Verifying Cisco IP Configuration

1. The Quick Check: show ip interface brief

When you first log in to a router or switch to troubleshoot a connectivity problem, your first "go-to" command should almost always be show ip interface brief. It provides a concise, table-based summary of the status of all IP interfaces on the device, allowing you to spot problems at a glance.

The output of this command is packed with valuable information. Let's break it down column by column.

Looking at the output in the image above, you see several columns:

• Interface: The name of the interface (e.g., FastEthernet0/0, Vlan1).
• IP-Address: The IP address assigned to the interface. "unassigned" means no IP address is configured.
• OK?: This indicates if the configuration is considered valid. It's almost always YES.
• Method: Shows how the IP address was configured (e.g., manual for static assignment, DHCP).
• Status: This column reflects the Layer 1 (Physical) status.
  • up: The interface is physically active (e.g., a cable is plugged in and the device on the other end is on).
  • down: There is a physical layer issue (e.g., no cable, bad cable, other end is off).
  • administratively down: The interface has been manually disabled with the shutdown command. You must use the no shutdown command in interface configuration mode to enable it.
• Protocol: This column reflects the Layer 2 (Data Link) status.
  • up: The data link layer protocol is functioning correctly (e.g., it's receiving keepalives).
  • down: There is a data link layer issue, such as a protocol mismatch (e.g., HDLC vs. PPP on a serial link) or a problem on the other side of the connection.

Interpreting Status and Protocol Pairs

The combination of the Status and Protocol columns is a powerful diagnostic indicator:

To see how this command is used in a real troubleshooting scenario, let's look at an example.

2. The Detailed View: show ip interface

While show ip interface brief is excellent for a quick overview, sometimes you need more detail. The show ip interface <interface_name> command provides a comprehensive look at a single interface's IP-related parameters.

This detailed view gives you information that the brief version omits, such as:

• The IP address and subnet mask in CIDR notation (e.g., 10.1.11.1/24).
• The broadcast address for the subnet (10.1.11.255).
• The Maximum Transmission Unit (MTU) size.
• Reliability and traffic counters (useful for detecting errors).
• Any helper addresses or access lists applied to the interface.

You can use show ip interface without specifying an interface to see this detailed output for all interfaces on the device.

3. Verifying IP Configuration on Switches

On a Layer 2 switch, you typically don't assign IP addresses to physical ports. Instead, you assign them to Switch Virtual Interfaces (SVIs), which are logical interfaces representing a VLAN. The verification commands are the same, but the interface name will be a VLAN, such as interface Vlan1 or interface Vlan20. This allows the switch to be managed remotely or to participate in routing.

The following video from Cisco demonstrates the process of configuring and verifying IP addresses on VLAN interfaces.

As you can see, the same commands work for both physical router interfaces and logical switch interfaces, making them versatile tools in your toolkit.

4. Viewing the Running Configuration

Finally, to see the actual commands that were typed to create the current state, you can use the show running-config command. This displays the entire active configuration file stored in the device's RAM.

While show running-config shows everything, it can be overwhelming. You can use a pipe (|) to filter the output for what you need. For example:

• show running-config | include ip address: Shows only the lines containing "ip address".
• show running-config interface GigabitEthernet0/1: Shows only the configuration specific to that interface.

This is extremely useful for confirming the exact IP address and subnet mask commands that have been applied, helping you spot typos or incorrect masks.

Conclusion

You are now equipped with the primary commands for verifying Layer 3 configurations on a Cisco device. This moves you beyond just identifying a problem's location with ping and traceroute to actively inspecting a device to find the root cause.

Key Takeaways:

• show ip interface brief is your first and most important command for a quick summary of interface status and IP addressing.
• Pay close attention to the Status (Layer 1) and Protocol (Layer 2) columns to rapidly diagnose where a problem lies. An "up/up" status is the goal.
• Use show ip interface <interface_name> to get a detailed view of a specific interface's configuration, including its subnet mask and broadcast address.
• These commands apply to both physical interfaces on routers and Switch Virtual Interfaces (SVIs) on switches.
• Use show running-config with filters to view the exact configuration commands that have been applied to an interface.

In our next lesson, we will build on these verification skills. Now that you know how to look at the configuration, we will focus on what to look for by diagnosing common IP addressing errors like incorrect subnet masks, duplicate IP addresses, and misconfigured default gateways.