1.5 Verify Directly Connected Networks

Interface Verification Commands

  There are several show commands that can be used to verify the operation and configuration of an interface. The following commands are especially useful to quickly identify the status of an interface:

• show ip interface brief and show ipv6 interface brief – These display a summary for all interfaces including the IPv4 or IPv6 address of the interface and current operational status.
• show running-config interface interface-id – This displays the commands applied to the specified interface.
• show ip route and show ipv6 route – These display the contents of the IPv4 or IPv6 routing table stored in RAM. In Cisco IOS 15, active interfaces should appear in the routing table with two related entries identified by the code ‘C’ (Connected) or ‘L’ (Local). In previous IOS versions, only a single entry with the code ‘C’ will appear.

Verify Interface Status

The output of the show ip interface brief and show ipv6 interface brief commands can be used to quickly reveal the status of all interfaces on the router. You can verify that the interfaces are active and operational as indicated by the Status of “up” and Protocol of “up”, as shown in the example. A different output would indicate a problem with either the configuration or the cabling.

Verify IPv6 Link Local and Multicast Addresses

The output of the show ipv6 interface brief command displays two configured IPv6 addresses per interface. One address is the IPv6 global unicast address that was manually entered. The other address, which begins with FE80, is the link-local unicast address for the interface. A link-local address is automatically added to an interface whenever a global unicast address is assigned. An IPv6 network interface is required to have a link-local address, but not necessarily a global unicast address. The show ipv6 interface gigabitethernet 0/0/0 command displays the interface status and all of the IPv6 addresses belonging to the interface. Along with the link local address and global unicast address, the output includes the multicast addresses assigned to the interface, beginning with prefix FF02, as shown in the example.

Verify Interface Configuration

The output of the show running-config interface command displays the current commands applied to the specified interface, as shown. The following two commands are used to gather more detailed interface information: show interfaces- Displays interface information and packet flow count for all interfaces on the device. show ip interface and show ipv6 interface – Displays the IPv4 and IPv6 related information for all interfaces on a router..

Verify Routes

The output of the show ip route and show ipv6 route commands reveal the three directly connected network entries and the three local host route interface entries, as shown in the example. The local host route has an administrative distance of 0. It also has a /32 mask for IPv4, and a /128 mask for IPv6. The local host route is for routes on the router that owns the IP address. It is used to allow the router to process packets destined to that IP.   A ‘C’ next to a route within the routing table indicates that this is a directly connected network. When the router interface is configured with a global unicast address and is in the “up/up” state, the IPv6 prefix and prefix length are added to the IPv6 routing table as a connected route. The IPv6 global unicast address applied to the interface is also installed in the routing table as a local route. The local route has a /128 prefix. Local routes are used by the routing table to efficiently process packets with the interface address of the router as the destination.

Filter Show Command Output

Commands that generate multiple screens of output are, by default, paused after 24 lines. At the end of the paused output, the –More– text displays. Pressing Enter displays the next line and pressing the spacebar displays the next set of lines. Use the terminal length command to specify the number of lines to be displayed. A value of 0 (zero) prevents the router from pausing between screens of output. Another very useful feature that improves the user experience in the CLI is the filtering of show output. Filtering commands can be used to display specific sections of output. To enable the filtering command, enter a pipe (|) character after the show command and then enter a filtering parameter and a filtering expression. There are four filtering parameters that can be configured after the pipe:

• section – Shows the entire section that starts with the filtering expression.
• include – Includes all output lines that match the filtering expression.
• exclude – Excludes all output lines that match the filtering expression.
• begin – Shows all the output lines from a certain point, starting with the line that matches the filtering expression

Command History Feature

The command history feature is useful because it temporarily stores the list of executed commands to be recalled.

• To recall commands in the history buffer, press Ctrl+P or the Up Arrow key. The command output begins with the most recent command. Repeat the key sequence to recall successively older commands. To return to more recent commands in the history buffer, press Ctrl+N or the Down Arrow key. Repeat the key sequence to recall successively more recent commands.
• By default, command history is enabled and the system captures the last 10 command lines in its history buffer. Use the show history privileged EXEC command to display the contents of the buffer.
• It is also practical to increase the number of command lines that the history buffer records during the current terminal session only. Use the terminal history size user EXEC command to increase or decrease the size of the buffer.

Other related topics

Other useful information

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