Understanding the network.pdf - Back to Home

the spanning tree is also recalculated and, if the blocked interface has
a better cost, it becomes primary. However, port trunks can readjust
the traffic flow to accommodate for lost member links. The need for
redundancy needs to be evaluated against the cost (in terms of
usefulness) of the idle ports.
Until Gigabit Ethernet became available, port trunking was the only
way to provide backbone network links beyond 200Mbps (100Mbps
full-duplex Ethernet). At the time of this writing, Gigabit Ethernet is
only available over single or multimode fiber optic cable. This allows it
to be easily implemented over existing 100BaseFX cabling
infrastructures. However, many early nonstandard Gigabit Ethernet
implementations only delivered around 400Mbps, and many of the
standards-based Gigabit implementations suffer packet loss when
utilization reaches around 80 percent. Alternatively, most
workgroup-class switches with today's Application-Specific
Integrated Circuits (ASICs) can provide "wire rate" full-duplex
transmission without any problem. So, right now, port trunking will
probably perform better than the current Gigabit offerings. Port
trunking also has the added benefit of redundancy, due to the load
balancing and recovery capabilities inherent in its functional design.
Port Mirroring
In the beginning of the chapter, a brief overview of packet sniffers was
given. Packet sniffers are hardware and/or software tools used to
monitor Layer 2 packets as they are transmitted across the shared
transmission medium. The operative word here is "shared." With
network switches, each port is a collision domain, which makes each
port on the switch effectively its own shared network segment (see
Figure 6.9).