Specifications

Configuring Routing Between VLANs
Information About Routing Between VLANs
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Broadcast Control
Just as switches isolate collision domains for attached hosts and only forward appropriate traffic out a
particular port, VLANs provide complete isolation between VLANs. A VLAN is a bridging domain, and
all broadcast and multicast traffic is contained within it.
VLAN Performance
The logical grouping of users allows an accounting group to make intensive use of a networked
accounting system assigned to a VLAN that contains just that accounting group and its servers.
That group’s work will not affect other users. The VLAN configuration improves general network
performance by not slowing down other users sharing the network.
Network Management
The logical grouping of users allows easier network management. It is not necessary to pull cables to
move a user from one network to another. Adds, moves, and changes are achieved by configuring a port
into the appropriate VLAN.
Network Monitoring Using SNMP
SNMP support has been added to provide mib-2 interfaces sparse table support for Fast Ethernet
subinterfaces. Monitor your VLAN subinterface using the show vlans EXEC command. For more
information on configuring SNMP on your Cisco network device or enabling an SNMP agent for remote
access, refer to the “Configuring SNMP” chapter in the Cisco IOS Configuration Fundamentals
Configuration Guide.
Communication Between VLANs
Communication between VLANs is accomplished through routing, and the traditional security and
filtering functions of the router can be used. Cisco IOS software provides network services such as
security filtering, quality of service (QoS), and accounting on a per-VLAN basis. As switched networks
evolve to distributed VLANs, Cisco IOS software provides key inter-VLAN communications and allows
the network to scale.
Before Cisco IOS Release 12.2, Cisco IOS support for interfaces that have 802.1Q encapsulation
configured is IP, IP multicast, and IPX routing between respective VLANs represented as subinterfaces
on a link. New functionality has been added in IEEE 802.1Q support for bridging on those interfaces and
the capability to configure and use integrated routing and bridging (IRB).
Relaying Function
The relaying function level, as displayed in Figure 74, is the lowest level in the architectural model
described in the IEEE 802.1Q standard and presents three types of rules:
• Ingress rules—Rules relevant to the classification of received frames belonging to a VLAN.
• Forwarding rules between ports—Rules decide whether to filter or forward the frame.
• Egress rules (output of frames from the switch)—Rules decide if the frame must be sent tagged or
untagged.