Switch Port Dynamic Modes

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Questions and Answers

What is the default dynamic mode for departmental-level or wiring closet-level switches?

auto (correct)
desirable
access
trunk

What happens when a switch port in auto mode negotiates with a neighboring interface?

It refuses to form a trunk link.
It changes its native VLAN.
It actively negotiates to operate in trunk mode.
It forms a trunk link only if negotiations are initiated by the neighboring interface. (correct)

What is the purpose of VLAN 0?

To configure a unique voice virtual VLAN.
To separate voice traffic and data traffic.
To change the native VLAN.
To indicate to an upstream switch that it is sending frames that have a configured 802.1p priority. (correct)

What is the native VLAN on a Cisco switch by default?

VLAN 1 (B)

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What is the range of VLAN IDs available on extended IOS images?

1 through 4094 (D)

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What happens when a switch port in desirable mode negotiates with a neighboring interface?

It actively negotiates to operate in trunk mode. (B)

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What is the purpose of the command 'switchport trunk native vlan vlan-id'?

To change the native VLAN. (D)

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What is VLAN 4094 used for?

It is not used for DTP frames unless it has been configured as the native VLAN. (B)

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What is the primary function of the CAM table in a switch?

To discover the relationship between Layer 2 addresses and physical ports (B)

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What is the purpose of the FIB table?

To contain all the prefixes from the IP routing table (B)

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Which table is used by a switch to make forwarding decisions based on destination MAC addresses?

CAM table (A)

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What is the relationship between the FIB and the IP routing table?

The FIB is synchronized with the IP routing table (A)

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What is the purpose of the adjacency table?

To store information about adjacent devices on a network (C)

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What is Cisco Express Forwarding (CEF)?

A hardware-based switching method (D)

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What are the two main components of Cisco Express Forwarding (CEF)?

The FIB and the adjacency table (B)

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Which command is used to configure DAI on VLANs 2 through 4?

ip arp inspection vlan 2,3,4 (A), ip arp inspection vlan 2-4 (D)

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What is the purpose of the ARP table?

To resolve IP addresses to MAC addresses (B)

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What happens by default to a port when DAI is enabled on that port?

It becomes an untrusted port (D)

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What is the purpose of the ip arp inspection trust command?

To override the default configuration for a given port (A)

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Which VLAN is used by DTP to negotiate a trunk link when 802.1Q encapsulation is configured on the interface?

The native VLAN (B)

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What happens if the native VLAN is not configured identically on both ends of a link?

A trunk is not dynamically formed (A)

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What is the default behavior of interfaces on a Cisco switch?

They use DTP to automatically negotiate whether an interface should be an IEEE 802.1Q trunk port or an access port (C)

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What is the effect of changing the native VLAN on a trunk link?

It can cause unexpected consequences (A)

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What is the purpose of DAI?

To inspect traffic sent from each host (D)

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Study Notes

Dynamic Modes of Operation for a Switch Port

There are two dynamic modes of operation for a switch port: auto and desirable.
In auto mode, the switch port operates in access mode unless the neighboring interface actively negotiates to operate as a trunk.
In desirable mode, the switch port actively negotiates to operate in trunk mode and will form a trunk link with a neighboring port that is configured to operate in trunk, desirable, or auto mode.

VLAN Configuration

VLAN 1 is the default native VLAN on a Cisco switch, but it can be changed using the switchport trunk native vlan vlan-id command.
VLAN 0 is a special VLAN used by IP phones to indicate to an upstream switch that it is sending frames that have a configured 802.1p priority.
VLAN IDs can be a value from 1 through 1005 or from 1006 through 4094, depending on the IOS image.

Dynamic Trunking Protocol (DTP)

DTP uses the native VLAN to negotiate a trunk link when IEEE 802.1Q encapsulation is configured on the interface.
Changing the native VLAN can have unexpected consequences, such as preventing a trunk from dynamically forming.

Switch Forwarding Decisions

A switch uses the Content Addressable Memory (CAM) table to discover the relationship between the Layer 2 address of a device and the physical port used to reach the device.
The CAM table is used to make forwarding decisions based on the destination MAC address contained in a frame's header.