Computer Networks
LAN Bridges and Switches
Autumn 2000
John Kristoff
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Where are we?
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Recall
Ñ LANs have physical distance limitations
Ñ Performance suffers when LAN utilization
increases
Ñ Separate LANs may eventually want to
connect to each other
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Motivation
Ñ
Users require arbitrary distance connections
Ñ Example: 2 computers across a corporate campus are
part of one workgroup
Ñ
Ñ
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
Ñ
Ñ
Ñ
Ñ
Ñ
Ñ
A hardware device with minimal software
Connects 2 or more LANs together
ÑGenerally same LAN types are connected
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
Ñ Listen to all LANs in promiscuous mode
Ñ Only move frames between LANs if
necessary
Ñ 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
Ñ 1. Watch all frames on each LAN
Ñ 2. For each frame, store the source address in a cache along with
the associated LAN the frame arrived on (bridge table)
Ñ 3. For each frame, the cache is queried for the destination address
Ñ a. If found, the frame is forwarded to the LAN associated with the
address, unless its the LAN the frame arrived on (filtered)
Ñ b. If not found, the frame is forwarded to all LAN interfaces except the
one on which the frame arrive (flooding)
Ñ 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
Ñ Access methods
Ñ Ethernet, Token Ring, FDDI
Ñ Frame formats
Ñ New fields, non existant fields
Ñ MTU
Ñ FCS
Ñ Bit ordering
Don't bother doing this, use routers!
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Will This Work?
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Introducing Spanning Tree
Ñ Allow a path between every LAN without
causing loops (loop-free environment)
Ñ Bridges communicate with special
configuration messages (BPDUs)
Ñ 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
Ñ Each bridge is assigned a unique identifier
ÑConsists of the MAC address and a priority
Ñ A group address for bridges on a LAN
Ñ A unique port identifier for all ports on all
bridges
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Spanning Tree Concepts:
Root Bridge
Ñ The bridge with the lowest bridge ID
value is elected the root bridge
Ñ One root bridge chosen among all bridges
Ñ Every other bridge calculates a path to
this root bridge
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Spanning Tree Concepts:
Path Cost
Ñ Associated with each port on each bridge
Ñ The cost associated with transmission
onto the LAN connected to the port
Ñ Can be manually or automatically
assigned
Ñ Can be used to alter the path to the root
bridge
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Spanning Tree Concepts:
Root Port
Ñ The port on each bridge that is on the
path towards the root bridge
Ñ The root port is part of the lowest cost
path towards the root bridge
Ñ 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
Ñ The minimum cost path to the root bridge
Ñ The cost starts at the root bridge
Ñ Each bridge computes root path cost
independently based on their view of the
network
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Spanning Tree Concepts:
Designated Bridge
Ñ Only one bridge on a LAN at one time is
chosen the designated bridge
Ñ This bridge provides the minimum cost
path to the root bridge for the LAN
Ñ 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
Ñ
Ñ
1. Determine the root bridge among all bridges
2. Each bridge determines its root port
Ñ The port in the direction of the root bridge
Ñ
3. Determine the designated port on each LAN
Ñ The port which accepts frames to forward towards
the root bridge
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Spanning Tree Algorithm:
Selecting Root Bridge
Ñ 1. Initially, each bridge considers itself to
be the root bridge
Ñ 2. Bridges send BDPU frames to its
attached LANs
Ñ a. The bridge and port ID of the sending bridge
Ñ b. The bridge and port ID of the bridge the sending bridge
considers root
Ñ c. The root path cost for the sending bridge
Ñ 3. Best one wins (lowest ID/cost/priority)
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Spanning Tree Algorithm:
Selecting Root Ports
Ñ Each bridge selects one of its ports which
has the minimal cost to the root bridge
Ñ In case of a tie, the lowest uplink
(transmitter) bridge ID is used
Ñ In case of another tie, the lowest port ID
is used
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Spanning Tree Algorithm:
Select Designated Bridges
Ñ 1. Initially, each bridge considers itself to
be the designated bridge
Ñ 2. Bridges send BDPU frames to its
attached LANs
Ñ a. The bridge and port ID of the sending bridge
Ñ b. The bridge and port ID of the bridge the sending bridge
considers root
Ñ c. The root path cost for the sending bridge
Ñ 3. Best one wins (lowest ID/cost/priority)
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Forwarding/Blocking State
Ñ Root and designated ports will forward
frames to and from their attached LANs
Ñ 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
Ñ Used in token ring environments
Ñ Alternative to transparent bridging
Ñ Bridge loops can exist
Ñ Defined by IBM and standardized by IEEE
802.5
Ñ Intelligence moves from bridges to end
stations
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Source Routing Bridging
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Source Route Destinations
Ñ Null - destination on the same LAN
Ñ Non-broadcast - includes a route to
destination
Ñ All routes broadcast - flooded to each
LAN, bridges record route along the way
Ñ Single route broadcast - only one frame
per LAN, spanning tree used
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Route Discovery
Ñ
Transmit "all-route’broadcast to destination
Ñ Destination sends non-broadcast response to the first
frame received (using that route)
Ñ
Transmit "single-route’broadcast to destination
Ñ Destination sends back an all-route broadcast
response
Ñ Sender 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
Ñ 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
Ñ Useful for controlling LAN traffic
Ñ Examines data link layer information
Ñ Examples
ÑDo not forward frames from MAC address X
ÑDo not forward Ethernet frames of type X
ÑDo not forward broadcast frames from X
ÑLimit source route hops to 6
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Switches
Ñ Physically similar to hubs
Ñ Logically similar to bridges
Ñ Takes advantage of improvements in ASIC
technology
Ñ Permits full duplex operation
Ñ Quickly replacing hub/bridge technology
Ñ The name switch is a marketing gimmick
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Inside a Switch
Ñ Conceptual operation
ÑOne LAN segment per host
ÑBridge interconnects each host/segment
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Switches: Final Notes
Ñ Store and Forward
Ñ Cut-through
Ñ Mixing interfaces
Ñ Network Management Issues
ÑPort Mirroring
ÑSecurity
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Virtual LANs - An
Introduction
Ñ Defines a broadcast domain on switches
Ñ Only difference from LAN is the packaging
Ñ To move between VLANs, you need a
route (layer 3 device)
Ñ Why have separate VLANs?
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VLANs Illustrated
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