LAN Bridges and Switches

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LAN Bridges and Switches

• Computer Networks

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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 connections
• Example 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 software
• Connects 2 or more similar LANs together
• Forwards frames between connected LANs
• Does not forward collisions, noise, beacons, etc.
• Examines data link layer information
• Allows 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 LAN
• 2. 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 address
• a. 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 dont even know the
  bridge is there!

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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 identifier
• Consists of the MAC address and a priority
• A group address for bridges on a LAN
• A 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 ConceptsPath 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 ConceptsRoot 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 ConceptsRoot Path Cost

• The minimum cost path to the root bridge
• The cost starts at the root bridge
• Each 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 ConceptsIllustrated
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Spanning Tree ConceptsIllustrated continued
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Spanning Tree AlgorithmAn Overview

• 1. Determine the root bridge among all bridges
• 2. Each bridge determines its root port
• The port in the direction of the root bridge
• 3. Determine the designated port on each LAN
• The port which accepts frames to forward towards
  the root bridge

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Spanning Tree AlgorithmSelecting Root Bridge

• 1. Initially, each bridge considers itself to be
  the root bridge
• 2. Bridges send BDPU frames to its attached LANs
• a. The bridge and port ID of the sending bridge
• b. The bridge and port ID of the bridge the
  sending bridge considers root
• c. The root path cost for the sending bridge
• 3. Best one wins (lowest ID/cost/priority)

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Spanning Tree AlgorithmSelecting 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 AlgorithmSelect Designated Bridges

• 1. Initially, each bridge considers itself to be
  the designated bridge
• 2. Bridges send BDPU frames to its attached LANs
• a. The bridge and port ID of the sending bridge
• b. The bridge and port ID of the bridge the
  sending bridge considers root
• c. 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 environments
• Alternative to transparent bridging
• Bridge loops can exist
• Defined by IBM and standardized by IEEE 802.5
• Intelligence 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 LAN
• Non-broadcast - includes a route to destination
• All 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 destination
• Destination sends non-broadcast response to the
  first frame received (using that route)
• Transmit single-route broadcast to destination
• Destination sends back an all-route broadcast
  response
• Sender picks the first response received from
  destination
• Routes can also be manually configured on stations

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Source Route DiscoveryIllustrated
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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 traffic
• Examines data link layer information
• Examples
• Do not forward frames from MAC address X
• Do not forward Ethernet frames of type X
• Do not forward broadcast frames from X
• Limit source route hops to 6

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Switches

• Physically similar to hubs
• Logically similar to bridges
• Takes advantage of improvements in ASIC
  technology
• Permits full duplex operation
• Quickly replacing hub/bridge technology
• The name switch is a marketing gimmick

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Inside a Switch

• Conceptual operation
• One LAN segment per host
• Bridge interconnects each host/segment

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Switches Final Notes

• Store and Forward
• Cut-through
• Mixing interfaces
• VLANs
• Network Management Issues
• Port Mirroring
• Security

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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