ISL/802.1Q subinterfaces

This section describes how to configure subinterfaces on the router for encapsulation.

This example is for a Cisco Catalyst 3640:

   2811# configure terminal

Enter configuration commands, one on each line. End with Ctrl+Z.

This configuration is using the Fast Ethernet connection on the router that is connected to the Switch Trunk port from the preceding trunking section. Also select FastEthernet 0/0 for the trunk configuration. This connector plugs into the Switch-port FastEthernet 0/1.

Notice that no IP address is specified on the main interface.

   Cat3640(config)# int FastEthernet 0/0
   Cat3640(config-if)# no shut
   Cat3640(config-if)# exit

Enable trunking on the subinterface FastEthernet 0/0.1.

The actual trunks are configured on the subinterfaces.

   Cat3640(config)# int FastEthernet 0/0.1

Enter the trunking encapsulation as either ISL or dot1q (if you were working with a Dell switch for instance)

   Cat3640(config-subif)# encapsulation isl 1

Give the subinterfaces IP addresses for routing over the network. Configure L3 information on the subinterface 0/0.1.

   Cat3640(config-subif)# ip address 10.70.0.6 255.255.0.0
   Cat3640(config-subif)# exit

This address (10.70.0.6) is the Default Gateway for VLAN1.

Now enable trunking on the subinterface FastEthernet 0/0.2.

   Cat3640(config)# int FastEthernet 0/0.2
   Cat3640(config-subif)# encapsulation isl 2

Enter the trunking encapsulation ISL. Notice that this trunking matches the second VLAN on the switch that is being routed. Again, if this trunking was for working with a Dell switch, the encapsulation would be dot1q.

Configure L3 information on the subinterface 0/0.2:

   Cat3640(config-subif)# ip address 10.60.0.6 255.255.0.0
   Cat3640(config-subif)# exit
   Cat3640(config)# ^Z

This address (10.60.0.6) is the Default Gateway for VLAN2.

You can check the configuration to make sure that you have 2-way communication with the following command:

   Cat3640# show vlans
   
   Virtual LAN ID: 1 (Inter Switch Link Encapsulation)
      vLAN Trunk Interface:   FastEthernet0/0.2
      Protocols Configured:   Address: 
              Received: 
        Transmitted:
              IP 
              10.70.0.6 
               274083 
              164096
   Virtual LAN ID:  5 (Inter Switch Link Encapsulation)
      vLAN Trunk Interface:   FastEthernet0/0.1
      Protocols Configured:   Address: 
              Received: 
        Transmitted:
              IP 
              10.60.0.6 
               370687 
              623107
   Cat3640#

Configuring a Dell switch trunk port requires 802.1Q encapsulation, as shown in this example:

   Delta# show vlans
   
   Virtual LAN ID:  1 (IEEE 802.1Q Encapsulation)
      vLAN Trunk Interface:   FastEthernet0/0.1

This switch trunk port is configured as native VLAN for the following interface(s):

   FastEthernet0/0
      Protocols Configured:   Address: 
              Received: 
        Transmitted:
              IP 
              10.10.70.4 
             1881760 
              483783
           Other 
                                           0 
                8027
      1890808 packets, 554876237 bytes input
      491810 packets, 110471839 bytes output
   Virtual LAN ID:  5 (IEEE 802.1Q Encapsulation)
      vLAN Trunk Interface:   FastEthernet0/0.2
      Protocols Configured:   Address: 
              Received: 
        Transmitted:
              IP 
              10.60.0.5 
               977968 
              495206
           Other 
                                           0 
               20078
      977968 packets, 680607031 bytes input
      515284 packets, 115591709 bytes output