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Computer Networks

LAN Bridges and Switches

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Where are we?

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Recall

LANs have physical distance limitations

Performance suffers when LAN utilization increases

Separate LANs may eventually want to connect to each other

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Motivation

Users require arbitrary distance connectionsExample: 2 computers across a corporate campus

are part of one workgroup

May not want to forward all transmissions to all workgroups for performance or security reasons

May want to avoid a single point of failure (redundancy/reliability)

The books: Interconnections - Radia Perlman, The Switch Book - Rich Seifert

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LAN Bridges/Switches

A hardware device with minimal softwareConnects 2 or more LANs together

Generally same LAN types are connectedForwards frames between connected LANsDoes not forward collisions, noise, beacons,

etc.Examines data link layer informationAllows each LAN to operate independently

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Bridge/Switch Operation

Listen to all LANs in promiscuous mode

Only move frames between LANs if necessary

Only act on layer 1/2 information

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Connections

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Transparent Bridging Illustrated

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Transparent Bridging Rules

1. Watch all frames on each LAN2. For each frame, store the source address in a cache along

with the associated LAN the frame arrived on (bridge table)3. For each frame, the cache is queried for the destination

addressa. If found, the frame is forwarded to the LAN associated with the

address, unless its the LAN the frame arrived on (filtered)b. If not found, the frame is forwarded to all LAN interfaces

except the one on which the frame arrive (flooding)

Transparent bridges make all the forwarding decisions, end stations don’t even know the bridge is there!

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Bridge Address Table

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Bridging between dissimiliar LANs

Access methods Ethernet, Token Ring, FDDI

Frame formats New fields, non existant fields

MTU FCS Bit ordering

Don't bother doing this, use routers!

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Will This Work?

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Introducing Spanning Tree

Allow a path between every LAN without causing loops (loop-free environment)

Bridges communicate with special configuration messages (BPDUs)

Standardized by IEEE 802.1D

Note: redundant paths are good, active redundant paths are bad (they cause loops)

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Spanning Tree Requirements

Each bridge is assigned a unique identifierConsists of the MAC address and a

priority

A group address for bridges on a LANA unique port identifier for all ports on

all bridges

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Spanning Tree Concepts: Root Bridge

The bridge with the lowest bridge ID value is elected the root bridge

One root bridge chosen among all bridges

Every other bridge calculates a path to this root bridge

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Spanning Tree Concepts:Path Cost

Associated with each port on each bridge

The cost associated with transmission onto the LAN connected to the port

Can be manually or automatically assigned

Can be used to alter the path to the root bridge

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Spanning Tree Concepts:Root Port

The port on each bridge that is on the path towards the root bridge

The root port is part of the lowest cost path towards the root bridge

If port costs are equal on a bridge, the port with the lowest ID becomes root port

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Spanning Tree Concepts:Root Path Cost

The minimum cost path to the root bridge

The cost starts at the root bridgeEach bridge computes root path cost

independently based on their view of the network

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Spanning Tree Concepts: Designated Bridge

Only one bridge on a LAN at one time is chosen the designated bridge

This bridge provides the minimum cost path to the root bridge for the LAN

Only the designated bridge passes frames towards the root bridge

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Spanning Tree Concepts:Illustrated

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Spanning Tree Concepts:Illustrated [continued]

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Spanning Tree Algorithm:An Overview

1. Determine the root bridge among all bridges

2. Each bridge determines its root portThe port in the direction of the root bridge

3. Determine the designated port on each LANThe port which accepts frames to forward

towards the root bridge

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Spanning Tree Algorithm:Selecting Root Bridge

1. Initially, each bridge considers itself to be the root bridge

2. Bridges send BDPU frames to its attached LANsa. The bridge and port ID of the sending bridgeb. The bridge and port ID of the bridge the sending bridge

considers rootc. The root path cost for the sending bridge

3. Best one wins (lowest ID/cost/priority)

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Spanning Tree Algorithm:Selecting Root Ports

Each bridge selects one of its ports which has the minimal cost to the root bridge

In case of a tie, the lowest uplink (transmitter) bridge ID is used

In case of another tie, the lowest port ID is used

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Spanning Tree Algorithm:Select Designated Bridges

1. Initially, each bridge considers itself to be the designated bridge

2. Bridges send BDPU frames to its attached LANsa. The bridge and port ID of the sending bridgeb. The bridge and port ID of the bridge the sending bridge

considers rootc. The root path cost for the sending bridge

3. Best one wins (lowest ID/cost/priority)

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Forwarding/Blocking State

Root and designated ports will forward frames to and from their attached LANs

All other ports are in the blocking state

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Configuration Messages

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Bridge Encapsulation

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Source Route Bridging

Used in token ring environmentsAlternative to transparent bridgingBridge loops can existDefined by IBM and standardized by

IEEE 802.5Intelligence moves from bridges to

end stations

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Source Routing Bridging

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Source Route Destinations

Null - destination on the same LANNon-broadcast - includes a route to

destinationAll routes broadcast - flooded to each

LAN, bridges record route along the way

Single route broadcast - only one frame per LAN, spanning tree used

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Route Discovery

Transmit "all-route” broadcast to destinationDestination sends non-broadcast response to the

first frame received (using that route)

Transmit "single-route” broadcast to destinationDestination sends back an all-route broadcast

responseSender picks the first response received from

destination

Routes can also be manually configured on stations

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Source Route Discovery:Illustrated

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Routing Information Field

If bit 0 of byte 0 in the source address is set to 1, then this frame is a source routed frame

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Bridge Filters

Useful for controlling LAN trafficExamines data link layer informationExamples

Do not forward frames from MAC address X

Do not forward Ethernet frames of type XDo not forward broadcast frames from XLimit source route hops to 6

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Switches

Physically similar to hubsLogically similar to bridgesTakes advantage of improvements in ASIC

technologyPermits full duplex operation Quickly replacing hub/bridge technologyThe name switch is a marketing gimmick

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Inside a Switch

Conceptual operationOne LAN segment per hostBridge interconnects each host/segment

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Switches: Final Notes

Store and ForwardCut-throughMixing interfacesNetwork Management Issues

Port MirroringSecurity

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Virtual LANs - An Introduction

Defines a broadcast domain on switches

Only difference from LAN is the packaging

To move between VLANs, you need a route (layer 3 device)

Why have separate VLANs?

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VLANs Illustrated