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MAC Address Tables
Share on LinkedIn Share. Share on Digg Share. In Cisco technology, the traceroute mac command output shows the Layer2 path when the specified source and destination addresses belong to the same VLAN. Fabio Semperboni. Join us on LinkedIn! Join us on Facebbook!
cisco - Can 'show mac address-table' be used inside user mode? - Server Fault
Cisco 7200 - No "Show Mac Address-Table" Command?
Many network admins break down network infrastructure problems by analyzing the Layer 3 path through the network, hop by hop, in both directions. This process helps them isolate the problem; once they determine which hop in the layer path fails, they can then look further into the details. There are various tools that can help with network troubleshooting. The Cisco Discovery Protocol CDP discovers basic information about neighbor routers and switches without needing to know the passwords for those Cisco network devices. This is possible because Cisco routers and switches routinely send out CDP messages that announce information about themselves.
Therefore, Cisco hardware that supports CDP can learn about other devices by listening for these messages. CDP discovers several useful details from neighboring Cisco devices:. This command lists each neighboring device, one per line. This interface command also lists the platform, identifying the specific model of the neighboring router or switch. To get additional details, such as the full name of the model of switch and the IP address configured on the neighboring device, add the detail parameter as follows:.
Determine your MAC address
Of course, being able to discover a lot of information about neighboring devices is a network security exposure. To toggle CDP off and on for an entire device, use the no cdp run and cdp run global commands. To toggle it on a specific interface, use the no cdp enable and cdp enable interface subcommands. It outputs the following information:.
The basic purpose for ping is to check for reachability, round trip time RTT and packet loss. In order to troubleshoot a device for these properties, we need to use specify the IP address of the device — for example, ping You can ping from the particular interface by adding the source parameter with the interface name at the end of the command — for example, ping Traceroute is a function that traces the path from one network to another, so it can help diagnose the source of many problems.
Then traceroute sends a set of three UDP datagrams with TTL 2, so they time out when they hit the second router, causing it to responds with timeout message. You can also use an extended traceroute command to test connectivity from a specified source — for example, traceroute When you use Telnet to connect to a remote device, it uses the default port You can use any port number from 1 to to test whether a remote device is listening to the specific port, for example, telnet Cisco switches use two different sets of interface status codes.
Both sets of status codes can determine whether an interface is working. When you first configure an interface in configure terminal mode, you must administratively enable the interface before the router can use it to transmit or receive packets. Use the Cisco no shutdown command to allow the IOS software to use the interface. Later, you might want to disable a specific interface to perform hardware maintenance on it or a segment of a network. You might also want to disable an interface if a problem exists on a specific network segment and you must isolate that segment from the rest of the network.
The shutdown command administratively enables an interface.
To restart the interface, use the no shutdown command. Most routing tables contain a combination of static routes and dynamic routes.
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However, before any static or dynamic routing can be used, the routing table must contain the directly connected networks that are used to access remote networks. To verify the static routes in the routing table, use the show ip route command, specifying the network address, subnet mask and IP address of next hop router or exit interface. If both of these are configured for an interface, the switch or router disables the IEEE-standard auto-negotiation process on that interface.
Both the show interfaces and show interfaces status commands list the speed and duplex settings on an interface, but only the show interfaces status command indicates how the switch determined the speed and duplex settings; it lists all autonegotiated settings with a prefix of a-.
For example, a-full means full-duplex as auto-negotiated, whereas full means full-duplex but as manually configured. While auto-negotiation works well, the default values allow for the possibility of a problem called a duplex mismatch , in which the devices considers the link to be up but one side would use half-duplex and the other side would use full-duplex. The number of input errors and the number of CRC errors are just two of the counters in the output of the show interfaces command.
The challenge is deciding which counters you need to see, which ones show that a problem is happening, and which ones are normal and of no concern. Here is the list of counters to help you to start understanding which ones point to problems and which ones are just counting normal events that are not problems:. To know exactly how a particular switch will forward an Ethernet frame, you need to examine the MAC address table on a Cisco switch.
This command lists all MAC addresses currently known by the switch. The output includes some static overhead MAC addresses used by the switch and any statically configured MAC addresses, such as those configured with the port security feature.