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Chapter 16

Connecting LANs,Backbone Networks,

and Virtual LANs

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Connecting devices A repeater connects segments of a LAN. A repeater forwards every frame; it has no

filtering capability. A repeater is a regenerator, not an amplifier.

Amplifier cannot discriminate between the intended signal and noise; it amplifies equally everything fed into it.

A repeater does not actually connect two LANs; it connects two segments of the same LAN.

A repeater is not a device that can connect two LANs of different protocols.

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Repeater

Repeater extends length and thus overcomes the length restriction of 10Base5

An amplifier cannot discriminate between the intended signal and noise; it amplifies equally everything fed into it. A repeater does not amplify the signal; it regenerates the signal.

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Function of a Repeater

When a repeater receives a weakened or corrupted signal, it creates a copy, bit for bit, at the original strength.

Repeater must be places so that a signal reaches it before any noise changes the meaning of any of its bits.

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Hubs

Hub is actually a multi-port repeater. Normally used to create connections between

stations in a physical star topology. Removes the length restriction of 10Base-T

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Operates in both the physical and data link layers

As physical layer device, it regenerates the signal it receives.

As a data link layer device, the bridge can check the MAC (source and destination) address contained in the frame.

A bridge has a table used in filtering decisions. Difference in functionality between a bridge and

a repeater? A bridge has filtering capability. It can check the destination address of a frame and

decide if the frame should be forwarded or dropped. If the frame is to be forwarded, the decision must

specify the port. A bridge has a table that maps addresses to ports.

A bridge does not change the physical (MAC) addresses in a frame.

Bridge

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Bridge

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Learning Bridge Transparent bridge is a bridge in which the stations

are completely unaware of the bridge’s existence. Frames must be forwarded from one station to another. Forwarding table is automatically made by learning frame

movements in the network. Otherwise, administrator should do static entries in the table.

Loops in the system must be prevented.

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Loop Problem

A frame sent out by bridge 1 is received by bridge 2, which has no idea about the destination.

Bridge 2’s copy is received by bridge 1 and sent out due to lack of information about destination.

Each frame is handled separately by using CSMA/CD.

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Prior to Spanning Tree Application

A bridge with smallest ID is selected as root bridge Mark one port of each bridge (except for the root bridge) as

the root port. A root port is the port with the least-cost path from the bridge to the root bridge.

Choose a designated bridge for each LAN. A designated bridge has the least-cost path between the LAN and the root bridge. Make the corresponding port the designated port. If two bridges have the same least-cost value, choose the one with the smaller ID.

Make the root port and designated port as forwarding ports, the others are blocking ports.

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Applying Spanning Tree

B1 has the least ID and so it is the root bridge. Root ports are marked with one star. Designated bridges have an arrow pointing to them from the

corresponding LAN. Designated ports are marked by two stars.

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Forwarding Ports and Blocking Ports

Blocking port is shown with a broken line. No loop exists.

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Source Routing Bridges

Source Routing bridge A sending station defines the bridges

that the frame must visit. The addresses of these bridges are

included in the frame. These addresses are obtained through

special exchange frames Source routers are used with Token Ring

LANs.

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Bridges connecting Different LANs There are many issues: (Ethernet, ATM, Wireless)

Frame Format: Each LAN has its own frame format.

Maximum Data Size: Bridges must discard any frames too large for its system.

Data Rate: The bridge must buffer the frame to compensate for the difference.

Bit Order: Each LAN has its own strategy in sending the bits; MSB or LSB.

Security: Wireless LANs implement security measures in the data link layer, however, Ethernet don’t. Bridge must encrypt a data received from wireless before forwarding to Ethernet LAN.

Multimedia Support: some LANs support multimedia and QoS needed for this type of communication; others do not.

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Bus Backbone

A backbone network allows several LANs to be connected. In a backbone network, no station is directly connected to the

backbone; the stations are part of a LAN, and the backbone connects the LANs.

In a bus backbone, the topology of the backbone is a bus. Ex.: A bus backbone is one that connects single or multiple

floor buildings on a campus. Only needed traffic is passed over from one bridge to another.

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Star Backbone Collapsed or switched backbone In a star backbone, the topology of the backbone is a star; the

backbone is just one switch Used as a distribution backbone inside a building. We often find a rack or chassis in the basement where the backbone

switch and all hubs or switches are installed.

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Connecting Remote LANs This type of backbone network is useful when

a company has several offices with LANs and needs to connect them.

The connection can be done through bridges, sometimes called remote bridges.

Point-to-point network in this case is considered a LAN without stations.

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A switch connecting three LANs LAN setup is physical and geographic.

Virtual LAN as a local area network configured by software, not by physical wiring.

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A switch using VLAN software VLAN technology is to divide a LAN into logical, instead

of physical, segments. A LAN can be divided into several logical LANs called

VLANs. Each VLAN is a work-group in the organization. If a person moves from one group to another, there is no need to change the physical configuration.

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Two switches in a backbone using VLAN software VLANs create broadcast domains All members belonging to a VLAN can receive broadcast

messages sent to that particular VLAN. Membership in VLAN can be based on physical port

numbers or MAC addresses or IP addresses or Multicast IP addresses or in combinations.

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Configuration of VLANs Manual configuration Automatic configuration Semiautomatic configuration

Communication between switches Using frame tagging (When a frame is travelling

between switches, an extra header is added to the MAC frame to define the destination VLAN)

802.1Q standard. Uses Time-Division Multiplexing

Advantages of VLANs Cost and Time reduction in configuring. Creating virtual workgroups Security: Any broadcast message sent by a group

members is received only by the specific group members.