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Troubleshooting Switched Connections

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The encapsulation isl 1 command defines that VLAN1 is using this

physical interface and is trunked via ISL. This command is placed on each
subinterface. Use the set trunk and clear trunk commands on the switch
to configure the switch side of the connection.

VLANs across Routers and Switches

VLAN implementation has been described on Catalyst switches and Catalyst
switches with Route Switch Modules. One VLAN implementation is left,
using a router and a switch.

The router plays many important roles in the implementation of VLANs

throughout a network. The overall role of a router is to provide communication
among VLANs. Also, routers are able to perform many functions that add to the
flexibility and scalability of VLAN deployment. Some of these major functions
are broadcast management, routing, policy control, VLAN switching, and
VLAN translation. Others include QoS management, redundancy, hierarchical
design, and traffic shaping and management.

Broadcast Management

Simply put, routers will not forward broadcasts. Switches also control
broadcasts by forwarding only to ports that are members of the source
VLAN. This property allows routers to lower broadcast traffic on the net-
work backbone.

Policy Control

Switches do not have the capability to apply policy control to individual ports or
VLANs on the switch. Use of a router provides the means to implement security
and policy control to and from connected VLANs. Access lists may be written and
applied to the VLAN subinterface on the router to provide this capability.

VLAN Switching

VLAN switching occurs when a packet destined for the same VLAN on a different
interface crosses the router. The header remains intact and the frame is switched
at Layer 2 to the destination interface where the VLAN resides.

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