EDUSAT SESSION FOR COMPUTER NETWORKSI CS64

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EDUSAT SESSION FOR COMPUTER NETWORKS-I
(CS64) Date 21.03.2006 Session IX Topic LAN
Bridges Faculty Anita Kanavalli MSRIT
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Hub Limitations

• single collision domain results in no increase in
  max throughput
• multi-tier throughput same as single segment
  throughput
• individual LAN restrictions pose limits on number
  of nodes in same collision domain and on total
  allowed geographical coverage
• cannot connect different Ethernet types (e.g.,
  10BaseT and 100baseT)

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Bridges

• A network component connecting LANs together.
• Operates only in the data link layer, thus is can
  handle any network protocol used.
• May be used
• to divide the large expensive and hard to manage
  network into smaller LANs.
• split networks that became loaded over time.
• to handle larger distances.
• to block some traffic leaking outside the network.

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Bridges
Link Layer devices operate on Ethernet frames,
examining frame header and selectively forwarding
frame based on its destination Bridge isolates
collision domains since it buffers frames When
frame is to be forwarded on segment, bridge uses
CSMA/CD to access segment and transmit
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Bridges

• Bridge advantages
• Isolates collision domains resulting in higher
  total max throughput, and does not limit the
  number of nodes nor geographical coverage
• Can connect different type Ethernet since it is a
  store and forward device
• Transparent no need for any change to hosts LAN
  adapters

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Bridges

• bridges filter packets
• same-LAN -segment frames not forwarded onto other
  LAN segments
• forwarding
• how to know which LAN segment on which to forward
  frame?
• looks like a routing problem (more shortly!)

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Bridges

• Reasons for bridges
• Limited number of stations on a LAN segment or
  ring
• Limited distance for executing CSMA / CD
  algorithm or distance one wants a token traveling
  on a ring
• Limited traffic on a single LAN available
  bandwidth must be shared by all stations
• Interconnecting networks
• Networks connected at the physical layer are
  connected by a repeater
• Networks connected at the MAC or link layer are
  connected by bridges

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Bridges

• Networks connected at the network layer are
  connected by routers
• Higher layer interconnection devices that perhaps
  execute additional functions such as protocol
  conversion are often called gateways
• Bridges
• Devices for gluing together LANs so that packets
  can be forwarded from one LAN to the other

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A Bridged LAN
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Interconnection by a Bridge
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Simple ideas for Bridges

• The no frills bridge simply transmit all
  traffic from one LAN segment onto all the other
  segments
• Advantages two stations can be transmitting at
  the same time. Bridge will buffer a packet until
  it can transmit on a LAN
• Disadvantages total bandwidth still that can be
  safely utilized is still the minimum bandwidth of
  each LAN segment
• Keeping a database of all stations on each LAN
  segment
• Manually enter addresses in such a database
• Partition addresses into ranges on each LAN

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Simple ideas for Bridges

• Eg. LAN 1 has 1-50, LAN 2 has 51-100, LAN 3 has
  101-150
• Have the MAC address be hierarchically divided
  into a LAN address and a station address (like
  the IP address)
• None of these solutions are really used
• Better solution the transparent learning bridge
• Learn on which segment a station resides
• Transmit a packet only onto the correct segment

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Example-Back Bone Bridge
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No-Back Bone Bridge

• Not recommended for two reasons
• - single point of failure at Computer Science hub
• - all traffic between EE and SE must path over CS
  segment

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Bridge Filtering

• bridges learn which hosts can be reached through
  which interfaces maintain filtering tables
• when frame received, bridge learns location of
  sender incoming LAN segment
• records sender location in filtering table
• filtering table entry
• (Node LAN Address, Bridge Interface, Time Stamp)
• stale entries in Filtering Table dropped (TTL can
  be 60 minutes)

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Bridge Filtering

• filtering procedure
• if destination is on LAN on which frame was
  received
• then drop the frame
• else lookup filtering table
• if entry found for destination
• then forward the frame on interface indicated
• else flood / forward on all but the
  interface on which
  the frame arrived/

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Transparent Bridge

• Main idea A bridge should easily connect any set
  of LANs together and make the connection
  transparent to the stations.
• No maintenance, software upgrade and routing
  table upload should be necessary.
• The bridge listens to both network at all times.
• Any frame received is buffered.
• Next the bridge should be able to decide if the
  frame was addressed to a station in the same
  network. If not, it should select the proper LAN
  and broadcast the frame there.

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Transparent Bridge

• Backward Learning
• The bridge keeps a table containing hashed
  (address, network) entry pairs.
• The bridge accepts any frame, if the destination
  address is in the table then the frame is
  forwarded to the proper network, otherwise the
  frame is broadcast onto all networks (except the
  one its coming from).
• For each incoming frame the bridge also read the
  source address and updates the hash table by
  inserting the source address and the network id
  into the tables.
• Entries in the table can live for a certain time,
  and if there is no packet traffic from or to that
  an address the entry is removed from the table.

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Transparent Bridge

• Maintain a forwarding database or cache of
  station MAC addresses and the bridge port that
  the stations are on
• Promiscuously listen to packets arriving on any
  port
• For each packet arriving at the bridge
• Store the stations source address and arriving
  port in the cache (if an entry already exists for
  an address update if different)

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Transparent Bridge

• determine if the destination address is in the
  cache
• If entry then forward only on the appropriate
  port unless the port is the same as the arrival
  port
• If no such entry then forward packet on all
  segments except the one the packet was received
  on.
• Age each entry in the cache and delete after an
  appropriate time

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Example-Initial Configuration
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Example-S1 sends a frame to S5
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Example-S3 sends a frame to S2
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Example-S4 sends a frame to S3
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Example-S2 sends a frame to S1
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Spanning Tree Bridge

• for increased reliability, desirable to have
  redundant, alternate paths from source to dest
• with multiple simultaneous paths, cycles result -
  bridges may multiply and forward frame forever
• solution organize bridges in a spanning tree by
  disabling subset of interfaces

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Spanning Tree Bridge
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Spanning Tree Bridge

• As the system grows a complex graph of many
  networks and many bridges appear.
• Frames may loop through networks!
• Bridges communicate to build dynamic spanning
  tree graph, showing the topology of the network.
• Spanning tree graphs avoid loops.

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Spanning Tree Bridge

• First the bridge with the smallest serial number
  becomes the root of the tree.
• Next the tree is constructed. LANs are placed on
  the nodes, and bridges are placed on the
  vertices.
• If a LAN or bridge is no longer present the tree
  is updated.
• All networks are on the tree but to prevent loops
  some of the bridges are left off the graph. This
  makes the graph a tree.

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Spanning Tree Bridge

• Elect a single bridge among all bridges as the
  root bridge. The algorithm will select the root
  bridge as the one with the lowest bridge id.
• Each bridge (except root) determines the least
  cost path (shortest path with respect to some
  metric, say hops) from itself to the root bridge
  through each of its ports. The port with least
  cost is the root port for that bridge. In case
  of ties use the smallest port id.

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Spanning Tree Bridge

• Elect a designated bridge for each LAN from the
  bridges directly connecting to that LAN. The
  designated bride is the one closest to the root
  bridge. In case of ties it is the one with the
  lowest bridge id. The port that connects the
  designated bridge and the LAN is the designated
  port for that LAN.
• Ports in the spanning tree are all root ports and
  designated ports. Other ports are in the
  blocking state.
• Data traffic is forwarded to and received from
  ports in the spanning tree only.

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Sample Topology
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Spanning Topology
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How algorithm works

• Bridges exchange bridge protocol data units
  (BPDUs). These have configuration messages
  consisting of
• Root ID, bridge assumed by sending bridge to be
  the root
• Transmitting bridge ID
• Cost of least cost path to the root of which the
  transmitting bridge is aware
• When a bridge receives a configuration message
  from a neighbor bridge, it compares this with
  what it would transmit over that port. Note that
  it will add the cost to the received message
  before comparison. It saves the best
  configuration message received for each port. If
  the saved configuration is better than what it
  would transmit it stops transmitting BPDUs over
  that port.

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How algorithm works

• All bridges start by transmitting on all ports
• Root id is own id
• Transmitting id is own id
• Cost is 0
• (Port id of port)
• Which is a better message?
• First compare root ID, lower is better
• If tie, next compare costs, lower is better
• If tie, next compare transmitting ID, lower is
  better
• If still tie, port id is tie breaker
• Eventually only the root bridge is transmitting.

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Source Routing Bridges

• CSMA/CD community preferred to use transparent
  bridges due to their simplicity.
• The Token Ring community, however, preferred
  source routing bridges.
• In source routing, the sending station knows
  whether the destination is on the same network.
  If it is not, the sender sets the higher
  destination address bit to 1 and includes the
  exact path to the frame header.

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Source Routing Bridges

• The path is a sequence of alternating bridge and
  LAN addresses (4bits/12bits).
• This requires that each machine know the topology
  and can construct a path to any receiver.
• Instead, the sender first broadcasts a discovery
  frame asking the receiver to signal himself. In
  the return trip bridges record their addresses in
  the frame header and the path is formed.
• Problem Too many frame loose in the network.

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Frame Format for Source Routing
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Frame Format for Source Routing

• The routing information field is inserted only
  if the stations are on different LANs
• if this field is present, then I/G bit in src
  addr field is 1 otherwise it is 0
• The routing control field defines type of
  frame, length of routing information field and
  direction of the route designator field(L to R or
  R to L)

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Route discovery

• First the src stn. Transmits the single route
  broadcast frame on its LAN without the route
  designator field.
• this frame should appear exactly once and hence
  selected bridges form spanning tree
• Once the selected bridge at the first hop
  receives this frame
• inserts an incoming LAN number
• bridge number
• outgoing LAN number in the routing information
  field
• Then forwards on outgoing LAN
• At the other hop when a selected bridge receives
  this frame

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Route discovery

• inserts bridge number and outgoing LAN number
  and forwards on outgoing LAN
• Non selected bridge simply ignore this frame
• Once the receiver gets this frame it broadcasts
  all routes broadcast frame with no route
  designator fields
• This frame generates all possible routes back to
  the src stn
• After collecting all routes the source station
  selects the best route and saves it
• To prevent all routes broad cast frames from
  circulating in the network, the bridge first
  checks whether the outgoing LAN number is already
  recorded, if so it does not forward the frame

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Source Routing Bridge
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Source Routing Bridge

• Assume that B1,B3,B4 and B6 are part of spanning
  tree
• S1 wants to send a frame to S3
• The next slide shows the routes followed by
  single route broadcast frame and all routes
  broadcast frames

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Example
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(No Transcript)
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Example
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Example

• Suppose C sends frame to D and D replies back
  with frame to C
• C sends frame, bridge has no info about D, so
  floods to both LANs
• bridge notes that C is on port 1
• frame ignored on upper LAN
• frame received by D
• D generates reply to C, sends
• bridge sees frame from D
• bridge notes that D is on interface 2
• bridge knows C on interface 1, so selectively
  forwards frame out via interface