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Catalyst 3750 SwitchSoftware Configuration Guide
OL-8550-09
Chapter12 Configuring Interface Characteristics
Understanding Interface Types
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.
By using the switch with routing enabled, when you configure both VLAN 20 and VLAN 30 with an
SVI to which an IP address is assigned, packets can be sent from Host A to Host B directly through the
switch with no need for an external router (Figure12-1).
Figure12-1 Connecting VLANs with a Layer 3 Switch
With the IP services image, the switch supports two methods of forwarding traffic between interfaces:
routing and fallback bridging. With the IP base image, only basic routing (static routing and RIP) is
supported. Whenever possible, to maintain high performance, forwarding is done by the switch
hardware. However, only IP Version 4 packets with Ethernet II encapsulation can be routed in hardware.
Non-IP traffic and traffic with other encapsulation methods can be fallback-bridged byhardware.
The routing function can be enabled on all SVIs and routed ports. The switch routes only IP traffic.
When IP routing protocol parameters and address configuration are added to an SVI or routed port,
any IP traffic received from these ports is routed.For more information, see Chapter38,
“Configuring IP Unicast Routing,” Chapter46, “Configuring IP Multicast Routing,” and
Chapter 47, “Configuring MSDP.”
Fallback bridging forwards traffic that the switch does not route or traffic belonging to a nonroutable
protocol, such as DECnet. Fallback bridging connects multiple VLANs into one bridge domain by
bridging between two or more SVIs or routed ports. When configuring fallback bridging, you assign
SVIs or routed ports to bridge groups with each SVI or routed port assigned to only one bridge
group. All interfaces in the same group belong to the same bridge domain. For more information,
see Chapter 48, “Configuring Fallback Bridging.”
Host A
SVI 1172.20.128.1 172.20.129.1SVI 2
Layer 3 switch
with routing enabled
VLAN 20
Host B
VLAN 30
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