Trunking with 802.1Q
One problem with having two users in the same VLAN but not on the same physical switch is how SW1 tells SW2 that a broadcast or unicast frame is supposed to be for VLAN 10. The answer is simple. For connections between two switches that contain ports in VLANs that exist in both switches, you configure specific trunk ports instead of configuring access ports. If the two switch ports are configured as trunks, they include additional information called a tag that identifies which VLAN each frame belongs to. 802.1Q is the standard protocol for this tagging. The most critical piece of information (for this discussion) in this tag is the VLAN ID.
Currently, the two users cannot communicate because they are in the same VLAN (VLAN 10), but the interswitch links (between the two switches) are not configured as trunks. To configure both sets of interfaces as trunks, you would specify the trunk method of 802.1Q and then turn on the feature, as shown in Example 9-2.
Example 9-2 Configure Interfaces as Trunk Ports
SW2(config)# interface range fa0/23-24
SW2(config-if-range)# switchport trunk encapsulation dot1q
SW2(config-if-range)# switchport mode trunk
SW2(config-if-range)#
! To verify the trunks:
SW2(config-if-range)# do show interface trunk
Port Mode Encapsulation Status Native vlan
Fa0/23 on 802.1Q trunking 1
Fa0/24 on 802.1Q trunking 1
Port Vlans allowed on trunk
Fa0/23 1-4094
Fa0/24 1-4094
Port Vlans allowed and active in management domain
Fa0/23 1,10
Fa0/24 1,10
Port Vlans in spanning tree forwarding state and not pruned
Fa0/23 1,10
Fa0/24 none
SW2(config-if-range)#
! Another way to verify the trunk:
SW2# show interface fa0/23 switchport
Name: Fa0/23
Switchport: Enabled
Administrative Mode: trunk
Operational Mode: trunk
Administrative Trunking Encapsulation: dot1q
Operational Trunking Encapsulation: dot1q
Negotiation of Trunking: On
Access Mode VLAN: 1 (default)
Trunking Native Mode VLAN: 1 (default)
Administrative Native VLAN tagging: enabled
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