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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter9 Creating and Maintaining VLANs
Understanding VLANs
Figure9-1 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 interfac es (SVIs). An SVI m ust
be explicitly configured and assigned an IP address to rout e t raffic bet we en V LAN s. For m ore
information, see the “Switch Virtual Interfaces” section on page8-4 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 a llo wed per V LAN .
The switch supports both Inter-Switch Link (ISL) and IEEE 802.1Q trunking methods for sending
VLAN traffic over Ethernet ports.
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