Ch' 9 Basic Router Troubleshooting

1
Ch. 9 Basic Router Troubleshooting

• CCNA 2
• Rick Graziani
• Cabrillo College

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

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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

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Static Routing
6
Dynamic Routing
7
Default Routes

• There a couple of items of misinformation in this
  section that we need to address.

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Default Routes - IGRP
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.

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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)

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Determining route source and destination
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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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
  (which is the best path to a particular
  destination) gt what is the best source of
  routing information to a particular destination.

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Routing Metrics - Corrections

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

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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 choose 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

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Network Testing
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Network Testing and Troubleshooting

• You most likely do troubleshooting already
• Cars, cooking, computer, etc.
• Approach might vary slightly depending upon the
  scenario
• Lab
• New implementation
• Existing network
• Change made
• No changes made
• Use all possible resources
• Support contracts
• Web sites and newsgroups
• Books
• Friends and other people
• Management

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Different Models
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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

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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

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Testing using the OSI Model

• Layer 3 errors can include
• Routing protocol not enabled
• Wrong routing protocol enabled
• Incorrect IP addresses
• Incorrect subnet masks

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Various commands

• These commands show various levels of
  connectivity or lack of connectivity
• Ping
• Traceroute
• Telnet
• Show interfaces
• Show cdp neighbors
• Show ip protocols
• Debug
• Show running-config
• What do these commands tell you?

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Ch. 9 Basic Router Troubleshooting

• CCNA 2 version 3.0
• Rick Graziani
• Cabrillo College