CCIE Routing & Switching v5 Workbook -
CCIE R&S v5 Advanced Technology Labs -
LAN Switching
Layer 2 Dynamic Switchports
A Note On Section Initial Configuration Files: You must load the
initial configuration files for the section, named Basic Layer2
Switching, which can be found in
. Reference the Virtual Routers & Physical
Switches Diagram to complete this task.
Task
Configure all inter-switch links on SW2, SW3, and SW4 to be in dynamic auto state.
Configure all inter-switch links on SW1 to be in dynamic desirable state.
For verification, ensure that:
SW1 Ethernet links to SW2, SW3, and SW4 are negotiated as trunks.
Ethernet links between SW2, SW3, and SW4 do not negotiate trunking and
fallback to access mode.
Configuration
SW1:
interface range FastEthernet0/19 - 24
switchport mode dynamic desirable
SW2:
interface range FastEthernet0/19 - 24
switchport mode dynamic auto
SW3:
interface range FastEthernet0/19 - 24
switchport mode dynamic auto
SW4:
interface range FastEthernet0/19 - 24
switchport mode dynamic auto
Verification
With SW1’s inter-switch links configured in dynamic desirable state, and all other
inter-switch links configured in dynamic auto state, trunks will only be negotiated
between SW1 to SW2, SW1 to SW3, and SW1 to SW4. This is because SW1
initiates trunking negotiation through DTP (desirable), and SW2, SW3, and SW4
only respond to DTP negotiation requests (auto). This can be verified as shown
below, note that the output may differ for the "Vlans in spanning tree forwarding
state and not pruned" based on which of the switches is the STP root bridge for
VLAN 1.
SW1#show interface trunk
PortModeEncapsulation StatusNative vlanFa0/19desirable n-isl trunking
1Fa0/20desirable n-isl trunking
1Fa0/21desirable n-isl trunking
1Fa0/22desirable n-isl trunking
1Fa0/23desirable n-isl trunking
1Fa0/24desirable n-isl trunking
1
PortVlans allowed on trunk
Fa0/191-4094
Fa0/201-4094
Fa0/211-4094
Fa0/221-4094
Fa0/231-4094
Fa0/241-4094
PortVlans allowed and active in management domain
Fa0/191
Fa0/201
Fa0/211
Fa0/221
Fa0/231
Fa0/241
PortVlans in spanning tree forwarding state and not pruned
Fa0/191
Fa0/201
Fa0/211
Fa0/221
Fa0/231
Fa0/241
The output on SW3 is the same as on SW2 and SW4. None of these switches are
trunking directly with each other, only with SW1.
SW3#show interfaces trunk
PortModeEncapsulation StatusNative vlanFa0/19auto n-isl trunking
1Fa0/20auto n-isl trunking
1
PortVlans allowed on trunk
Fa0/191-4094
Fa0/201-4094
PortVlans allowed and active in management domain
Fa0/191
Fa0/201
PortVlans in spanning tree forwarding state and not pruned
Fa0/191
Fa0/20none
As seen from above outputs, by default switches will also negotiate ISL instead of
802.1q as the trunking protocol. Verify the DTP port state of "dynamic desirable"
and "dynamic auto"; also note the difference between "Administrative Mode," which
defines how the port was configured to operate, and "Operational Mode," which
defines how the port actually operates after DTP negotiation.
SW3#show interfaces fastEthernet0/19 switchport
Name: Fa0/19
Switchport: EnabledAdministrative Mode: dynamic auto
Operational Mode: trunk
Administrative Trunking Encapsulation: negotiate
Operational Trunking Encapsulation: islNegotiation of Trunking: On
Access Mode VLAN: 1 (default)
Trunking Native Mode VLAN: 1 (default)
Administrative Native VLAN tagging: enabled
Voice VLAN: none
Administrative private-vlan host-association: none
Administrative private-vlan mapping: none
Administrative private-vlan trunk native VLAN: none
Administrative private-vlan trunk Native VLAN tagging: enabled
Administrative private-vlan trunk encapsulation: dot1q
Administrative private-vlan trunk normal VLANs: none
Administrative private-vlan trunk associations: none
Administrative private-vlan trunk mappings: none
Operational private-vlan: none
Trunking VLANs Enabled: ALL
Pruning VLANs Enabled: 2-1001
Capture Mode Disabled
Capture VLANs Allowed: ALL
Protected: false
Unknown unicast blocked: disabled
Unknown multicast blocked: disabled
Appliance trust: none
!
!SW3#show interfaces fastEthernet0/21 switchport
Name: Fa0/21
Switchport: EnabledAdministrative Mode: dynamic auto
Operational Mode: static access
Administrative Trunking Encapsulation: negotiate
Operational Trunking Encapsulation: nativeNegotiation of Trunking: On
Access Mode VLAN: 1 (default)
Trunking Native Mode VLAN: 1 (default)
Administrative Native VLAN tagging: enabled
Voice VLAN: none
Administrative private-vlan host-association: none
Administrative private-vlan mapping: none
Administrative private-vlan trunk native VLAN: none
Administrative private-vlan trunk Native VLAN tagging: enabled
Administrative private-vlan trunk encapsulation: dot1q
Administrative private-vlan trunk normal VLANs: none
Administrative private-vlan trunk associations: none
Administrative private-vlan trunk mappings: none
Operational private-vlan: none
Trunking VLANs Enabled: ALL
Pruning VLANs Enabled: 2-1001
Capture Mode Disabled
Capture VLANs Allowed: ALL
Protected: false
Unknown unicast blocked: disabled
Unknown multicast blocked: disabled
Appliance trust: none
!
!SW1#show interfaces fastEthernet0/19 switchport
Name: Fa0/19
Switchport: EnabledAdministrative Mode: dynamic desirable
Operational Mode: trunk
Administrative Trunking Encapsulation: negotiate
Operational Trunking Encapsulation: islNegotiation of Trunking: On
Access Mode VLAN: 1 (default)
Trunking Native Mode VLAN: 1 (default)
Administrative Native VLAN tagging: enabled
Voice VLAN: none
Administrative private-vlan host-association: none
Administrative private-vlan mapping: none
Administrative private-vlan trunk native VLAN: none
Administrative private-vlan trunk Native VLAN tagging: enabled
Administrative private-vlan trunk encapsulation: dot1q
Administrative private-vlan trunk normal VLANs: none
Administrative private-vlan trunk associations: none
Administrative private-vlan trunk mappings: none
Operational private-vlan: none
Trunking VLANs Enabled: ALL
Pruning VLANs Enabled: 2-1001
Capture Mode Disabled
Capture VLANs Allowed: ALL
Protected: false
Unknown unicast blocked: disabled
Unknown multicast blocked: disabled
Appliance trust: none