CCNA 200-301 switching and access explained
Switching and Network Access is also worth 25% of the v2.0 CCNA. It covers switch-to-switch and switch-to-router links, edge port configuration, CDP and LLDP, Layer 2 and Layer 3 troubleshooting with show commands, and Rapid PVST+. Questions mix configuration snippets with output, and the traps are small details: native VLANs, LACP modes and bridge priority arithmetic.
Infrastructure connectivity
| Topic | Key facts |
|---|---|
| 802.1Q trunks | Tag every frame except the native VLAN; both ends must agree on the native VLAN and the allowed list |
| EtherChannel | LACP modes active and passive; two passive ends never form a bundle; member ports need matching speed, duplex and VLAN or trunk settings |
| SVI | A virtual interface per VLAN; used for management and for inter-VLAN routing on a Layer 3 switch |
| Routed ports | no switchport turns a physical port into a Layer 3 interface |
Inter-VLAN routing on a Layer 3 switch needs ip routing plus an up/up SVI for each VLAN. Router-on-a-stick uses subinterfaces with encapsulation dot1q.
Edge ports
A host port is an access port in one VLAN. Specific devices need specific settings:
- IP phone and PC: one data VLAN with
switchport access vlanand a voice VLAN withswitchport voice vlan. - Access points: a standalone AP sits on an access port; a controller-based AP in local mode may use a trunk, depending on design.
- Virtualised hosts: often a trunk so several VLANs reach the hypervisor.
- PoE: the switch can power phones and APs; check with
show power inline.
CDP and LLDP
CDP is Cisco proprietary and LLDP is the open standard. Both let you confirm neighbor, port and platform, so you can verify that documentation matches reality. Use LLDP when the neighbor is not a Cisco device. show cdp neighbors detail and show lldp neighbors are the commands.
Troubleshooting with show commands
The objectives name show commands including logs, ping, extended ping, traceroute and packet capture output. A systematic order works: check the interface (show interfaces status), then VLAN membership (show vlan brief), the trunk (show interfaces trunk), the MAC table, and finally Layer 3 reachability.
Rapid PVST+
Rapid PVST+ runs one spanning tree instance per VLAN and converges in seconds.
| Concept | Rule |
|---|---|
| Root bridge | Lowest bridge ID: priority (plus VLAN ID) first, then lowest MAC |
| Default priority | 32768 plus the VLAN number, so VLAN 10 is 32778 |
| Priority steps | Multiples of 4096 |
| Port roles | Root, designated, alternate, backup |
| Port states | Discarding, learning, forwarding |
| PortFast | Edge ports go straight to forwarding |
| BPDU guard | Err-disables a port that receives a BPDU |
| Root guard | Stops a neighbor from becoming root |
| Loop guard | Protects against unidirectional links on non-designated ports |
On every non-root switch there is exactly one root port, the one with the lowest path cost to the root. On each segment the designated port is on the switch with the lowest cost to the root.
Example questions
Question 1. Switch A trunks to Switch B. A has native VLAN 1 on the trunk and B has native VLAN 99. What is the result?
- A. The trunk will not form
- B. All VLANs pass normally because tagging hides the mismatch
- C. Untagged frames land in different VLANs on each side and the mismatch is reported
- D. The switch with the higher VLAN number wins and the other adjusts
Show answer
Answer: C
A native VLAN mismatch causes CDP and spanning tree to report the inconsistency, and untagged traffic from one VLAN is placed into a different VLAN on the other side, a security and connectivity problem. The trunk does not simply fail to form, and the mismatch does not cause the native VLAN to be tagged or switch roles to be negotiated.
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Question 2. Two switches are configured with an EtherChannel using LACP. Which mode combination brings the bundle up?
- A. Passive on both switches
- B. Active on one switch and passive on the other
- C. Active on one switch and desirable on the other
- D. Passive on one switch and on on the other
Show answer
Answer: B
LACP forms a channel when at least one side actively sends LACP packets: active/active or active/passive. Passive/passive never starts the negotiation, so no bundle forms. On/on is static and does not use LACP. Desirable is a PAgP mode, not LACP.
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Question 3. For VLAN 10, SW1 has the default priority and a MAC of 0000.0c11.1111, SW2 has spanning-tree vlan 10 priority 4096, and SW3 has the default priority with a MAC of 0000.0c00.0001. Which switch becomes the root bridge?
- A. SW1
- B. SW3
- C. They tie, so the oldest switch is root
- D. SW2
Show answer
Answer: D
The lowest bridge ID wins and priority is compared before MAC. SW2 has priority 4096 plus the VLAN extension (4106), lower than the 32778 of SW1 and SW3. The MAC only breaks ties between equal priorities, so SW3’s lower MAC does not matter.
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Revision tips
Draw every spanning tree scenario. Mark the root, label each switch’s root port, then the designated ports; whatever remains is alternate. For EtherChannel, memorise the one rule that passive meets passive fails. For trunks, check native VLAN and allowed VLANs in that order.
Continue with IP routing, or revisit infrastructure.