Ethernet networks tend to be susceptible to performance problems as they grow, based on the CSMA/CD method of media access that they use. While implementing Layer 2 switching goes a long way towards better Ethernet performance, there are still a number of issues to consider when an Ethernet network begins to experience performance problems. Examples of reasons for congestion on Ethernet networks include having too many hosts on a given segment, not enough bandwidth available, broadcast storms, along with excessive broadcast or multicast traffic.
A few key metrics are used by Cisco to help decide when an Ethernet network is not performing at an appropriate level. These include:
Network utilization. A network utilization of over 40% on shared Ethernet segments represents that the network is saturated.
Broadcasts/multicasts. No Ethernet segment should have more than 20% combined broadcast and multicast traffic.
CRC Errors. There should be less than 1 CRC error per MB of network traffic.
Collisions. Less than 0.1% of network packets should be involved in collisions
While this data may seem difficult to obtain, a variety of network management tools can provide these particular metrics, along with many others. Examples include CiscoWorks and Cisco Netsys Performance Service Manager.