Example 114 Status of a Trunk Line

3550_switch#show interface gigabitEthernet 0/1 switchport

Name: Gi0/1 Switchport: Enabled Administrative Mode: trunk Operational Mode: trunk

Administrative Trunking Encapsulation: dotlq Operational Trunking Encapsulation: dotlq Negotiation of Trunking: On Access Mode VLAN: 1 (default) Trunking Native Mode VLAN: 1 (default) Trunking VLANs Enabled: ALL Pruning VLANs Enabled: 2-1001 Protected: false

Unknown unicast blocked: disabled Unknown multicast blocked: disabled

Voice VLAN: none (Inactive)

Appliance trust 3550 switch#


In 802.1Q networks, it is critical to ensure that the native VLAN is the same throughout the entire 1 This is because 802.1Q uses Mono Spanning Tree. Mono Spanning Tree makes the entire VTP doma a single-bridged domain to all third-party 802.1Q switches. Cisco ensures compatibility with MST d implementing PVST+ along with MST. This is an extended version of Per VLAN Spanning Tree Plus which provides sea mless transparent integration for 802.1Q networks. Mono Spanning Tree runs oi VLAN. For this rea son, it is important to have the same native VLAN throughout the entire internet' default VLAN is 1, which is also the default native VLAN. To change the native VLAN, use the follow command on to e trunk:

Switch#(config-if)#switchport trunk native vlanvlan-id

Theshow interface with the trunk keyword command will also list the VLANs that are prune eligib confuseprune eligible VLANs with VLAN propagation. Prune eligible means that unnecessary broadc and unknown unicast traffic wiil n ot be forwarded ove r ttunk libes to switches that do not have an a that particular VLAN. By default, all VLAN information and spanning-tree frames for each VLAN are out a ll trunkin g interfaces. VLA Ns and STP ca c only be removed ftom a trunk by using the clear tri on th e Pisco Catal yst 5500n6500 series or by changing the VLA Ns allowed on a tmsk o n the Pisco 3 switch. You wo i Il le arn more ab out teese functions in the upcoming section "Step 4: Oontrollinc] STP Pro pagation ."

Exampje 1-15 lists th e output of t he s Sow trnnk comman d. The trunk kevword displays similar in theswitchport keyword. This command, however, focuses more on the VLAN information of the tri

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