Bridges and LAN Switches

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• Bridges and LAN Switches

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Last time Datagrams
Connectionless Each datagram is forwarded on its
own.
forwarding table
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Virtual Circuit Switching
Establish a connection between source and
destination between any data is communicated.

• Virtual Circuit Table
• virtual circuit identifier
• incoming interface
• outgoing interface
• a different VCI for outgoing packets

Two part process (1) Connection setup (2) Data
transfer
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Source Routing

• The sender of a packet knows the precise route
  that will take data across the network, all the
  way to the destination host.
• Each packet carries with it the entire route from
  source to destination.

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Bridges and LAN Switches
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Bridges
You want to interconnect two Ethernets. You could
use repeaters as we discussed previously
LAN 1
LAN 2


You could also use bridges, i. e. special
multiport nodes or switches that can forward
packets from one LAN to the other.


Question Is this solution always going to work?
Why not?
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Leaning Bridges
LAN 1
LAN 2
Question Does a bridge need to forward all the
packets it receives?
A bridge can inspect all the packets it receives
looking at the source addresses it receives, it
can map hosts to ports building a table like the
one above.
Question Do you need human intervention to
configure the bridge? How could it be automated?
Questions What should the bridge do if it
receives a packet to a destination it doesnt
know?
Questions How many entries should the bridge
have? Should it keep entries forever after
created?
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Spanning Tree
Extended LANs can have cycles because either they
were built without global knowledge or for the
sake of having redundant paths between LANs.
In order to populate routing tables so that
packets dont end up looping forever, first we
have to remove cycles from the network graph and
find a spanning tree.
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Spanning Tree Algorithm
Question How can you find a spanning tree of a
graph if you dont know the full graph?
Bridges have ids, just as nodes do. When a bridge
is started up, it exchanges configuration
messages with others and elects one bridge to be
the root of the spanning tree.
Initially every bridge thinks its the root and
sends out configuration messages on all its
ports. When it starts receiving messages from
other bridges, it checks to see if the new
message improves the configuration recorded for
that port.
configuration message
(1) id of sender bridge (2) id of believed
root (3) distance in hops to root
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Spanning Tree Algorithm

• A message is considered better than the recorded
  info if
• it ids a root with smaller id or
• it ids a root with equal id but shorter distance
  or
• the root id and distance are equal, but the
  sending bridge has smaller id.
• Before updating the info with the received
  message, the bridge adds 1 to the hop count.
• When a bridge discovers it is not the root, it
  stops sending out messages of its own (only
  forwards those from other bridges after adding 1
  to the hop count).

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Spanning Tree Algorithm
When a bridge discovers its not the designated
bridge for that port, it stops sending
configuration messages over that
port. Eventually, the system stabilizes only the
root sends out configuration messages and the
other bridges only forward them around.
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Broadcast and Multicast

• Broadcast messages should still reach all the
  nodes in the extended LAN.
• Multicast messages should reach all the nodes in
  a multicast group independently of the LAN to
  which the nodes belong.

Question How can one implement these kinds of
messages on the extended LAN?
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Limitations of bridges?