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 Signup and view all the answers

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

1 through 4094 Signup and view all the answers

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

It actively negotiates to operate in trunk mode. Signup and view all the answers

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

To change the native VLAN. Signup and view all the answers

What is VLAN 4094 used for?

It is not used for DTP frames unless it has been configured as the native VLAN. Signup and view all the answers

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

To discover the relationship between Layer 2 addresses and physical ports Signup and view all the answers

What is the purpose of the FIB table?

To contain all the prefixes from the IP routing table Signup and view all the answers

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

CAM table Signup and view all the answers

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

The FIB is synchronized with the IP routing table Signup and view all the answers

What is the purpose of the adjacency table?

To store information about adjacent devices on a network Signup and view all the answers

What is Cisco Express Forwarding (CEF)?

A hardware-based switching method Signup and view all the answers

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

The FIB and the adjacency table Signup and view all the answers

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

ip arp inspection vlan 2,3,4 Signup and view all the answers

What is the purpose of the ARP table?

To resolve IP addresses to MAC addresses Signup and view all the answers

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

It becomes an untrusted port Signup and view all the answers

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

To override the default configuration for a given port Signup and view all the answers

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

The native VLAN Signup and view all the answers

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

A trunk is not dynamically formed Signup and view all the answers

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 Signup and view all the answers

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

It can cause unexpected consequences Signup and view all the answers

What is the purpose of DAI?

To inspect traffic sent from each host Signup and view all the answers

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.