Switches 

For these pages we will use a fairly common switche, the Cisco Catalyst 1900.  As the image above hints at - a switch can break apart a Layer 2 Ethernet LAN into two segments.  It also can connect to a router, which in turn connects the LAN to a WAN.  In that configuration, the switch sits between IP Hosts and the Router.    The Catalyst 1900 has 25 ethernet ports (for access lines) and 2 fast ethernet ports (f0/26 and f0/27) for trunks.  The switch interchanges the words “port” and “interface” – but they mean the same thing (sh run will list interfaces, and “sh spantree” will list ports).  For example, the switch may list the port/interface in two different ways :

 

Port Ethernet 0/1 is forwarding

 

or

 

Interface Ethernet 0/1

 

The switch ports which transmit frames (Layer 2), and also has a layer 3 interface which connects to the router.  But it’s internals are all Layer 2. 

 

Routers have no IP address – but their interfaces do

Switches and Hosts have an IP address – their interfaces do not.  Therefore the “interface” has no IP address, and the “sh IP” command will display the IP address, but the “sh interface” will not.  In addition, with a switch there is no “sh ip route” command (since a switch cannot route) – just a “sh ip” command.