CCNA: Ethernet Switch Troubleshooting

Verifying the Network Topology with Cisco Discovery Protocol
The proprietary Cisco Discovery Protocol (CDP) discovers basic information about
neighboring routers and switches without needing to know the passwords for the
neighboring devices.

CDP discovers several useful details from the neighboring Cisco devices:
■ Device identifier: Typically the hostname
■ Address list: Network and data-link addresses
■ Local interface: The interface on the router or switch issuing the show cdp command
with which the neighbor was discovered
■ Port identifier: Text that identifies the port used by the neighboring device to send
CDP messages to the local device
■ Capabilities list: Information on what type of device it is (for instance, a router or a
switch)
■ Platform: The model and OS level running in the device


CDP can be enabled per interface using the no cdp enable interface subcommand. (The cdp enable interface subcommand re-enables CDP.)

The no cdp run global command disables CDP for the entire switch, with the cdp run global command re-enabling CDP globally.


Interface Status Codes and Reasons for Nonworking States
You can verify the interface status by show interface and show interface status.

















Interface Speed and Duplex Issues
You can verify the speed and duplex status by show interface and show interface status.
The command output lists autonegotiated settings with a prefix of a-.
For example, a-full means full duplex as autonegotiated, whereas full means full duplex but as manually configured.

When the IEEE autonegotiation process works on both devices, both devices agree to the
fastest speed supported by both devices.

When one device has disabled autonegotiation, and the other device uses autonegotiation, the device using autonegotiation chooses the default duplex setting based on the current speed. The defaults are as follows:
■ If the speed is not known, use 10 Mbps, half duplex.
■ If the speed is somehow known to be 10 or 100 Mbps, default to use half duplex.
■ If the speed is somehow known to be 1000 Mbps, default to use full duplex.


*Finding a duplex mismatch 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 connect (up/up) state.

To identify duplex mismatch problems, check the duplex setting on each end of the link,
and watch for incrementing collision and late collision counters











































Common Layer 1 Problems on Working Interfaces
First, consider a couple of common reasons why Ethernet frames experience errors during
transmission. When an Ethernet frame passes over a UTP cable, the electrical signal may
encounter problems. The cable could be damaged, for example, if it lies under carpet. If
the user’s chair keeps squashing the cable, eventually the electrical signal can degrade.

Additionally, many sources of electromagnetic interference (EMI) exist; for example, a
nearby electrical power cable can cause EMI.

Regardless of the root cause, whenever the electrical signal degrades, the receiving device
may receive a frame whose bits have changed value. These frames do not pass the error
detection logic as implemented in the FCS field in the Ethernet trailer.

Cisco switches list this error as a CRC error (cyclic redundancy check [CRC] is an older
term referring to the frame check sequence [FCS] concept).


Ethernet collision versus a late collision
Collisions occur as a normal part of the half-duplex logic imposed by CSMA/CD, so a switch interface with an increasing collisions counter may not even have a problem, all collisions should occur by the end of the 64th byte of any frame.

When a switch has already sent 64 bytes of a frame, and the switch receives a frame on that same interface, the switch senses a collision. In this case, the collision is a late collision, and the
switch increments the late collision counter.

Three common LAN problems can be found using these counters:
Excessive interference on the cable can cause the various input error counters to keep growing larger, especially the CRC counter. If the CRC errors grow, but the collisions counters do not, the problem may simply be interference on the cable.

Duplex mismatches and jabber can be partially identified by looking at the collisions
and late collision counters. Jabber refers to cases in which the NIC ignores Ethernet rules
and sends frame after frame without a break between the frames.

Incrementing late collisions counter typically means one of two things:
■ The interface is connected to a collision domain whose cabling exceeds Ethernet cable
length standards.
■ The interface is using half duplex, and the device on the other end of the cable is using
full duplex.