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Chapter 11: Switching and VLANsInstructor:

Chapter 11 Objectives• The Following CompTIA Network+ Exam

Objectives Are Covered in This Chapter:

1.4 Explain the purpose and properties of routing and switching.

– Spanning Tree Protocol– VLAN (802.1q)– Port mirroring

2.1 Given a scenario, install and configure routers and switches.

– VLAN (trunking)– Interface configurations– MAC filtering– PoE– VTP configuration– QoS– Port mirroring

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Switching and Virtual LANs(VLANs)

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• Switching breaks up large collision domains into smaller ones and that a collision domain is a network segment with two or more devices sharing the same bandwidth.

• Switched design is properly implemented, it will result in a clean, cost-effective, and resilient internetwork.

The first LAN

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The first LAN had each hub placed into a router port.

Switched network design

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• A typical, contemporary, and complete switched network design/implementation would look something like this.

• There is a router implemented.

• The router creates and handles logical segmentation.

• What makes Layer 2 switching so efficient is that no modification to the data packet takes place.

Switching Services

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• Layer 2 switching provides the following benefits:– Hardware-based bridging (ASIC)

– Wire speed

– Low latency

– Low cost

Switch Functions at Layer 2

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• There are three distinct functions of Layer 2 switching– Address learning– Forward/filter decisions– Loop avoidance

Address Learning

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• Layer 2 switches and bridges are capable of address learning; they remember the source hardware address of each frame received on an interface and enter this information into a MAC database known as a forward/filter table.

• Initially there is no address information in the table.

Forwarding/Filter Table

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When the hosts start communicating, the switch places the source hardware address of each frame in the table along with the corresponding port

1.Host A sends a frame to Host B. Host A’s MAC address is 0000.8c01.000A; Host B’s MAC address is 0000.8c01.000B.2.The switch receives the frame on the E0/0 interface and places the source address in the MAC address table.3.Because the destination address is not in the MAC database, the frame is forwarded out all interfaces—except the source port.4.Host B receives the frame and responds to Host A. The switch receives this frame on interface E0/1 and places the source hardware address in the MAC database.5.Host A and Host B can now make a point-to-point connection, and only the two devices will receive the frames. Hosts C and D will not see the frames, nor are their MAC addresses found in the database because they haven’t yet sent a frame to the switch.

Forwarding/Filter Table Evaluation

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• Host A is sending a data frame to Host D.

• What will the switch do when it receives the frame from Host A?

Broadcast Storm

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• Redundant links between switches can be a wise thing to implement because they help prevent complete network failures in the event that one link stops working.

• There is a drawback; frames can be flooded down all redundant links simultaneously creating network loops.

Multiple Frame Copies

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The MAC address filter table could be totally confused about the device’s location because the switch can receive the frame from more than one link.

The switch could get so caught up in constantly updating the MAC filter table with source hardware-address locations that it may fail to forward a frame. This is called thrashing the MAC table.

Spanning Tree Protocol (STP)

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STP’s main task is to stop network loops from occurring on your Layer 2 network (bridges or switches).

It achieves this feat by vigilantly monitoring the network to find all links and making sure that no loops occur by shutting down any redundant ones.

STP uses the spanning-tree algorithm (STA) to first create a topology database and then search out and destroy redundant links.

With STP running, frames will be forwarded only on the premium, STP-picked links.

Switches transmit Bridge Protocol Data Units (BPDUs) out all ports so that all links between switches can be found.

Switching Loops

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• Switched network with a redundant topology (switching loops) without some type of Layer 2 mechanism to stop network loops will fail.

Spanning-Tree Port States

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The ports on a bridge or switch running STP can transition through five different states:

• Blocking– A blocked port won’t forward frames; it just listens to BPDUs and will

drop all other frames.

• Listening– The port listens to BPDUs to make sure no loops occur on the network

before passing data frames without populating the MAC address table.

• Learning– 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 the forwarding state.

• Disabled– A port in the disabled state (administratively) does not

participate in the frame forwarding or STP.

Switching Design

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• There are ways to design really great ways to implement your switched network so that STP converges efficiently.

Rapid Spanning Tree Protocol 802.1w

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The 802.1w is defined in these different port states (compared to 802.1d):

• Disabled = Discarding

• Blocking = Discarding

• Listening = Discarding

• Learning = Learning

• Forwarding = Forwarding

VLAN Basics

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• Layer 2 switched networks are typically designed as flat networks.

• Every broadcast packet transmitted is seen by every device on the network regardless of whether the device needs to receive that data or not.

• VLANs will let us control our broadcast domains.

Benefits of a Switched Network

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• Host A is sending a frame with Host D as its destination.

• The frame is only forwarded out of the port where Host D is located. This is a huge improvement over hubbed networks.

Physical LANs Connected to a Router

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• Each network is attached with a hub port to the router (each segment also has its own logical network number.

• Each department has its own LAN, so if we needed to add new users we would just plug them into the appropriate LAN.

Switches Removing the Physical Boundary

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• Switches remove the physical boundary to solve the problem of hubs.

• Six VLANs (numbered 2 through 7) are used to create a broadcast domain for each department.

• Each switch port is then administratively assigned a VLAN membership, depending on the host and which broadcast domain it’s placed in.

Quality of Service

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QoS methods focus on one of five problems that can affect data as it traverses network cable:

• Delay• Dropped packets• Error• Jitter• Out-of-order delivery

VLAN Memberships

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• Static VLANs– Creating static VLANs is the most

common way to create a VLAN, and one of the reasons for that is because static VLANs are the most secure

• Dynamic VLANs– On the other hand, a dynamic VLAN

determines a host’s VLAN assignment automatically. Using intelligent management software, you can base VLAN assignments on hardware (MAC) addresses, protocols, or even applications that work to create dynamic VLANs.

Access and Trunk Links

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• Different link types are used in a switched network. – Access Links– Trunk Links

• All hosts connected to the switches can communicate to all ports in their VLAN because of the trunk link between them.

• If we used an access link between the switches, this would allow only one VLAN to communicate between switches.

VLAN Identification Methods

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• Inter-Switch Link (ISL)– Proprietary to Cisco switches, and it’s used for

Fast Ethernet and Gigabit Ethernet links only. ISL routing is pretty versatile and can be used on a switch port, on router interfaces, and on server interface cards to trunk a server

• IEEE 802.1Q– Created by the IEEE as a standard method of

frame tagging, IEEE 802.1Q 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’ve got to use 802.1Q for the trunk to work.

VLAN Trunking Protocol

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• Consistent VLAN configuration across all switches in the network

• Accurate tracking and monitoring of VLANs

• Dynamic reporting of added VLANs to all switches in the VTP domain

• Adding VLANs using Plug and Play

VTP Modes of Operation

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• Server

• Client

• Transparent

Configuring VTP

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Switch#config t

Switch#(config)#vtp mode server

Device mode already VTP SERVER.

Switch(config)#vtp domain Lammle

(ChangesVTP domain name from null to Lammle)

Switch(config)#vtp password todd

(Sets device VLAN database password to todd)

Advanced Features of Switches

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• Switches really expand our flexibility when designing our networks. There are features which enhance the functionality or the switch networks.

– Power over Ethernet (PoE)– Port Mirroring/Spanning

Power over Ethernet (PoE)

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• Switches can provide power to end devices by injecting power into the Ethernet cabling.

• If PoE switches are not implemented, power can be injected into the cabling outside the switch.

Port Mirroring/Spanning

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• Port mirroring, also called Switch Port Analyzer (SPAN), allows you to sniff traffic on a network when using a switch.

• A problem with this arises when you need to sniff traffic on a switched network. The sniffer cannot see data going from Host A to Host B.

• To solve this little snag, you could temporarily place a hub between Host A and Host B.

Port Mirroring

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• The port-mirroring option allows you to place a port in spanning mode so that every frame from Host A is captured by both Host B and the sniffer.

Summary

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• Summary

• Exam Essentials Section

• Written Labs

• Review Questions