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VLANs can exist either as end-to-end
networks, which span the entire switch fabric, or they can exist
inside of geographic boundaries.
An end-to-end VLAN network comprises
the following characteristics:
- Users are grouped into VLANs
independent of physical location and dependent on group or job
function.
- All users in a VLAN should have
the same 80/20 traffic flow patterns.
- As a user moves around the campus,
VLAN membership for that user should not change.
- Each VLAN has a common set of
security requirements for all members.
In the Figure, starting in the wiring
closet, 10-megabit-per-second (Mbps) dedicated Ethernet ports are
provisioned for each user. Each color represents a subnet and,
because people have moved around over time, each switch eventually
becomes a member of all VLANs. Fast Ethernet Inter-Switch Link (ISL)
or IEEE 802.1Q is used to carry multiple VLAN information between
the wiring closets and the distribution-layer switches.
Note: ISL is a Cisco-proprietary protocol that maintains VLAN information as traffic flows between switches and routers.
IEEE 802.1Q is an open-standard (IEEE) VLAN tagging mechanism that predominates in modern switching installations.
Workgroup servers operate in a client/server model, and attempts have been made to keep users in the same VLAN as their server to maximize the performance of Layer 2 switching and keep traffic localized.
In the core, a router allows
inter-subnet communication. The network is engineered, based on
traffic flow patterns, to have 80 percent of the traffic within a
VLAN and 20 percent crossing the router to the enterprise servers
and to the Internet and WAN.
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