Ch. 9 – Basic Router
Troubleshooting
CCNA 2 version 3.0
Rick Graziani
Cabrillo College
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
•Rick Use
debug commands to monitor router activity
Grazianibasic
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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
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Dynamic Routing
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Default Routes
•
There a couple of items of misinformation in this section
that we need to address.
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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!
Rick Graziani graziani@cabrillo.edu
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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.
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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
192.168.1.0/24
.1
e0
192.168.1.10/24
X
RTA
192.168.2.0/24
.1
.2
s0
s0
RTB
Data Link Header
Data link destination address
Data link source address Other data link fields
192.168.3.0/24
.1
.2
s1
s0
RTC
Y
192.168.4.0/24
.1
e0
192.168.4.10/24
IP (Network layer) Packet
IP Destination Address
IP Source Address Other IP fields and data
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
•
•
•
Let’s 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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X
192.168.1.0/24
.1
e0
192.168.1.10/24
00-10
0A-10
Data link destination address
00-10
RTA
192.168.2.0/24
.1
.2
e1
e0
00-20
0B-31
RTB
Data link source address Other data link fields
0A-10
192.168.3.0/24
.1
.2
s0
s0
RTC
IP Destination Address
192.168.4.0/24
Y
.1
e0
192.168.4.10/24
0C-22
0B-20
IP Source Address Other IP fields and data
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 RTA’s Ethernet 0 interface’s 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 RTA’s Ethernet address?
– Remember, it looks at the packet’s 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.
•Rick IfGraziani
yougraziani@cabrillo.edu
do not remember, be sure to review our previous presentation, “ARP –
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X
192.168.1.0/24
.1
e0
192.168.1.10/24
00-10
0A-10
Data link destination address
0B-31
RTA
192.168.2.0/24
.1
.2
e1
e0
00-20
0B-31
192.168.3.0/24
.1
.2
s0
s0
RTB
Data link source address Other data link fields
00-20
IP Destination Address
192.168.4.0/24
Y
.1
e0
192.168.4.10/24
0C-22
0B-20
IP Source Address Other IP fields and data
192.168.4.10 192.168.1.10
1
3
RTA ARP Cache
IP Address
MAC Address
192.168.2.2
0B-31
RTC
2
RTA Routing Table
Network
Hops Next-hop-ip Exit-interface
192.168.1.0/24 0
Dir.Conn.
e0
192.168.2.0/24 0
Dir.Conn
e1
192.168.3.0/24 1
192.168.2.2
e1
192.168.4.0/24 2
192.168.2.2
e1
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 nexthop-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.
Rick Graziani graziani@cabrillo.edu
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X
192.168.1.0/24
.1
e0
192.168.1.10/24
00-10
0A-10
Data link destination address
0B-31
RTA
192.168.2.0/24
.1
.2
e1
e0
00-20
0B-31
192.168.3.0/24
.1
.2
s0
s0
RTB
Data link source address Other data link fields
00-20
IP Destination Address
192.168.4.0/24
Y
.1
e0
192.168.4.10/24
0C-22
0B-20
IP Source Address Other IP fields and data
192.168.4.10 192.168.1.10
1
3
RTA ARP Cache
IP Address
MAC Address
192.168.2.2
0B-31
RTC
2
RTA Routing Table
Network
Hops Next-hop-ip Exit-interface
192.168.1.0/24 0
Dir.Conn.
e0
192.168.2.0/24 0
Dir.Conn
e1
192.168.3.0/24 1
192.168.2.2
e1
192.168.4.0/24 2
192.168.2.2
e1
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 reencapsulated Ethernet frame.
The frame is now forwarded out Ethernet 1 (as specified in RTA’s 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.
Rick Graziani graziani@cabrillo.edu
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X
192.168.1.0/24
.1
e0
192.168.1.10/24
00-10
0A-10
Data link destination address
RTA
192.168.2.0/24
.1
.2
e1
e0
00-20
0B-31
RTB
192.168.3.0/24
.1
.2
s0
s0
Data link source address Other data link fields
FFFF
RTC
IP Destination Address
192.168.4.0/24
Y
.1
e0
192.168.4.10/24
0C-22
0B-20
IP Source Address Other IP fields and data
192.168.4.10 192.168.1.10
1
2
Network
192.168.1.0/24
192.168.2.0/24
192.168.3.0/24
192.168.4.0/24
RTB Routing Table
Hops Next-hop-ip Exit-interface
1
192.168.2.1
e0
0
Dir.Conn
e0
0
Dir.Conn
s0
1
192.168.3.2
s0
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.
Rick Graziani graziani@cabrillo.edu
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X
192.168.1.0/24
.1
e0
192.168.1.10/24
00-10
0A-10
Data link destination address
RTA
192.168.2.0/24
.1
.2
e1
e0
00-20
0B-31
RTB
192.168.3.0/24
.1
.2
s0
s0
Data link source address Other data link fields
FFFF
RTC
IP Destination Address
192.168.4.0/24
Y
.1
e0
192.168.4.10/24
0C-22
0B-20
IP Source Address Other IP fields and data
192.168.4.10 192.168.1.10
1
2
Network
192.168.1.0/24
192.168.2.0/24
192.168.3.0/24
192.168.4.0/24
RTB Routing Table
Hops Next-hop-ip Exit-interface
1
192.168.2.1
e0
0
Dir.Conn
e0
0
Dir.Conn
s0
1
192.168.3.2
s0
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.
Rick Graziani graziani@cabrillo.edu
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X
192.168.1.0/24
.1
e0
192.168.1.10/24
00-10
0A-10
Data link destination address
0B-20
RTA
192.168.2.0/24
.1
.2
e1
e0
00-20
0B-31
192.168.3.0/24
.1
.2
s0
s0
RTB
Data link source address Other data link fields
0C-22
IP Destination Address
192.168.4.0/24
Y
.1
e0
192.168.4.10/24
0C-22
0B-20
IP Source Address Other IP fields and data
192.168.4.10 192.168.1.10
1
3
RTC ARP Cache
IP Address
MAC Address
192.168.4.10
0B-20
RTC
2
Network
192.168.1.0/24
192.168.2.0/24
192.168.3.0/24
192.168.4.0/24
RTC Routing Table
Hops Next-hop-ip Exit-interface
2
192.168.3.1
s0
1
192.168.3.1
s0
0
Dir.Conn
s0
0
Dir.Conn
e0
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.
Rick Graziani graziani@cabrillo.edu
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X
192.168.1.0/24
.1
e0
192.168.1.10/24
00-10
0A-10
Data link destination address
0B-20
RTA
192.168.2.0/24
.1
.2
e1
e0
00-20
0B-31
192.168.3.0/24
.1
.2
s0
s0
RTB
Data link source address Other data link fields
0C-22
IP Destination Address
192.168.4.0/24
Y
.1
e0
192.168.4.10/24
0C-22
0B-20
IP Source Address Other IP fields and data
192.168.4.10 192.168.1.10
1
3
RTC ARP Cache
IP Address
MAC Address
192.168.4.10
0B-20
RTC
2
Network
192.168.1.0/24
192.168.2.0/24
192.168.3.0/24
192.168.4.0/24
RTC Routing Table
Hops Next-hop-ip Exit-interface
2
192.168.3.1
s0
1
192.168.3.1
s0
0
Dir.Conn
s0
0
Dir.Conn
e0
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 reencapsulated Ethernet frame.
The frame is now forwarded out Ethernet 0 (as specified in RTA’s 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) –> 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/technologies_tech_n
ote09186a008009437d.shtml
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Network Testing
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