Switching and Network Access practice questions

switchport nonegotiate and static trunks

switchport nonegotiate stops a port sending DTP frames. SW1 is a static trunk that never asks, and SW2 is dynamic auto, which only answers, so SW2 stays an access port. The only way to trunk without DTP is a static trunk on SW2 as well, with nonegotiate to keep DTP off. Removing nonegotiate from SW1 would work technically but violates the policy; dynamic desirable both keeps DTP and gets no answer from SW1.

1. Identify that SW1 sends no DTP frames because of nonegotiate.
2. Recall that dynamic auto never initiates, so SW2 stays access.
3. Apply the policy: DTP must be off on both ends.
4. Configure SW2 as a static trunk with nonegotiate.

Exam tip: With DTP disabled, both ends of a trunk must be configured as static trunks.

Editing a trunk's allowed VLAN list

switchport trunk allowed vlan with a bare list overwrites the existing list. The add, remove and except keywords modify it instead. Adding VLAN 30 with add keeps VLANs 10 and 20 forwarding; the bare form would drop them, and the native and access VLAN commands do not change which VLANs a trunk allows.

1. Note the trunk already restricts its allowed list to 10 and 20.
2. Recall that the bare allowed vlan command replaces the list.
3. Use the add keyword to append VLAN 30.

Exam tip: Use allowed vlan add to extend a trunk list; the bare form replaces it.

Allowed versus allowed-and-active VLANs on a trunk

show interfaces trunk lists VLANs in stages. 'Allowed on trunk' is the configured list; 'allowed and active' keeps only VLANs that exist and are active in the local VLAN database; the last stage removes VLANs that spanning tree blocks or pruning removes. VLAN 30 passes the first stage but not the second, which points to VLAN 30 not being created on SW1. Creating it with the vlan 30 command fixes the problem.

1. Check whether VLAN 30 is in the allowed list: it is (1-4094).
2. Check the allowed-and-active list: VLAN 30 is missing.
3. Recall that only locally existing VLANs are active.
4. Create VLAN 30 on SW1.

Exam tip: Allowed but not active on a trunk means the VLAN does not exist on that switch.

Native VLAN agreement on an 802.1Q trunk

The CDP message lists the local interface and native VLAN first (SW1, 99) and the neighbour second (SW2, 1). Frames in the native VLAN cross untagged, so each end places untagged frames into its own native VLAN; when they differ, traffic leaks between VLANs. Both ends must use the same native VLAN. Changing SW1 to 1 would also fix the mismatch but breaks the stated standard, so SW2 is the end to change.

1. Read the log: SW1 uses 99, SW2 uses 1.
2. Apply the constraint: the standard is VLAN 99.
3. Change the native VLAN on the end that is non-standard, SW2.

Exam tip: Both ends of a trunk must use the same native VLAN; CDP reports when they don't.

Native VLAN handling on a router-on-a-stick trunk

On an 802.1Q trunk the native VLAN is sent untagged. SW1's native VLAN is 10, so VLAN 10 traffic reaches R1 without a tag. R1's subinterface .10 is configured with encapsulation dot1Q 10 (no native keyword), so it only matches frames tagged with VLAN 10; the untagged frames are not delivered to it. VLAN 20 is tagged and works. Fix it by changing SW1's native VLAN to an unused VLAN, or by adding the native keyword on R1 (encapsulation dot1Q 10 native).

1. Compare which VLAN works (20) with which fails (10).
2. Notice VLAN 10 is the switch's native VLAN, sent untagged.
3. Notice the router subinterface expects tagged VLAN 10 frames.
4. Align them: change the native VLAN or use the native keyword on R1.

Exam tip: A router subinterface without the native keyword only accepts tagged frames.

Layer 2 switch ports versus Layer 3 routed ports

Catalyst switch ports default to Layer 2 (switchport). Only Layer 3 interfaces, such as routed ports and SVIs, accept an IP address. no switchport converts the physical interface into a routed port, after which the ip address command works. ip routing is still needed to route between interfaces, but it is a global command and does not change the port type.

1. Recognise that the port is still a Layer 2 switch port.
2. Recall that only Layer 3 interfaces take IP addresses.
3. Convert the port with no switchport, then add the address.

Exam tip: A switch port takes an IP address only after no switchport makes it a routed port.