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Level 10

Troubleshooting Ethernet LANs


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Analyzing/predicting normal operation
Predict the details of what should happen if the network is working correctly, based on documentation, configuration, and show and debug command output.
Problem isolation
Determine how far along the expected path the frame/packet goes before it cannot be forwarded any farther, again based on documentation, configuration, and show and debug command output.
Root cause analysis
Identify the underlying causes of the problems identified in the preceding step—specifically, the causes that have a specific action with which the problem can be fixed.
show
__ commands can either confirm that a specific root cause is the problem or at least give some hints as to the root cause.
show
You need to remember the theory of how networks should work, as well as how to interpret the __ command output that confirms how the devices are currently behaving.
hop by hop
Many engineers break down network problems as in this list, analyzing the Layer 3 path through the network, __, in both directions.
Cisco Discovery Protocol
The proprietary __ discovers basic information about neighboring routers and switches without needing to know the passwords for the neighboring devices.
CDP messages
To discover information, routers and switches send __ out each of their interfaces.
announce information
The messages essentially __ about the device that sent the CDP message.
advertisements
Devices that support CDP learn information about others by listening for the __ sent by other devices.
proprietary solution, Link Layer Discovery Protocol
Cisco created CDP as a __ to meet a need for Cisco customers. Since that time, the IEEE has standardized the __, which serves the same role.
LLDP
most enterprises that use Cisco routers and switches use CDP, with __ as an option
CDP
From a troubleshooting perspective, __ can be used to either confirm or fix the documentation shown in a network diagram, or even discover the devices and interfaces used in a network.
multicast frames, copy of the CDP update
On media that support multicasts at the data link layer (like Ethernet), CDP uses __; on other media, CDP sends a __ to any known data link addresses.
Device identifier
CDP discovers several useful details from the neighboring Cisco devices [the host name]
Address list
CDP discovers several useful details (the host name) from the neighboring Cisco devices [Network and data link addresses]
Port identifier
CDP discovers several useful details from the neighboring Cisco devices [The interface on the remote router or switch on the other end of the link that sent the CDP advertisement]
Capabilities list
CDP discovers several useful details from the neighboring Cisco devices [Information on what type of device it is (for example, a router or a switch)]
Platform
CDP discovers several useful details from the neighboring Cisco devices [The model and OS level running on the device]
show cdp neighbors {type number}
Lists one summary line of information about each neighbor, or just the neighbor found on a specific interface if an interface was listed.
show cdp neighbors detail
Lists one large set (approximately 15 lines) of information, one set for every neighbor.
show cdp entry {name}
Lists the same information as the show cdp neighbors detail command, but only for the named neighbor (case sensitive).
routers, switches
Cisco __ and __ support the same CDP commands, with the same parameters and same types of output.
one line per neighbor
The show cdp neighbors command, which lists __.
show cdp neighbors detail
The show cdp entry {name} command lists the exact same details shown in the output of the __ command, but for only the one neighbor listed in the command.
show cdp
States whether CDP is enabled globally, and lists the default update and holdtime timers.
show cdp interface {type number}
States whether CDP is enabled on each interface, or a single interface if the interface is listed, and states update and holdtime timers on those interfaces.
show cdp traffic
Lists global statistics for the number of CDP advertisements sent and received.
two codes, single code
Cisco switches actually use two different sets of interface status codes—one set of __ (words) that use the same conventions as do router interface status codes, and another set with a __ (word).
two-code status, line status, protocol status
The switch show interfaces and show interfaces description commands list the __ just like routers. The two codes are named the __ and __.
line status, protocol status
They generally refer to whether Layer 1 is working (__) and whether Layer 2 is working (__).
up, down
LAN switch interfaces typically show an interface with both codes with the same value, either “__” or “__.”
connected
The show interfaces status command lists a “__” state for working interfaces.
connected, up/up
Any interface state other than __ or __ means that the switch will not forward or receive frames on the interface.
root causes
Each nonworking interface state has a small set of __.
Administratively Down - Down
The interface is configured with the shutdown command.
Down - Down
No cable; bad cable; wrong cable pinouts; the speeds are mismatched on the two connected devices; the device on the other end of the cable is (a) powered off, (b) shutdown, or (c) error disabled.
Up - Down
An interface up/down state is not expected on LAN switch physical interfaces.
Down - Down (err-disabled)
Port security has disabled the interface.
Up - Up
The interface is working.
Administratively Down - Down
disabled
Down - Down
notconnect
Up - Down
notconnect
Down - Down (err-disabled)
err-disabled
Up - Up
connected
interfere
The installation of any equipment that uses electricity, even non-IT equipment, can __ with the transmission on the cabling, and make the link fail.
damaged
The cable could be __, for example, if it lies under carpet. If the user’s chair keeps squashing the cable, eventually the electrical signal can degrade.
bending
While optical cables do not suffer from EMI, someone can try to be helpful and move a fiber-optic cable out of the way,__ it too much.
show interfaces status
the __ command implies how the switch determined the speed and duplex settings.
show interfaces
The __ fa0/13 command (without the status option) simply lists the speed and duplex for interface Fast Ethernet 0/13, with nothing implying that the values were learned through autonegotiation.
duplex mismatch
While autonegotiation works well, these defaults allow for the possibility of a difficult-totroubleshoot problem called a __.
duplex mismatch
Finding a __ can be much more difficult than finding a speed mismatch, because if the duplex settings do not match on the ends of an Ethernet segment, the switch interface will still be in a connected (up/up) state.
interface counters
These __ can help identify problems that can occur even though the interface is in a connect state.
input error
The receiving device might receive a frame whose bits have changed values. It discards the frame and counts it as some kind of __.
Cyclic redundancy check
__ is a term related to how the FCS math detects an error.
input errors, CRC errors
The number of __, and the number of __, are just a few of the counters in the output of the show interfaces command.
Runts
Frames that did not meet the minimum frame size requirement (64 bytes, including the 18-byte destination MAC, source MAC, Type, and FCS). Can be caused by collisions.
Giants
Frames that exceed the maximum frame size requirement (1518 bytes, including the 18-byte destination MAC, source MAC, Type, and FCS).
Input Errors
A total of many counters, including runts, giants, no buffer, CRC, frame, overrun, and ignored counts.
CRC
Received frames that did not pass the FCS math; can be caused by collisions.
Frame
Received frames that have an illegal format, for example, ending with a partial byte; can be caused by collisions.
Packets Output
Total number of packets (frames) forwarded out the interface.
Output Errors
Total number of packets (frames) that the switch port tried to transmit, but for which some problem occurred.
Collisions
Counter of all collisions that occur when the interface is transmitting a frame.
Late Collisions
The subset of all collisions that happen after the 64th byte of the frame has been transmitted.
duplex mismatch
One problem, called late collisions, points to the classic __ problem.
64th byte
If a LAN design follows cabling guidelines, all collisions should occur by the end of the __ of any frame.
CRC counter
Excessive interference on the cable can cause the various input error counters to keep growing larger, especially the __.
interference on the cable
If the CRC errors grow, but the collisions counters do not, the problem might simply be __.
mental process
The more formal troubleshooting process begins with a __ where you predict where frames should flow in the LAN.
show mac address-table dynamic
This command lists all dynamically learned MAC table entries on a switch, for all VLANs.
predict
After you __ the expected contents of the MAC address tables, you can then examine what is actually happening on the switches.
Troubleshooting
__ revolves around three big ideas: predicting what should happen, determining what is happening that is different than what should happen, and figuring out why that different behavior is happening.
filters
When tracing the path a frame takes through LAN switches, different kinds of __ can discard frames, even when all the interfaces are up.
access control lists
LAN switches can use filters called __ that filter based on the source and destination MAC address, discarding some frames.
port security
In some cases, you can easily tell that __ has taken action, because it shuts down the interface.
port security status
A port security configuration that leaves the interface up, but still discards frames, requires the network engineer to look closely at __, rather than just looking at interfaces and the MAC address table.
shutdown
As a reminder, port security allows three violation modes (shutdown, protect, and restrict), but only the default setting of __ causes the switch to err-disable the interface.
Port security
__ manages the MAC addresses, any MAC addresses associated with a port on which port security is enabled show up as static MAC addresses.
know, active, allow
Before a switch can forward frames in a particular VLAN, the switch must __ about a VLAN and the VLAN must be __. And before a switch can forward a frame over a VLAN trunk, the trunk must currently __ that VLAN to pass over the trunk.
reassign
[1] Identify all access interfaces and their assigned access VLANs and __ into the correct VLANs as needed.
configured, learned with VTP
[2] Determine whether the VLANs both exist (__ or __) and are active on each switch. If not, configure and activate the VLANs to resolve problems as needed.
trunk, same
[3] Check the allowed VLAN lists, on the switches on both ends of the __, and ensure that the lists of allowed VLANs are the __.
trunking
[4] Ensure that for any links that should use __, one switch does not think it is trunking, while the other switch does not think it is trunking because of an unfortunate choice of configuration settings.
show vlan brief
Lists each VLAN and all interfaces assigned to that VLAN (but does not include operational trunks)
show vlan id {number}
Lists both access and trunk ports in the VLAN
show interfaces {type number} switchport
Identifies the interface’s access VLAN and voice VLAN, plus the configured and operational mode (access or trunk)
show mac address-table
Lists MAC table entries, including the associated VLAN
show vlan, show vlan brief
The __ and __ commands, they list all the known VLANs and the access interfaces assigned to each VLAN.
show mac address-table
If the show vlan and show interface switchport commands are not available in a particular exam question, the __ command can also help identify the access VLAN.
not configured, configured but disabled
Switches do not forward frames for VLANs that are (a) __ or (b) __ (shutdown).
vlan {id}, learned, VTP
A VLAN can be defined to a switch in two ways: using the __ global configuration command, or it can be __ from another switch using __.
show running- config
The show vlan command always lists all VLANs known to the switch, but the __ command does not.
VTP servers, clients
Switches configured as __ and __ do not list the vlan commands in the running-config nor the startup-config file.
VTP transparent
Switches configured to use __ mode, or that disable VTP, list the vlan configuration commands in the configuration files.
allowed VLAN list
It is possible to configure a different __ on the opposite ends of a VLAN trunk. When mismatched, the trunk cannot pass traffic for that VLAN.
show interfaces trunk
The __ command output on both sides looks completely normal. You can only notice the problem by comparing the allowed lists on both ends of the trunk.
switchport mode dynamic auto
The most common incorrect configuration—which results in both switches not trunking—is a configuration that uses the __ command on both switches on the link.
passive
The word “auto” in switchport mode dynamic auto command, just makes us all want to think that the link would trunk automatically, but this command is both automatic and __.
static access
The show interfaces switchport command on both switches confirms both the administrative state (auto), as well as the fact that both switches operate as “__” ports.
show interfaces trunk, show interfaces switchport
Always check the trunk’s operational state on both sides of the trunk. The best commands to check trunking-related facts are __ and __.
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