Interconnecting LAN segments

1
Interconnecting LAN segments

• Repeaters
• Hubs
• Bridges
• Switches

2
Interconnecting with repeaters

• Repeaters used to connect multiple LAN segments
• A repeater repeats bits it hears on one interface
  to its other interfaces physical layer device
  only!
• Ethernet Max 4 repeaters per LAN
• Total 5 LAN segments ? 530 150 nodes max.
• Repeaters have become a legacy technology

3
Interconnecting with hubs

• Effectively a physical layer device
• Multi-port repeater
• Operates at bit level
• Repeat received bits on one interface to all
  other interfaces

4
Interconnecting with hubs

• Hubs can be arranged in a hierarchy (or
  multi-tier design), with backbone hub at its top
• Better than repeaters
• Hubs can detect malfunctioning node adapters and
  disconnect them from the network thereby
  increasing reliability
• Can collect statistics such as collision rate,
  network usage, average frame size
• Provide network management functionality

5
Advantages of hubs

• Easy to Understand
• Easy to Implement
• so theyre cheap

6
Limitation of hubs

• Cant interconnect 10BaseT 100BaseT
• Individual segment collision domains become one
  large collision domain
• if a node in CS and a node EE transmit at same
  time collision
• Poor security
• Why should host B get to share its link with a
  conversation between A and D?
• Packet sniffer on one port can monitor the
  traffic of all of the ports
• Can we do better?
• Use bridges

7
Interconnecting with bridges

• Link layer device
• stores and forwards LL, e.g., Ethernet, frames
• examines frame header and selectively forwards
  frame based on MAC destination address
• when frame is to be forwarded on segment, uses
  CSMA/CD to access segment
• segments become separate collision domains
• Transparent hosts are unaware of presence of
  bridges
• Plug-and-play, self-learning bridges do not need
  to be configured

8
Backbone Bridge

• Recommended configuration
• Notice that a bridge can connect a 10BaseT LAN
  with a 100BaseT LAN, while a hub can not!

9
Bridges Forwarding

• How does the bridge determine to which LAN
  segment to forward a frame to?
• Notice that this has to be done transparent to
  the hosts. That is, hosts should not be aware
  that there is a bridge connecting several LANs
  together

10
Bridges Self Learning

• Basic idea Build cache (called the bridge table)
  of which nodes are downstream of which ports
• entry in bridge table
• (Node MAC Address, Bridge Interface, Time Stamp)
• stale entries in table dropped (TTL can be 60
  min)
• How? Bridge monitors source MAC address on all
  packets that it forwards
• when frame received, bridge learns location of
  sender incoming LAN segment
• records sender/location pair in bridge table
• What to do with unknown sources?
• Flood network, i.e., forward the frame on all
  interfaces except over the one from which the
  frame was received

11
Bridge Learning 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

12
Bridge Learning Example

• 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 1

13
Bridges Filtering/Forwarding

• When bridge receives a frame
• index bridge table using destination MAC address
• if entry found for destinationthen
• if dest on segment from which frame arrived
  then drop the frame
• else forward the frame on interface
  indicated
• else floodforward on all but the interface
  on which the frame arrived
• If destination MAC is FF-FF-FF-FF-FF-FF, that is,
  the packet is being broadcast to all hosts, then
• forward the frame on all but the interface on
  which the frame arrived

14
Eliminating Loops in Bridged Networks Spanning
Tree

• Desirable to have redundant, alternate paths from
  source to destination for increased reliability,
  availability
• 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

15
Interconnecting with Switches

• Switches
• multi-port bridge
• Each port acts as a bridge
• Each port determines MAC addresses connected to
  itself
• Master list within switch determines forwarding
  behavior

16
Switches (more)

• A-to-B and A-to-B communication simultaneously
  no collisions
• large number of interfaces versus bridges (which
  typically have only two)
• Typically star-shaped topology
• Cut-through switching frame forwarded from input
  to output port without awaiting for assembly of
  entire frame
• slight reduction in latency
• Combinations of shared/dedicated, 10/100/1000
  Mbps interfaces
• LAN, e.g., Ethernet, but no collisions!

17
Switched Network Advantages

• Higher link bandwidth
• Point to point electrically simpler than bus
• Much greater aggregate bandwidth
• Separate segments can send simultaneously
• Data backplane of switches typically large to
  support simultaneous transfers amongst ports
• Challenge
• Learning which packets to copy across links
• Forwarding table based on destination MAC address
• Avoiding forwarding loops
• Perlmans Spanning Tree Algorithm

18
Summary

• Covered how to extend LAN segments
• Repeaters
• Physical Layer Devices
• Hubs
• Multi-port repeaters
• Bridges
• Link Layer Devices Store forward frames based
  on the destination MAC address of the frame
• Build packet forwarding table on the fly by
  observing passing packets
• Spanning Tree to eliminate loops
• Switches
• Multi-port bridges