CSEE W4140 Networking Laboratory

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CSEE W4140Networking Laboratory

• Lecture 8 LAN Switching
• Jong Yul Kim
• 03.25.2009

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Announcements

• Reminder of lab rules
• Labs are mandatory.
• Dont connect rack machines to Internet.
• Dont bring food / drinks to the lab.

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Announcements

• Grades
• Will be uploaded today
• Field Trip
• Two separate groups / dates
• Projects
• Projects in place of finals?
• Please come see me after class

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Short review of midterm
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Todays lecture

• Hubs
• Switches
• Learning algorithm
• Spanning Tree Protocol

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Various equipments are used to interconnect
networks
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We already know routers

• Routers operate at the Network Layer (Layer 3)
• Interconnect different subnetworks

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Weve heard about gateways

• The term Gateway is used with different meanings
  in different contexts
• Gateway is a generic term for routers (Level 3)
• Default gateway
• Gateway is also used for a device that
  interconnects different Layer 3 networks and
  which performs translation of protocols
  (Multi-protocol router)

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Ethernet Hub

• A simple repeater (extends the physical cable)
• Frame collisions are propagated
• Good for sniffing traffic in a network we want to
  monitor

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Bridges/LAN switches

• A bridge or LAN switch is a device that
  interconnects two or more Local Area Networks
  (LANs) and forwards packets between these
  networks.
• Bridges/LAN switches operate at the Data Link
  Layer (Layer 2)

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Terminology Bridge, LAN switch, Ethernet switch

• There are different terms to refer to a data-link
  layer interconnection device
• The term bridge was coined in the early 1980s.
• Today, the terms LAN switch or (in the context of
  Ethernet) Ethernet switch are used.
• Convention
• Since many of the concepts, configuration
  commands, and protocols for LAN switches were
  developed in the 1980s, and commonly use the old
  term bridge, we will, with few exceptions,
  refer to LAN switches as bridges.

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A Switched Enterprise Network
Router
Switch
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Bridges versus Routers

• Routers
• Each hosts IP address must be configured
• If network is reconfigured, IP addresses may need
  to be reassigned
• Routing done via RIP or OSPF
• Each router manipulates packet header (e.g.,
  reduces TTL field)

• Bridges
• MAC addresses are hardwired
• No network configuration needed
• plug-and-play!
• No routing protocol needed (sort of)
• learning bridge algorithm
• spanning tree algorithm
• Bridges do not manipulate frames

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Frame Forwarding

• Each bridge maintains a forwarding table with
  entries
• lt MAC address, port, agegt
• MAC address host name or group address
• port port number of bridge
• age aging time of entry
• with interpretation
• a machine with MAC address lies in direction of
  the port number from the bridge. The entry is age
  time units old.

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Frame Forwarding

• Assume a MAC frame arrives on port x.

Is MAC address of destination in
forwardingtable for ports A, B, or C ?
Notfound ?
Found?
Forward the frame on theappropriate port
Flood the frame, i.e., send the frame on all
ports except port x.
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Learning Algorithm

• Routing tables entries are set automatically with
  a simple heuristic
• The source field of a frame that arrives on a
  port tells which hosts are reachable from this
  port.

Port 1
Port 4
x is at Port 3
y is at Port 4
Port 2
Port 5
Port 3
Port 6
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Learning Algorithm

• Algorithm
• For each frame received, the source stores the
  source field in the forwarding database together
  with the port where the frame was received.
• All entries are deleted after some time (default
  is 15 seconds).

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Learning Algorithm Example

• Consider the following packets (SrcA, DestF),
  (SrcC, DestA), (SrcE, DestC)
• What have the bridges learned?

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Danger of Loops

• Consider the two LANs that are connected by two
  bridges.
• Assume host n is transmitting a frame F with
  unknown destination.
• What is happening?
• Bridges A and B flood the frame to LAN 2.
• Bridge B sees F on LAN 2 (with unknown
  destination), and copies the frame back to LAN 1
• Bridge A does the same.
• The copying continues
• Wheres the problem? Whats the solution ?

F
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Spanning Tree Protocol (STP)

• A solution is to prevent loops in the topology
  
• IEEE 802.1d has an algorithm that builds and
  maintains a spanning tree in a dynamic
  environment
• Bridges that run 802.1d are called transparent
  bridges
• Bridges exchange messages to configure the bridge
  (Configuration Bridge Protocol Data Unit,
  Configuration BPDUs) to build the tree.
• Using the BPDUs, each bridges makes a local
  decision which of its ports are part of the
  spanning tree

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Diagram of a spanning tree
Disabled ports
Forwarding ports
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Concepts

• Each bridge as a unique identifier
• Bridge ID ltpriority number MAC addressgt
• Note that a bridge has several MAC addresses
  (one for each port), but only one ID
• lower priority number has higher priority
• (The lower the better!!)
• Each port within a bridge has a unique identifier
  (port ID).
• Root Bridge
• The bridge with the lowest identifier is the root
  of the spanning tree.
• Root Port
• Each bridge has a root port which identifies the
  next hop from a bridge to the root.

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Concepts

• Root Path Cost
• For each bridge, the cost of the min-cost path to
  the root.
• The lower the better!!
• Designated Bridge, Designated Port
• Lowest cost bridge on the segment is the
  designated bridge.
• On the designated bridge, the port that is
  attached to the segment is the designated port.
• if two bridges have the same cost, select the one
  with highest priority
• if the min-cost bridge has two or more ports on
  the LAN, select the port with the lowest
  identifier

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Configuration BPDUs
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Steps of Spanning Tree Algorithm

• 1. Determine the root bridge
• 2. Determine the root port on all other bridges
• 3. Determine the designated port on each LAN
• Each bridge is sending out BPDUs that contain the
  following information

root ID
cost
bridge ID/port ID
root bridge (what the sender thinks it is) root
path cost for sending bridgeIdentifies sending
bridge
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Determine the Root Bridge

• Initially, all bridges assume they are the root
  bridge.
• Each bridge B sends BPDUs of this form on its
  LANs
• Each bridge looks at the BPDUs received on all
  its ports and its own transmitted BPDUs.
• Root bridge is the smallest received root ID that
  has been received so far (Whenever a smaller ID
  arrives, the root is updated)

B
0
B
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Calculate the Root Path CostDetermine the Root
Port

• At this time A bridge B has a belief of who the
  root is, say R.
• Bridge B determines the Root Path Cost (Cost) as
  follows
• If B R Cost 0.
• If B ? R Cost Smallest Cost in any of BPDUs
  that were received
  cost of Bs interface where this
  BPDU was received
• Bs root port is the port from which B received
  the lowest cost path to R
• Knowing R and Cost, B can generate its BPDU (but
  will not necessarily send it out)

R
Cost
B
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Default Cost
Ethernet Speed Cost
10 Mbps 100
100 Mbps 19
1 Gbps 4
10 Gbps 2
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Calculate the Root Path CostDetermine the Root
Port

• At this time B has generated its BPDU
• B will send this BPDU on one of its ports, say
  port x, only if its BPDU is lower than any BPDU
  that B received from port x.
• In this case, B also assumes that it is the
  designated bridge for the LAN to which the port
  connects.

R
Cost
B
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Selecting the Ports for Spanning Tree

• Each bridges makes a local decision which of its
  ports are part of the spanning tree
• B will decide which ports are in the spanning
  tree
• Bs root port is part of the spanning tree
• All designated ports are part of the spanning
  tree
• All other ports are not part of the spanning tree
• Bs ports that are in the spanning tree will
  forward packets (forwarding state)
• Bs ports that are not in the spanning tree will
  not forward packets (blocking state)

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Homework

• Prelab 6 due this Friday
• Lab reports due this week

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Main Points of Lab 6

• Hubs vs. switches
• Switches
• Learning algorithm
• Spanning Tree Protocol