Cisco CCNA (200-301) - Routing & Switching Essentials Flashcards

Question 1

IEEE 802.1Q Encapsulation

Accepted Answer

An industry-standard trunking protocol that inserts a **4-byte VLAN tag** into the original Ethernet frame header. Traffic belonging to the **Native VLAN** is transmitted untagged across the trunk link.

Question 2

Dynamic Trunking Protocol (DTP) Modes

Accepted Answer

Cisco proprietary protocol to negotiate trunking:
- `switchport mode access`: Disables DTP negotiation; forces port into access mode.
- `switchport mode trunk`: Forces port into trunk mode and sends DTP frames.
- `switchport mode dynamic desirable`: Actively seeks to convert link to a trunk.
- `switchport mode dynamic auto`: Passive; becomes a trunk only if neighbor is set to trunk or desirable.

*Best Practice:* Disable DTP using `switchport nonegotiate` on static trunks.

Question 3

Native VLAN Security Risk & Best Practice

Accepted Answer

If the Native VLAN mismatch occurs on a trunk, traffic can leak between VLANs. Attackers can exploit default untagged VLANs via **VLAN Hopping** (double tagging).

*Best Practice:* Change the native VLAN from the default `VLAN 1` to an unused VLAN and explicitly tag native VLAN traffic (`vlan dot1q tag native`).

Question 4

Spanning Tree Protocol (STP) Port States (802.1D)

Accepted Answer

1. **Blocking:** Receives BPDUs only; discards data frames and does not learn MAC addresses.
2. **Listening:** Processes BPDUs to determine topology; does not forward data or learn MACs (15s).
3. **Learning:** Learns MAC addresses; does not forward data frames (15s).
4. **Forwarding:** Sends and receives data frames; processes BPDUs.
5. **Disabled:** Administrative shutdown state.

Question 5

STP Root Bridge Selection Formula

Accepted Answer

The switch with the **lowest Bridge ID (BID)** becomes the Root Bridge.

$$	ext{Bridge ID} = 	ext{Bridge Priority} + 	ext{System ID Extension (VLAN ID)} + 	ext{MAC Address}$$

Default Priority is **32768** (configurable in increments of 4096).

Question 6

PortFast

Accepted Answer

An STP feature configured on edge switch ports connected to end devices (PCs, printers). It immediately transitions the interface from **Blocking** to **Forwarding**, bypassing Listening and Learning states to prevent DHCP timeouts.

Command: `spanning-tree portfast`

Question 7

BPDU Guard

Accepted Answer

An STP security feature typically paired with PortFast. If an unexpected BPDU is received on a BPDU Guard-enabled port, the switch immediately puts the port into the **`err-disabled`** state to prevent rogue switches from altering the STP topology.

Command: `spanning-tree bpduguard enable`

Question 8

Router-on-a-Stick (ROAS)

Accepted Answer

An inter-VLAN routing method using a single physical router interface connected to a trunk switchport. The physical interface is divided into multiple logical **subinterfaces**, each assigned an IP address and 802.1Q tag matching its specific VLAN:
```text
interface GigabitEthernet0/0/0.10
 encapsulation dot1Q 10
 ip address 192.168.10.1 255.255.255.0
```

Question 9

Switch Virtual Interface (SVI)

Accepted Answer

A Layer 3 logical interface configured on a Multilayer Switch to perform inter-VLAN routing without requiring an external router.

```text
ip routing
interface vlan 10
 ip address 192.168.10.1 255.255.255.0
 no shutdown
```

Question 10

Administrative Distance (AD) Comparison

Accepted Answer

AD measures route trustworthiness (lower is better):
- **Connected:** 0
- **Static:** 1
- **eBGP:** 20
- **OSPF:** 110
- **RIP:** 120

Question 11

OSPFv2 Path Cost Calculation

Accepted Answer

OSPF uses **Cost** as its metric, based on interface bandwidth:

$$	ext{Cost} = \frac{	ext{Reference Bandwidth}}{	ext{Interface Bandwidth}}$$

Default Reference Bandwidth = $10^8	ext{ bps}$ ($100	ext{ Mbps}$). FastEthernet cost = $1$, GigabitEthernet cost = $1$ (requires manual adjustment using `auto-cost reference-bandwidth` for higher speeds).

Question 12

OSPF Router ID (RID) Selection Order

Accepted Answer

1. Explicitly configured RID (`router-id X.X.X.X`)
2. Highest IP address among configured **Loopback interfaces**
3. Highest IP address among active **Physical interfaces**

Question 13

OSPF Neighbor Adjacency States

Accepted Answer

1. **Down:** No BPDUs/Hellos received.
2. **Init:** Hello received, but local Router ID not seen in Hello packet.
3. **2-Way:** Bi-directional communication established; DR/BDR election occurs here on broadcast networks.
4. **ExStart:** Master/Slave relationship established for LSDB exchange.
5. **Exchange:** Switches Database Description (DBD) packets.
6. **Loading:** Requests missing link-state information using LSRs and LSUs.
7. **Full:** Link-state databases are fully synchronized.

Question 14

Standard vs. Extended Access Control Lists (ACLs)

Accepted Answer

- **Standard ACLs:** Filter traffic based **only on Source IP address**. Number ranges: `1-99` and `1300-1399`. Place as **close to the destination** as possible.
- **Extended ACLs:** Filter traffic based on **Source IP, Destination IP, Protocol (TCP/UDP/ICMP), and Port numbers**. Number ranges: `100-199` and `2000-2699`. Place as **close to the source** as possible.

Question 15

Implicit Deny in ACLs

Accepted Answer

Every ACL ends with an invisible, automatic statement: **`deny ip any any`** (or `deny any` for standard ACLs). If a packet does not match any explicit `permit` rule in the ACL, it is automatically dropped.

Question 16

Wildcard Mask Calculation

Accepted Answer

Wildcard masks inverse subnet masks (`0` = must match exact bit, `255` = ignore bit value).

To derive a wildcard mask, subtract the subnet mask from `255.255.255.255`:
$$	ext{Wildcard} = (255.255.255.255) - (255.255.255.0) = 0.0.0.255$$

Question 17

Static NAT vs. Dynamic NAT vs. PAT

Accepted Answer

- **Static NAT:** One-to-one mapping of a private IP address to a fixed public IP address.
- **Dynamic NAT:** Many-to-many mapping using a pool of public IP addresses on a first-come, first-served basis.
- **PAT (Port Address Translation / NAT Overload):** Many-to-one mapping that translates multiple private IPs to a single public IP using unique **TCP/UDP source port numbers**.

Question 18

Port Address Translation (PAT) Configuration Syntax

Accepted Answer

Configuring NAT Overload (PAT) using an outside interface IP:
```text
ip nat inside source list 1 interface GigabitEthernet0/0/0 overload
access-list 1 permit 192.168.1.0 0.0.0.255

interface g0/0/0
 ip nat outside
interface g0/0/1
 ip nat inside
```

Question 19

Switch Port Security Violation Modes

Accepted Answer

- **Protect:** Drops frames with unauthorized MAC addresses; does *not* log or increment violation counter.
- **Restrict:** Drops violating frames; **logs warning and increments counter**.
- **Shutdown (Default):** Immediately puts interface into **`err-disabled`** state, turns off port LED, and logs event.

Question 20

Floating Static Route

Accepted Answer

A static route configured with a **higher Administrative Distance** than the primary routing protocol (e.g., AD = 120 vs OSPF = 110). It remains inactive in the routing table until the primary path fails, providing backup redundancy.

Command: `ip route 0.0.0.0 0.0.0.0 203.0.113.1 120`

Cisco CCNA (200-301) - Routing & Switching Essentials

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