Network Verification Commands Review
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When working with Cisco equipment, there are a number of different commands that an engineer must be familiar with to figure out the current status of the device. Which ones to use depend on the specific type of operation being tested. This article takes a look at some of the most basic network verification commands that can be used on Cisco equipment and discusses the information that can be obtained from these commands.
show ip interface brief
While it may seem odd that to review a more specific command over the more basic form, the show ip interface brief command is so commonly used by many engineers as a first step that it really needs to be discussed first. This command is used to provide a concise output of the current status of the local IP interfaces and their status. The important information to use includes the interface, the interface IP address, status (physical) and protocol status (data link). Figure 1 shows that only one interface is configured with an IP Address (192.168.1.150) and is operational (Fastethernet0/0).
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Figure 1 show ip interface brief
show interface
Like the show ip interface brief command, the show interface command includes the interface, interface status (both physical and data link), and IP address. It also includes additional information, including the interface IP subnet mask, bandwidth settings, delay settings, queuing configuration, data link protocol information (in this case duplex, ARP type), and a number of different counters that can be used to monitor the interface. Figure 2 shows the output for the operation interface shown in the above output (fastethernet0/0) and includes information about the subnet mask (/24), the bandwidth (100 Mbps), delay (100 usec), queuing (First In-First Out (FIFO)), duplex (Full), and ARP type (ARPA).
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Figure 2 show interface
show ip interface
The show ip interface command is the full version of the show ip interface brief command and includes all of the settings that are IP specific, including IP address and mask information, access list configuration (ACL), type of switching used (how the IP traffic is processed by the device), and compression settings, among others.
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Figure 3 show ip interface
show ip arp
The show ip arp commands focuses on the information obtained from the Address Resolution Protocol (ARP) that is used to map IP addresses to MAC addresses. These mappings are used by the device when traffic is received and is destined for a local host. The device will look up the MAC address as it requires it to send the traffic to the host on the local network. Figure 4 shows three different devices that are known by ARP, including the local device (192.168.1.150) all out interface fastethernet0/0.
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Figure 4 show ip arp
show ip protocols
The show ip protocols command is used when a dynamic routing protocol is being run on the device. The output from this command can be used to verify that the routing protocol configuration is being processed as expected. The exact output from this command depends on the dynamic routing protocol that is configured. The output shown in Figure 5 is for the Open Shortest Path First (OSPF) dynamic routing protocol. The output shows that the Router ID (RID) for the device being used by OSPF is 192.168.1.150 and that it is routing traffic for the 192.168.1.0/24 network using area 0.
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Figure 5 show ip protocols
show ip route
A vital command that is used by every engineer using a Cisco device is the show ip route command; this command is used to display the current content of the IP routing table. The output shown in the figure is basic but the output from this command can be very long when multiple networks are managed by the device. Figure 6 shows two different routes that exist within this routing table, one that is a connected network (the 192.168.1.0/24 network), and the other a default static route that sends all unknown prefixes to the device with the IP address 192.168.1.1.
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Figure 6 show ip route
show logging
When a networking device is set up for logging, it can be used for the verification of a number of different things. The show logging command is used to access this log and display it for review. In Figure 7, an interface is manually brought down and back up to show the resulting log.
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Figure 7 show logging
ping
One of the most popular utility that is used by network engineers for quick reachability verification is the ping command. At its most simple, the ping command is used to send a group of five Internet Control Message Protocol (ICMP) packets to a destination which in turn will return five packets (should reachability exist). Since a normal routing or switching device typically has many outgoing interfaces, the command can be extended and customized with a number of different options, including source interface, count, datagram size, timeout, pattern, and Type of Service (ToS), among others. The examples shown in Figure 8 include a basic ping with no additional options with a destination of 192.168.1.103 and an extended version (also using the default parameters).
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Figure 8 ping
traceroute
The traceroute command is another one of the commonly used tools for engineers to verify correct network operation. The traceroute command will send a number of packets out to determine the path from source to destination, which is done by taking advantage of the Time to Live (TTL) functionality built into the IP header. The TTL field allows a source to set the number of “hops” that a packet is allowed to travel before being dropped. The typical reaction of a device that is processing a packet where the TTL has expired is to return a ICMP port unreachable message; the traceroute utility receives this packet and notes the source address. The traceroute utility will continue sending packets until the source address matches the intended destination device starting with a TTL set to 1, then 2, then 3 and so on. The example shown in Figure 9 uses a local host that is only one hop away.
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Figure 9 traceroute
Summary
There are a number of different commands that can be used on a Cisco device to verify operations; the commands shown in this article are some of the most basic that are used by almost all Cisco engineers in most situations. Take the time to check out these commands on a Cisco device (or dynamips); none of them will affect the operations of the device and will provide a learning experience to any upcoming network engineer.