Cisco Certification: Switching Terms

Terminologies

Layer 2 Switching & VLAN

TERM

DEFINITION

Address Learning

Layer 2 switches and bridges remember the source hardware address of each frame received on an interface, and they

enter this information into a MAC database called a forward/filter table.

Forward / filter decisions

When a frame is received on an interface, the switch looks at the destination hardware address and finds the exit

interface in the MAC database. The frame is only forwarded out the specified destination port.

Loop Avoidance

If multiple connections between switches are created for redundancy purposes, network loops can occur. Spanning

Tree Protocol (STP) is used to stop network loops while still permitting redundancy.

Spanning Tree Protocol

STP is a bridge protocol that uses the STA to find redundant links dynamically and create a spanning-tree topology database. Bridges exchange BPDU messages with other bridges to detect loops, and then remove them by shutting

down selected bridge interfaces.

Root Bridge

The root bridge is the bridge with the best bridge ID. With STP, the key is for all the switches in the network to elect

a root bridge that becomes the focal point in the network. All other decisions in the network—such as which port is to

be blocked and which port is to be put in forwarding mode—are made from the perspective of this root bridge.

BPDU

All the switches exchange information to use in the selection of the root switch, as well as in subsequent configuration

of the network. Each switch compares the parameters in the Bridge Protocol Data Unit (BPDU) that they send to one neighbor with the one that they receive from another neighbor.

Bridge ID

The bridge ID is how STP keeps track of all the switches in the network. It is determined by a combination of the

bridge priority (32,768 by default on all Cisco switches) and the base MAC address. The bridge with the lowest

bridge ID becomes the root bridge in the network.

Terminologies

Layer 2 Switching & VLAN

TERM

DEFINITION

Nonroot Bridge

These are all bridges that are not the root bridge. Nonroot bridges exchange BPDUs with all bridges and update the

STP topology database on all switches, preventing loops and providing a measure of defense against link failures.

Root Port

The root port is always the link directly connected to the root bridge, or the shortest path to the root bridge. If more

than one link connects to the root bridge, then a port cost is determined by checking the bandwidth of each link. The lowest-cost port becomes the root port. If multiple links have the same cost, Since multiple links can be from the

same device, the lowest port number will be used.

Designated Port

A designated port is one that has been determined as having the best (lowest) cost. A designated port will be marked

as a forwarding port.

Port Cost

Port cost determines when multiple links are used between two switches and none are root ports. The cost of a link is determined by the bandwidth of a link.

Nondesignated Port

A nondesignated port is one with a higher cost than the designated port. Nondesignated ports are put in blocking

mode—they are not forwarding ports.

Forwarding Port

A forwarding port forwards frames.

Blocked Port

A blocked port is the port that will not forward frames, in order to prevent loops. However, a blocked port will always listen to frames.

Terminologies

Layer 2 Switching & VLAN

TERM

DEFINITION

Blocking

A blocked port won’t forward frames; it just listens to BPDUs. The purpose of the blocking state is to prevent the use

of looped paths. All ports are in blocking state by default when the switch is powered up.

Listening

The port listens to BPDUs to make sure no loops occur on the network before passing data frames. A port in listening

state prepares to forward data frames without populating the MAC address table.

Learning

The switch port listens to BPDUs and learns all the paths in the switched network. A port in learning state populates

the MAC address table but doesn’t forward data frames.

Forwarding

The port sends and receives all data frames on the bridged port. If the port is still a designated or root port at the end

of the learning state, it enters this state.

Disabled

A port in the disabled state (administratively) does not participate in the frame forwarding or STP. A port in the

disabled state is virtually nonoperational.

Convergence

Convergence occurs when all ports on bridges and switches have transitioned to either the forwarding

or blocking modes.

Cut Through (Fast Forward)

When in this mode, the switch only waits for the destination hardware address to be received before it looks up the destination address in the MAC filter table. Cisco sometimes calls this the FastForward method.

Fragment Free (modified cut-through)

This is the default mode for the Catalyst 1900 switch, and it’s sometimes referred to as modified cut-through. In

Fragment Free mode, the switch checks the first 64 bytes of a frame before forwarding it for fragmentation, thus

guarding against forwarding runts, which are caused by collisions.

Terminologies

Layer 2 Switching & VLAN

TERM

DEFINITION

Store and Forward

In this mode, the complete data frame is received on the switch’s buffer, a CRC is run, and, if the CRC passes, the

switch looks up the destination address in the MAC filter table.

Access Links

This type of link is only part of one VLAN, and it’s referred to as the native VLAN of the port. Any device attached to

An access link is unaware of a VLAN membership— the device just assumes it’s part of a broadcast domain, but it

has no understanding of the physical network.

Trunk Links

Trunks can carry multiple VLANs and originally gained their name after the telephone system trunks that carry

multiple telephone conversations. A trunk link is a 100- or 1000Mbps point-to-point link between two switches,

between a switch and router, or between a switch and server. These carry the traffic of multiple VLANs—from 1 to

1005 at a time.

Inter-Switch Link (ISL)

This is proprietary to Cisco switches, and it’s used for Fast Ethernet and Gigabit Ethernet links only. ISL routing

can be used on a switch port, router interfaces, and server interface cards to trunk a server. This is a very good

approach if you’re creating functional VLANs and you don’t want to break the 80/20 rule.

IEEE 802.1Q

Created by the IEEE as a standard method of frame tagging, it actually inserts a field into the frame to identify the

VLAN. If you’re trunking between a Cisco switched link and a different brand of switch, you have to use 802.1Q for

the trunk to work.

VLAN Trunking Protocol (VTP)

The basic goals of VLAN Trunking Protocol (VTP) are to manage all configured VLANs across a switched

internetwork and to maintain consistency throughout that network. VTP allows an administrator to add, delete, and

rename VLANs—information that is then propagated to all other switches in the VTP domain.

Terminologies

Layer 2 Switching & VLAN

TERM

DEFINITION

Server

This is the default for all Catalyst switches. You need at least one server in your VTP domain to propagate VLAN information throughout the domain. The switch must be in server mode to be able to create, add, or delete VLANs in

a VTP domain. Changing VTP information must also be done in server mode, and any change made to a switch in

server mode will be advertised to the entire VTP domain.

Client

In client mode, switches receive information from VTP servers, and they also send and receive updates. But they

can’t make any changes. Plus, none of the ports on a client switch can be added to a new VLAN before the VTP

server notifies the client switch of the new VLAN.

VTP Pruning

VTP pruning only sends broadcasts to trunk links that truly must have the information. This provides a way for you to preserve bandwidth by configuring it to reduce the amount of broadcasts, multicasts, and unicast packets.

Cisco Discovery Protocol (CDP)

A proprietary protocol designed by Cisco to help administrators collect information about both locally attached and

remote devices. By using CDP, you can gather hardware and protocol information about neighbor devices.

CDP Timer

How often CDP packets are transmitted to all active interfaces

CDP Holdtime

The amount of time that the device will hold packets received from neighbor devices