Understanding the network.pdf - Back to Home

Layer 2 infrastructures to operate multiple discrete Layer 3 segments.
By using VLANs, you can reduce Layer 2 collision domains and token
contention problems, as well as control the extent of Layer 3
broadcast traffic. When it exceeds the normal operating percentage of
8 to 10 percent of Layer 3 segmentoperating bandwidth, this kind of
traffic impacts network performance and increases the chance of
Layer 3 broadcast storms. VLANs can also reduce unneeded router
processing overhead by eliminating the need to have geographically
distant end-stations (with common resource access requirements)
homed on different Layer 3 segments. Instead, dynamic VLANs can be
created to have end-stations and servers in different locations
function as if they were on the same local segment.
NOTE
Remember, when configuring 802.1q switches, each switch involved
within a VLAN group must be configured using the same VID
designation. In other words, if you want a port on switch B to belong
to the same VLAN as another port on switch A, you need to make the
VID for both of those ports identical between the two switches.
NOTE
Broadcast storms occur when the network is too large or when
defective hardware or network configurations exist. Network
broadcasts are essential for any network to operate, and when the
network is of a reasonable size, broadcasts should not result in any
significant impact on network performance. As Layer 3 networks
increase in size, however, the broadcast domains (the possible
number of hosts that can send or receive broadcast traffic) also
increase. This results in higher network bandwidth utilization rates for
broadcast traffic and less for "real" network traffic. Broadcast
bandwidth utilization rates up to 25 percent are tolerable, but not
optimal. When Layer 3 networks grow too large, broadcast storms can
be triggered by minor network events and cause excessive broadcast
traffic between hosts on top of an already high network broadcast rate.
Broadcast storms will also occur regardless of the underlying Layer 3
broadcast when an end-station NIC, switch, or bridge port fails and
randomly generates broadcast traffic, or if two or more bridge ports
exist between two Layer 2 network segments and spanning tree is
disabled.