Information Technology Reference
In-Depth Information
Both optimum switching and distributed optimum switching are no longer supported
beginning with Cisco IOS software Release 12.0.
NOTE
NetFlow Switching
NetFlow switching
is another form of cache-based switching. The NetFlow cache is built
by processing the first packet of a flow through the standard switching mechanism. As a
result, each flow is associated with an incoming and outgoing interface and with a specific
security access permission and encryption policy. The cache also includes entries for traffic
statistics that are updated with the switching of subsequent packets.
A
flow
is defined as a specific conversation between two hosts. Source and destination
addresses, ports, and the IP packet type define a flow. For TCP communication, a conver-
sation starts and stops with various TCP control messages. For UDP, a conversation is con-
sidered to have ceased after a timer has expired. Subsequent packets that match the flow tag
are considered to be members of the same flow and are simply switched through the out-
bound interface, bypassing further checking against access lists, queuing, and so on.
NetFlow switching is designed to provide a highly efficient mechanism with which to
process extended or complex access lists without paying as much of a performance penalty
as with other switching methods. With NetFlow switching, detailed accounting information
is collected for each flow. In fact, information collection has become so important that in
newer IOS releases, NetFlow switching is being used exclusively for that purpose and is no
longer used to switch packets.
NOTE
With both CEF and NetFlow switching enabled, CEF provides the switching path for IP
packets and populates the flow cache. NetFlow is used to export the statistics to a flow
collector. The flow information includes per-user, per-protocol, per-port, and per-type of
service statistics, which can be used for a wide variety of purposes, such as network
analysis and planning, accounting, and billing.
Shortcomings of Cached-Based Switching Methods
The following are some of the shortcomings of the cache-based switching methods:
•
They are all traffic-driven, in that they are dependent on receipt of the first packet to
populate the cache. This packet is switched in the slow path, leading to low perfor-
mance and high CPU usage. In a network with large and constantly changing traffic
patterns, such as the Internet, the processing of the first packet can cause significant
