Components of Troubleshooting End-to-End Connectivity

Bottom-up approach steps when there is no end-to-end connectivity are as follows:
1.Check physical connectivity at the point where network communication stops
2.Check for duplex mismatches.
3.Check data link and network layer addressing on the local network.
4.Verify that the default gateway is correct.
5.Ensure that devices are determining the correct path from the source to the destination.
6.Verify the transport layer is functioning properly.
7.Verify that there are no ACLs blocking traffic.
8.Ensure that DNS settings are correct.

End-to-End Connectivity Problem Initiates Troubleshooting

Usually what initiates a troubleshooting effort is the discovery that there is a problem with end-to-end connectivity.

Two of the most common utilities used to verify a problem with end-to-end connectivity are ping and traceroute.

Step 1 - Verify the Physical Layer

The show interfaces command is useful when troubleshooting performance-related issues and hardware is suspected to be at fault.

Of interest in the output are the:

  • Interface status
  • Input queue drops
  • Output queue drops
  • Input errors
  • Output errors

Step 2 - Check for Duplex Mismatches

The IEEE 802.3ab Gigabit Ethernet standard mandates the use of autonegotiation for speed and duplex and practically all Fast Ethernet NICs also use autonegotiation by default.

Problems can occur when there is a duplex mismatch.

Step 3 - Verify Addressing on the Local Network

The arp Windows command displays and modifies entries in the ARP cache that are used to store IPv4 addresses and their resolved Ethernet physical (MAC) addresses.

Troubleshoot VLAN Assignment Example

For example, the MAC address on Fa0/1 should be in VLAN 10 instead of VLAN 1.

Troubleshoot VLAN Assignment Example

The following configuration changes Fa0/1 to VLAN 10 and verifies the change.

Step 4 - Verify Default Gateway

Misconfigured or missing default gateways can cause connectivity problems.

In the figure for example, the default gateways for:

  • R1 is 192.168.1.2 (R2)
  • PC1 is 10.1.10.1 (R1 G0/0/0)

Useful commands to verify the default gateway on:

  • R1: show ip route
  • PC1: route print (or netstat –r)
Step 5 - Verify Correct Path
  • The figure describes the process for both the IPv4 and IPv6 routing tables.
  • The process of forwarding IPv4 and IPv6 packets is based on the longest bit match or longest prefix match.
  • The routing table process will attempt to forward the packet using an entry in the routing table with the greatest number of leftmost matching bits.
  • The number of matching bits is indicated by the prefix length of the route.
Step 6 - Verify the Transport Layer

  • Two of the most common issues that affect transport layer connectivity include ACL configurations and NAT configurations.

    • A common tool for testing transport layer functionality is the Telnet utility.
    • For example, the administrator attempts to Telnet to R2 using port 80.
Step 7 - Verify ACLs

  • In this example, ACL 100 has been incorrectly configured inbound on the G0/0/0 instead of inbound on S0/1/1.

Correction

  • The ACL is removed from G0/0/0 and configured inbound on S0/1/1.

Step 8 - Verify DNS

  • The DNS protocol controls the DNS, a distributed database with which you can map hostnames to IP addresses.

    • When you configure DNS on the device, you can substitute the hostname for the IP address with all IP commands, such as ping or telnet. command output.
  • Use the ip host global configuration command to enter a name to be used instead of the IPv4 address of the switch or router, as shown in the command output.
    •  
    • Use the nslookup Windows command to display the name-to-IP-address mapping information.
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Troubleshooting Quiz

Test You Understanding

1 / 8

Which statement describes the physical topology for a LAN?

2 / 8

What network troubleshooting capability is provided by a cable analyzer?

3 / 8

While designing changes to a data center because of a new IoE implementation, a network administrator has the job of diagramming the new physical topology of the area. What should be included in the physical topology diagram?

4 / 8

A company is setting up a web site with SSL technology to protect the authentication credentials required to access the web site. A network engineer needs to verify that the setup is correct and that the authentication is indeed encrypted. Which tool should be used?

5 / 8

What is the purpose of establishing a network baseline?

6 / 8

A networked PC is having trouble accessing the Internet, but can print to a local printer and ping other computers in the area. Other computers on the same network are not having any issues. What is the problem?

7 / 8

When should a network performance baseline be measured?

8 / 8

A network engineer is troubleshooting a network problem and can successfully ping between two devices. However, Telnet between the same two devices does not work. Which OSI layers should the administrator investigate next?

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