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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter 9 Creating and Maintaining VLANs
Understanding VLANs
Figure 9-1
VLANs as Logically Defined Networks
VLANs are often associated with IP subnetworks. For example, all the end stations in a particular IP
subnet belong to the same VLAN. Interface VLAN membership on the switch is assigned manually on
an interface-by-interface basis. When you assign switch interfaces to VLANs by using this method, it is
known as interface-based, or static, VLAN membership.
Traffic between VLANs must be routed. A Catalyst 3550 switch with the enhanced mutilayer software
image installed can route traffic between VLANs by using switch virtual interfaces (SVIs). An SVI must
be explicitly configured and assigned an IP address to route traffic between VLANs. For more
information, see the
“Switch Virtual Interfaces” section on page 8-4
and the
“Configuring Layer 3
Interfaces” section on page 8-22
.
Number of Supported VLANs
The Catalyst 3550 switch supports 1005 VLANs in VTP client, server, and transparent modes. VLANs
are identified with a number between 1 and 1001. VLAN numbers 1002 through 1005 are reserved for
Token Ring and FDDI VLANs. The switch supports per-VLAN spanning tree (PVST) with a maximum
of 128 spanning-tree instances. One spanning-tree instance is allowed per VLAN.
The switch supports both Inter-Switch Link (ISL) and IEEE 802.1Q trunking methods for sending
VLAN traffic over Ethernet ports.
Floor 1
Floor 2
Engineering
VLAN
Cisco router
Fast
Ethernet
Floor 3
Marketing
VLAN
Accounting
VLAN
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