Section 18: Connectivity Troubleshooting

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116. Introduction

https://www.udemy.com/course/cisco-icnd1/learn/lecture/8605642#overview

  • Ping
  • Traceroute

117. Basic Connectivity Troubleshooting

https://www.udemy.com/cisco-icnd1/learn/lecture/8605640#overview

Ping

  • Uses ICMP
    • Internet Control Message Protocol

Message Replies

  • ! (Exclamation Point): The Ping reached its destination and returned successfully.
    • It is not uncommon to miss the first 1 (or even 2) pings if a devices has not updated its ARP cache.
  • . (Period): Indicates the Ping has failed.
    • Router does not have a corresponding route
    • Destination IP address is not responding
  • U (Unreachable): The packet has been discarded
    • Blocked by an Access Control List

Extended Ping

PC1> ping 10.1.2.10
10.1.2.10 icmp_seq=1 timeout
10.1.2.10 icmp_seq=2 timeout
...
10.1.2.10 icmp_seq=5 timeout
R1> ping 10.1.2.10
Sending 5 100-byte ICMP Echos to 10.1.2.10, timeout is 2 seconds:
!!!!!
Success rate is 100% (5/5)

Scenario:

  • PC1 complains he cannot access services on PC3
  • R4 does not have a route to 10.0.1.0/24
  • Traffic which originates from a router always uses the IP address on the outgoing interface as the source address.
  • A ping from R1 to 10.1.2.10 will succeed because R4 has a route to 10.0.0.1

Solution:

  • Force the PING to source from the same subnet the PC is on (10.0.1.0/24)
    • You should always do this when checking connectivity from a subnet!
R1#ping
Protocol [ip]
Target IP address: 10.1.2.10
Repeat count [5]:
Datagram size [100]:
Timeout in seconds [2]:
Extended commands [n]: y
Source address or interface: 10.0.1.1
... (Default the remaining options)
Sending 5 100-byte ICMP Echos to 10.1.2.10, timeout is 2 seconds:
Packet sent with a source address of 10.0.1.1
.....
Success rate is 100% (5/5)

Extended Ping option descriptions

  • Repeat count: Set this to a higher value if you suspect a connection is going up and down
  • Datagram size [100]: Good to troubleshoot a suspected MTU (Maximum Transmission Unit) issue.
    • This allows you to set the packet to different sizes to check this.
  • Timeout in seconds [2]: Possibly increase this if running over very slow connections.
  • Extended commands [n]: Required ‘y’ if you want to manually set a source address or interface.
    • Source address or interface: Use the IP address of the interface that corresponds to the subnet experiencing the issue.

Traceroute

  • Very similar to PING.
  • Great for checking connectivity issues between 2 points.
  • 1st. attempt is set with a TTL (time to live, although really means ‘maximum hop count’) of 1
    • TTL is actually used as a route loop prevention mechanism.
    • This prevents a packet from getting stuck in eternal loops.
    • Each time a packet is passed, it decrements the TTL by one.
    • Once the TTL reaches 0:
      • The router drops the packet
      • It replies to the sender with a ‘time exceeded’ message
  • Each successive attempt adds one more to the TTL.
    • 2nd attempt = 2 hops
    • 3rd attempt = 3 hops
  • Each ‘hop’ replies with the IP address it reached before returning and total time taken to run reach and return.

Successful Traceroute:

R1#traceroute 10.1.2.1
Type escape sequence to abort.
Tracing the route to 10.1.2.1
VRF info: (vrf in name/id, vrf out name/id)
  1 10.0.0.2 20 msec 16 msec 16 msec
  2 10.1.0.1 36 msec 40 msec 40 msec
  3 10.1.1.1 60 msec 54 msec 60 msec

Traceroute Responses

  • First run Ping to verify the connection is failing
  • Then run Traceroute to find out where the failure starts.
  • In this example, the packet gets as far as 10.1.0.1.  Start troubleshooting there.
  • Press <Ctrl><Shift><6> to abort.
R1#traceroute 10.1.2.1
Type escape sequence to abort.
Tracing the route to 10.1.2.1
VRF info: (vrf in name/id, vrf out name/id)
  1 10.0.0.2 28 msec 16 msec 16 msec
  2 10.1.0.1 36 msec 36 msec 40 msec
  3  *  *  *
  4  *  *  *

Other basic tools

  • Layer 1
    • show ip interface brief
    • show interface
  • Layer 2
    • show arp
    • show mac address-table
  • Layer 4
    • telnet <destination IP> <destination Port>
  • DNS
    • nslookup (like dig)
    • Ping by FQDN

 

118. Connectivity Troubleshooting – Lab Exercises

https://www.udemy.com/course/cisco-icnd1/learn/lecture/8631424#overview

18 Connectivity Troubleshooting Lab Exercise 
18 Connectivity Troubleshooting Answer Key

1. Use ping to test connectivity from PC1 to PC3.

PC1> ping 10.1.2.10
*10.1.0.1 icmp_seq=1 ttl=253 time=108.341 ms (ICMP type:3, code:1, Destination host unreachable)
*10.1.0.1 icmp_seq=2 ttl=253 time=82.847 ms (ICMP type:3, code:1, Destination host unreachable)
*10.1.0.1 icmp_seq=3 ttl=253 time=98.612 ms (ICMP type:3, code:1, Destination host unreachable)

 

2. Use traceroute to determine where the problem is likely to be.

PC1> trace 10.1.2.10
trace to 10.1.2.10, 8 hops max, press Ctrl+C to stop
1 10.0.1.1 33.801 ms 16.563 ms 26.429 ms
2 10.0.0.2 50.240 ms 98.177 ms 75.777 ms
3 10.1.0.1 121.613 ms 121.023 ms 109.946 ms
4 *10.1.0.1 106.448 ms (ICMP type:3, code:1, Destination host unreachable

Problem between R3 and R4

3. Determine the issue and fix it to restore connectivity between PC1 and PC3

Check R3 Interfaces Up – OK

R3#sh ip int bri
Interface IP-Address OK? Method Status Protocol
FastEthernet0/0 10.1.1.2 YES NVRAM up up
FastEthernet1/0 10.1.0.1 YES NVRAM up up
FastEthernet2/0 unassigned YES NVRAM administratively down down
FastEthernet3/0 unassigned YES NVRAM administratively down down

Check R3 for routes to 10.1.2.0/24 – FAIL

R3#show ip route
10.0.0.0/8 is variably subnetted, 9 subnets, 2 masks
S 10.0.0.0/24 [1/0] via 10.1.0.2
S 10.0.1.0/24 [1/0] via 10.1.0.2
S 10.0.2.0/24 [1/0] via 10.1.0.2
S 10.0.3.0/24 [1/0] via 10.1.0.2
C 10.1.0.0/24 is directly connected, FastEthernet1/0
L 10.1.0.1/32 is directly connected, FastEthernet1/0
C 10.1.1.0/24 is directly connected, FastEthernet0/0
L 10.1.1.2/32 is directly connected, FastEthernet0/0
S 10.1.3.0/24 [1/0] via 10.1.1.1

Add static route to 10.1.2.0/24 via 10.1.1.1

R3#conf t
R3(config)#ip route 10.1.2.0 255.255.255.0 10.1.1.1

Verify ping from PC1

PC1> ping 10.1.2.10
84 bytes from 10.1.2.10 icmp_seq=1 ttl=60 time=170.952 ms
84 bytes from 10.1.2.10 icmp_seq=2 ttl=60 time=122.586 ms
84 bytes from 10.1.2.10 icmp_seq=3 ttl=60 time=93.745 ms
84 bytes from 10.1.2.10 icmp_seq=4 ttl=60 time=146.945 ms
84 bytes from 10.1.2.10 icmp_seq=5 ttl=60 time=141.337 ms

Done!

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