Configuring Trunk Interface

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Trunking is method for a system to provide network access to many clients or users by sharing multiple lines or frequencies. Its point to point link between one or more Ethernet switch interfaces and other networking devices i.e router. Trunks can carry the traffic of multiple VLANs over a single link.
Ethernet trunk interfaces support different trunking modes.  We can set interface as trunking or nontrunking or to negotiate trunking with the neighbouring interface. To autonegotiate trunking, the interface must be in the same VTP domain. Trunk negotiation is managed by the point to point Dynamic Trunking Protocol (DTP).
The VLAN Trunking Protocol (VTP) is a very useful protocol to create, manage and maintain a large network with many interconnected switches. The VLAN Trunking Protocol (VTP) can manage the addition, deletion, and renaming of VLANs from a central point without manual intervention and VLAN Trunk Protocol (VTP) thus reduces network administration in a switched network.
Layer 2 interface Modes
·         Switchport mode access  
·         Switchport mode dynamic desirable
·         Switchport mode trunk
·         Switchport nonegotiable
Important Points:
802. IQ trunk impose some limitation on the trunking strategy for a network
·         In a Cisco switches, the switch maintain one instance of spanning tree for each VLAN allowed on trunks.
·         When you connect Cisco switch to a non Cisco device, the Cisco switch combines the spanning tree instance of the VLAN of the trunk with spanning tree instance of the non Cisco switch.
·         Native VLAN trunk should be same on both ends of the trunk link.  If not spanning tree loops might occur.
·         Disabling spanning tree on the native VLAN trunk without disabling spanning tree on every VLAN in the network can cause spanning tree loops.
·         Interface whose trunk encapsulation is Auto can’t be configured to trunk mode
Configuring Trunk Ports
To configure an interface as a static trunk, we shall first need to configure the encapsulation type and shutdown interfaces on both switches.
Switch 1 Console:
 Switch1# enable
Switch1# configure terminal
Switch1# interface range Fa0/10 – 22
Switch1# shutdown
Switch1# interface fa0/10
Swith1# switchport trunk encapsulation dot1q
Switch1# switchport mode trunk
Switch1# no shut
Switch# end
Switch 2 Console:
                Switch2# enable
                Switch2# configure terminal
                Switch2# Interface range Fa0/10 – 02
                Switch2# shutdown
                Switch2# interface fa0/10
                Swtich2# switchport trunk encapsulation dot1q
                Switch2# Switchport mode trunk
                Switch2# no shut
                Switch2# end
Configuring the Native VLAN
If we do not configure this parameter, the trunk port uses the default VLAN as the native VLAN ID.
                Switch# configuring terminal
                Switch# interface fast ehternet 0/9
                Switch# switchport trunk native vlan 10
Configuring the Allowed VLANs for Trunking Ports
By default all VLANs are allowed for the trunk interface. Following command can be used to set specific VLAN to be allowed for the trunk interface.
                Switch#interface fast Ethernet 0/3
                Switch# switchport trunk allowed vlan {vlan-list all}
Useful Commands
                Show interface Fastethernet 0/10 trunk

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