11-5
Catalyst 3750 Switch Software Configuration Guide
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Chapter 11 Configuring Interface Characteristics
Understanding Interface Types
Although the switch stack supports a total or 1005 VLANs (and SVIs), the interrelationship between the
number of SVIs and routed ports and the number of other features being configured might impact CPU
performance because of hardware limitations. See the “Configuring Layer 3 Interfaces” section on
page 11-21 for information about what happens when hardware resource limitations are reached.
SVIs are created the first time that you enter the vlan interface configuration command for a VLAN
interface. The VLAN corresponds to the VLAN tag associated with data frames on an ISL or 802.1Q
encapsulated trunk or the VLAN ID configured for an access port. Configure a VLAN interface for each
VLAN for which you want to route traffic, and assign it an IP address. For more information, see the
“Manually Assigning IP Information” section on page 4-10.
Note When you create an SVI, it does not become active until it is associated with a physical port.
SVIs support routing protocols and bridging configurations. For more information about configuring IP
routing, see Chapter 34, “Configuring IP Unicast Routing,” Chapter 36, “Configuring IP Multicast
Routing,”and Chapter 38, “Configuring Fallback Bridging.”
Note The SMI supports static routing and RIP; for more advanced routing or for fallback bridging, you must
have the EMI installed on the stack master.
EtherChannel Port Groups
EtherChannel port groups provide the ability to treat multiple switch ports as one switch port. These port
groups act as a single logical port for high-bandwidth connections between switches or between switches
and servers. An EtherChannel balances the traffic load across the links in the channel. If a link within
the EtherChannel fails, traffic previously carried over the failed link changes to the remaining links. You
can group multiple trunk ports into one logical trunk port, group multiple access ports into one logical
access port, or group multiple routed ports into one logical routed port. Most protocols operate over
either single ports or aggregated switch ports and do not recognize the physical ports within the
port group. Exceptions are the DTP, the Cisco Discovery Protocol (CDP), and the Port Aggregation
Protocol (PAgP), which operate only on physical ports.
When you configure an EtherChannel, you create a port-channel logical interface and assign an interface
to the EtherChannel. For Layer 3 interfaces, you manually create the logical interface by using the
interface port-channel global configuration command. Then you manually assign an interface to the
EtherChannel by using the channel-group interface configuration command. For Layer 2 interfaces, use
the channel-group interface configuration command to dynamically create the port-channel logical
interface. This command binds the physical and logical ports together. For more information, see
Chapter 33, “Configuring EtherChannels.”
Connecting Interfaces
Devices within a single VLAN can communicate directly through any switch. Ports in different VLANs
cannot exchange data without going through a routing device. With a standard Layer 2 switch, ports in
different VLANs have to exchange information through a router. In the configuration shown in
Figure 11-1, when Host A in VLAN 20 sends data to Host B in VLAN 30, it must go from Host A to the
switch, to the router, back to the switch, and then to Host B.
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