McAfee Secure

Troubleshoot Access-ports for VLAN Based Solution

Exam: Cisco 642-832 - Troubleshooting and Maintaining Cisco IP Networks (TSHOOT v1.0)

Access ports are used to connect end point device to the network.

Access port troubleshooting is usually related to performance issues on the end point device or the end point device not getting a link. Following points describe some of the common issues and their possible troubleshooting steps:

  • If the cable is plugged into the access port but the port does not get a link (port is in up/down state), then this is related to speed/duplex negotiation issues between the switch port and the end point device. If you have hard coded speed/duplex settings on the port, try to set them to auto/auto and check if the port comes up. (Auto/Auto is good for troubelshooting for getting the link up but it is not a recommended setting, especially for FastEthernet ports. Make sure you read the next steps for recommended speed/duplex settings)
  • Sometimes port is up but users complain of slowness issues. This is mostly related to port negotiating to half duplex. Note that on FastEthernet ports, if one side is hard codes while the other side is auto/auto, then the hard coded side will always negotiate to half duplex. To prevent this problem, make sure that you hard code the settings on both sides to 100/full. Gigabit interfaces cannot be half duplex so they can easily negotiate without problems but still it is recommeded to hard code them to 1000/full.
  • The "show interface" command displays useful information about the switch port. A sample output is displayed below:

FastEthernet0 is up, line protocol is up
Hardware is Fast Ethernet, address is 1c17.d373.e2fa (bia 1c17.d373.e2fa)
MTU 1500 bytes, BW 100000 Kbit, DLY 100 usec,
reliability 255/255, txload 2/255, rxload 1/255
Encapsulation ARPA, loopback not set
Keepalive set (10 sec)
Full-duplex, 100Mb/s
ARP type: ARPA, ARP Timeout 04:00:00
Last input never, output never, output hang never
Last clearing of "show interface" counters never
Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
Queueing strategy: fifo
Output queue: 0/40 (size/max)
5 minute input rate 51000 bits/sec, 81 packets/sec
5 minute output rate 953000 bits/sec, 84 packets/sec
48951426 packets input, 3577032354 bytes, 0 no buffer
Received 501 broadcasts, 0 runts, 0 giants, 0 throttles
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored

0 input packets with dribble condition detected
74863786 packets output, 1108797321 bytes, 0 underruns
0 output errors, 0 collisions, 2 interface resets
0 babbles, 0 late collision, 0 deferred

0 lost carrier, 0 no carrier
0 output buffer failures, 0 output buffers swapped out

The higlighted fields can be checked for common problems.

Speed/Duplex: In most cases, this must be 100/Full as described above. However, in some cases like connecting to ATA (Analog Telephone Adapters), the port cannot be full duplex. If you know that a device supports only half duplex then its recommended to hard code half duplex on the switch side to prevent issues.

Collisions/Late Collisions: This is related to speed/duplexing issues.

Runts/Giants/CRCs: These indicate that the sending side NIC might have problems.

If everything looks ok but still you see CRCs and errors increase then this might point to a cable issue.