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Helpful hints
Re-routing the Router
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show interfaces
This command displays a wealth of information about the interfaces of the device - some of which is common to all interfaces and some of which is specific to the type of interface. For example, you can learn the status of the interface, status of the line protocol, hardware address, IP address, frame type and packet rates (including error counters). "One time, one of our core routers had become inoperable because it was getting pounded by an enormous amount of traffic and had been brought to it knees. By using the 'show interface' command, I was able to see that the traffic was coming in on the ATM interface on which we were using [LAN Emulation]," says Edward Johnson, information services engineer with Capital One in Glen Ellen, Va. He discovered that his router was unable to handle this tremendous number of packets in its input buffer. Note that in Cisco IOS Release 12.0(1)T or later, you can filter the output from the show command using the following syntax: Show | {begin | include | exclude} | regular-expression For example, the command "show interface | include protocol" will output only lines where the word "protocol" appears. debug ip routing (and other related debug commands)
Real-time status messages defining current routing processes and updates. "This command allows me to watch the routing table updates to see if conflicting routes are being processed," says Ken Diliberto, a network system specialist with the city of Fresno, Calif. However, note that the debug commands consume significant CPU cycles on the router. Don't execute the debug commands on a heavily congested network. Know "undebug all," which turns off all debugging on the router. show version
Displays the IOS version information, uptime, configuration of system hardware, reason for last boot, source of configuration files and boot images. show arp
Displays the entries in the ARP table. "I'm constantly chasing down specific workstations or servers to find where a device is plugged into the network," says Chloris J. Cheng, technical analyst for Effem Services, a division of M&M Mars in Mount Olive, N.J. " 'Show arp' allows me to find the [media access control] address of a device via its IP address. I then use the MAC address against the 'show cam' command on a switch, and voila! A port location is revealed." show tech-support
Strings together the results of several useful troubleshooting commands, including show buffers, show controllers, show interfaces, show running-config, show processes cpu, show processes memory, show stacks and show version. show processes
Displays CPU utilization for the last 5 seconds and its routing update process timers, route entry hold-down timers, incoming and outgoing filter lists, metrics/distances, learned routes and routing neighbor list. This command is used to isolate routing problems caused by incorrect information. show cdp neighbors
Shows active neighbors using the Cisco Discovery Protocol (CDP). "I frequently find myself using 'show cdp neighbor' commands to get familiar with what is attached to the device. Having a good mental picture of what the device does and what other areas it serves is one of the first tools I reach for in my troubleshooting bag of goodies," says Scott Gund, a network engineer with San Diego Data Processing Corp. show ip route
Shows all static or dynamic IP routes as well as directly connected networks and the default gateway. ping (extended) and traceroute
Performs simple through complex connectivity tests and route path exploration. (Click for more details on these) show ip interface
Displays myriad details regarding the usability status of interfaces configured for IP, including the maximum transmission unit, helper address setting, directed broadcast forwarding, multicast groups joined, inbound/outbound access lists, proxy address resolution protocol setting, security level and Internet control message protocol mask replies. The command "show ip interface brief" provides a quick summary of IP status and configuration. show logging
Displays the state of syslog error and event logging, and SNMP configuration parameters and protocol activity. The command "show logging summary" charts the error messages and notification messages associated with each card in the device. show running-config
Displays the current running configuration of the device. "This is the absolute starting point. You must know how the router is configured before you start trouble-shooting," says Kevin Beaver, a partner at iSpeed Solutions in Roswell, Ga.
Related links
Packet-level.com
Cisco IOS reference site
Cisco training and certification
direct from Cisco. Hands-on learning
Online training programs let students tinker with real equipment. Re-routing the Router
When Layer 3 switches burst on the scene, industry pundits began predicting the demise of the slower, more costly router as a mainstay of enterprise networks. Has that happened? Not exactly.
Router commands
Here are a dozen indispensable commands you need to know for troubleshooting Cisco routers.
 
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Helpful hints
Re-routing the Router
Subscribe to the Product Review e-mail newsletter
show interfaces
This command displays a wealth of information about the interfaces of the device - some of which is common to all interfaces and some of which is specific to the type of interface. For example, you can learn the status of the interface, status of the line protocol, hardware address, IP address, frame type and packet rates (including error counters). "One time, one of our core routers had become inoperable because it was getting pounded by an enormous amount of traffic and had been brought to it knees. By using the 'show interface' command, I was able to see that the traffic was coming in on the ATM interface on which we were using [LAN Emulation]," says Edward Johnson, information services engineer with Capital One in Glen Ellen, Va. He discovered that his router was unable to handle this tremendous number of packets in its input buffer. Note that in Cisco IOS Release 12.0(1)T or later, you can filter the output from the show command using the following syntax: Show | {begin | include | exclude} | regular-expression For example, the command "show interface | include protocol" will output only lines where the word "protocol" appears. debug ip routing (and other related debug commands)
Real-time status messages defining current routing processes and updates. "This command allows me to watch the routing table updates to see if conflicting routes are being processed," says Ken Diliberto, a network system specialist with the city of Fresno, Calif. However, note that the debug commands consume significant CPU cycles on the router. Don't execute the debug commands on a heavily congested network. Know "undebug all," which turns off all debugging on the router. show version
Displays the IOS version information, uptime, configuration of system hardware, reason for last boot, source of configuration files and boot images. show arp
Displays the entries in the ARP table. "I'm constantly chasing down specific workstations or servers to find where a device is plugged into the network," says Chloris J. Cheng, technical analyst for Effem Services, a division of M&M Mars in Mount Olive, N.J. " 'Show arp' allows me to find the [media access control] address of a device via its IP address. I then use the MAC address against the 'show cam' command on a switch, and voila! A port location is revealed." show tech-support
Strings together the results of several useful troubleshooting commands, including show buffers, show controllers, show interfaces, show running-config, show processes cpu, show processes memory, show stacks and show version. show processes
Displays CPU utilization for the last 5 seconds and its routing update process timers, route entry hold-down timers, incoming and outgoing filter lists, metrics/distances, learned routes and routing neighbor list. This command is used to isolate routing problems caused by incorrect information. show cdp neighbors
Shows active neighbors using the Cisco Discovery Protocol (CDP). "I frequently find myself using 'show cdp neighbor' commands to get familiar with what is attached to the device. Having a good mental picture of what the device does and what other areas it serves is one of the first tools I reach for in my troubleshooting bag of goodies," says Scott Gund, a network engineer with San Diego Data Processing Corp. show ip route
Shows all static or dynamic IP routes as well as directly connected networks and the default gateway. ping (extended) and traceroute
Performs simple through complex connectivity tests and route path exploration. (Click for more details on these) show ip interface
Displays myriad details regarding the usability status of interfaces configured for IP, including the maximum transmission unit, helper address setting, directed broadcast forwarding, multicast groups joined, inbound/outbound access lists, proxy address resolution protocol setting, security level and Internet control message protocol mask replies. The command "show ip interface brief" provides a quick summary of IP status and configuration. show logging
Displays the state of syslog error and event logging, and SNMP configuration parameters and protocol activity. The command "show logging summary" charts the error messages and notification messages associated with each card in the device. show running-config
Displays the current running configuration of the device. "This is the absolute starting point. You must know how the router is configured before you start trouble-shooting," says Kevin Beaver, a partner at iSpeed Solutions in Roswell, Ga.
Related links
Chappell is a senior protocol analyst with Protocol Analysis Institute, a network analysis, research and training firm in Saratoga, Calif. She can be reached at lchappell@packet-level.com.
Firefighting: Command and troubleshooting tips Learn to use it: Find out more about Cisco training:Packet-level.com
Cisco IOS reference site
Cisco training and certification
direct from Cisco. Hands-on learning
Online training programs let students tinker with real equipment. Re-routing the Router
When Layer 3 switches burst on the scene, industry pundits began predicting the demise of the slower, more costly router as a mainstay of enterprise networks. Has that happened? Not exactly.
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