spring 2000john kristoff1 lan bridges and switches computer networks
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Spring 2000 John Kristoff 3
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 softwareConnects 2 or more similar LANs togetherForwards 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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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 don’t even know the bridge is there!
Spring 2000 John Kristoff 11
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
priorityA 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 Algorithm:An 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 Algorithm:Selecting 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 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 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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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 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 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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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 X Do not forward broadcast frames from X Limit 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 operation One LAN segment per host Bridge interconnects each host/segment
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Switches: Final Notes
Store and ForwardCut-throughMixing interfacesVLANsNetwork 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?