ip protocols
  • View routing protocol configuration and activity

38
Understand show ip protocols
  • Update timer values
  • Routing protocol configured
  • Networks advertised.
  • Interfaces sending and receiving updates

39
Layer 7 testing
40
Layer 7 telnet
  • telnet is a simple, fast, remote-access command
    line utility.
  • You must have client and server telnet software
    to function. Most network devices include both.
  • telnet indicates successful layer 7 functionality
    of the router, switch or server.
  • Failure is generally a DNS or application layer
    failure.
  • Corrupted config file, IOS, or hardware failure

41
Various commands
  • These commands show various levels of
    connectivity or lack of connectivity
  • Ping
  • Traceroute
  • Telnet
  • Show interfaces
  • Show cdp neighbors (Layer 2)
  • Show ip protocols
  • Debug
  • Show running-config

42
Ch. 9 Basic Router Troubleshooting
Finished!
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