Troubleshooting guide

Local Area Network
175
Using Layer 2 Statistics
The System Administration Tool provides various Layer 2 (L2) Ethernet traffic counters that can
help with debugging LAN problems. The counters appear in the output of the “L2 Stat Port”
maintenance command.
A LAN that is not operating correctly can cause IP voice quality issues ranging from minor
annoyance to an inability to hold an intelligible phone call.
Incorrectly operating LANs can also cause data transfer problems for computer users such as
slow response times.
The following is general information regarding LANs and what the L2 traffic parameters mean.
The table at the end indicates probable causes of a particular L2 error and actions to take to
try and resolve the error.
Shared versus Switched LANs
Shared LANs are LANs in which network devices share the same medium to communicate
with other network devices. Examples of shared LANs are networks based on the 10Base5,
10Base2 standards or wireless LAN standards. LANs based on the twisted pair standards can
also use a shared communication medium if the LAN does not support L2 switching.In shared
LANs all network devices communicate on a shared medium. In the case of10Base5 and
10Base2, a coaxial cable is the shared communication media. In the case of wireless LANs,
the ether or radio spectrum is the shared communication medium. In twisted pair LANs that
employ ethernet hubs rather than L2 switches, the network devices communicate over a shared
medium — in this instance the shared medium is the ethernet hub itself.
Some basic differences between shared LANs and switched or non-shared LANs are:
• In a shared LAN all network devices must use half duplex communication.
• In a switched LAN network devices can use half-duplex or full-duplex communication.
• In a shared LAN only one network device can transmit at any given time; otherwise, a
collision will occur.
• In a switched LAN in which the network devices are configured for full-duplex operation,
multiple devices can transmit simultaneously, as a result a device can transmit and receive
simultaneously.
• In a shared LAN collisions are a normal occurrence. Collisions will cause alignment errors,
FCS errors and, runt packets to occur on the network.
• In a switched LAN in which all network devices are configured for full-duplex operation,
collisions should be minimal or non-existent. On a network that has zero collisions, it is still
possible to have alignment errors, FCS errors, and runt packets due to other issues.
• As a result of the behavioral differences between shared LANs and switched LANs, the
traffic counters will behave differently and error counts will have different connotations
depending on whether the ICP is deployed in a shared LAN or a switched LAN.