Chapter 3 Ethernet Bridges

1
Chapter 3Ethernet Bridges Switches, ATM
Switching

• Professor Rick Han
• University of Colorado at Boulder
• rhan_at_cs.colorado.edu

2
Announcements

• Previous lecture online
• Reminder Programming assignment 1 is due Feb.
  19
• Homework 2 will be available on the Web site on
  Thurs. Feb. 7
• Shifted Office Hours Today 430-530 pm
• Reading in Chapter 3
• 3.2 Ethernet bridges and switches
• 3.1, 3.3 ATM packet switching
• Skip 3.4
• Next, Ethernet bridges, switches, and ATM

3
Recap of Previous Lecture

• Interconnecting Ethernet LANs
• Ethernet Repeaters Hubs Physical Layer
• Amplify analog signal
• Problems
• Limited range
• Amplify noise
• Same collision domain
• Cant connect LANs with different bit rates
• Ethernet Bridges Layer 2
• Forward Ethernet frames
• Construct a table for frame forwarding
• When a frame arrives, put ltsrc addr, src LANgt in
  table

4
Recap of Previous Lecture (2)

• Ethernet Bridges Layer 2
• Frame forwarding rules
• If dest. and src on same LAN, dont forward frame
• If dest. and src on diff. LAN, route frame to
  dest. LAN
• If dest. unknown, forward to all outgoing
  interfaces
• Advantages
• Can connect LANs with different bit rates
• Separate collision domains
• Indefinite range
• No noise amplification
• Problems
• Loops can develop, causing endless packet
  forwarding
• Packet multiplication effect
• Solution spanning trees

5
Problems With Bridges

• Bridges can interconnect LANs and have multiple
  paths between every node
• Inadvertent

Layer 2 Bridge
Bridge

• Purposely for robustness, in case highest tier
  fails

Bridge

• Problem Frames can cycle forever in a loop and
  multiply to crash LAN!

Bridge
6
Problems With Bridges Packet Multiplication
Effect

• Suppose all bridges have just booted
• Suppose A wants to send to Z

Bridge 4
Bridge 1

• Bridge 1 sends As frame to LAN 5 4
• These two frames propagate to Bridge 3, where
  they multiply into 4 copies

LAN4
Z
LAN1
A
LAN3
LAN5
Bridge 2
LAN2
Bridge 3

• Exponentially multiplying copies!

7
Problems With BridgesEndless Looping

• Suppose all bridges have just booted
• Suppose A wants to send to Z

Bridge 4
Bridge 1

• Bridge 2 sends frame to LAN 2
• Bridge 3 sends frame to LAN 3
• Bridge 4 -gt LAN 4
• Back to LAN 1

LAN4
A
Z
LAN1
LAN3
Bridge 2
LAN2
Bridge 3

• Frames can cycle forever!

8
Solution Spanning Tree Algorithm

• Invented by Radia Perlman, modified into 802.1d
  spanning tree standard
• Bridges communicate with each other to set up a
  spanning tree that has no loops

Bridge 4
Bridge 1

• Disconnect some interfaces, though physical link
  exists
• Some frames may take long route though shorter
  direct route exists

LAN4
A
Z
LAN3
LAN1
Bridge 2
LAN2
Bridge 3

• Some bridges may become orphans

9
Rules to Build Spanning Tree

1. Elect a root bridge with the smallest global id
2. Each bridge computes its shortest distance to
   root
3. Each LAN selects a forwarding/designated bridge
   closest to root

Bridge 4
Bridge 3

• Spanning tree root forwarding bridges
• Root forwards frames on all outgoing ports
• If dest. not on LAN, send via forwarding bridge
• Eliminates loops!

LAN A
LAN C
LAN D
LAN E
Bridge1
Bridge 2
Root
10
Control Messages to Build Spanning Tree

• Each bridge creates a configuration message
• ltbridge source id, distance to root, root bridge
  idgt
• Each bridge floods its initial configuration
  message on each of its ports/LANs
• ltsrcmy id, dist.0, rootmy idgt
• Each bridge stores best config msg for each
  port/LAN
• A config msg C1 is better than stored config msg
  C2 if
• Root id of C1 lt root id of C2
• Root ids equal and distance of C1 lt distance of
  C2
• If root ids and distances equal, C1 is better
  than C2 if transmitting bridge on C1 is lower
  than C2

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First, Elect the Root

• If advertised root of new config msg C1 has
  smaller id, then
• Stop sending out its own bridge id config msgs
• Forward new smaller id on all outgoing ports

• Higher id config messages are discarded.
• Eventually, lowest ID bridge suppresses all other
  bridges config msgs
• Root bridge knows it is the root because the
  lowest ID is its own

Bridge 4
Bridge 3
LAN A
LAN C
LAN D
LAN E
Bridge1
Bridge 2
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First, Elect the Root (2)

• Example
• Regardless of the config msgs exchanged by
  Bridges 2,3, and 4, as soon as Bridge 1 floods
  its config msg to Bridge 2 and 4, they both

• stop sending out their own bridge id config msgs
  and
• Begin forwarding Bridge 1s config msg on all
  outgoing ports
• Eventually, Bridge 3 also stops sending its
  config msgs

Bridge 4
Bridge 3
LAN A
LAN C
LAN D
LAN E
Bridge1
Bridge 2
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Next, Build Shortest-Path Forwarding Tree to Root

• Conceptually, build shortest-path forwarding tree
  after electing the root
• But, as the roots config msg floods the network,
  notice that the shortest-path tree can
  simultaneously be calculated
• Thus, piggyback on Bridge 1s config msg flooding
  to set up the shortest path tree to root
• Each bridge increments by one the distance, as it
  receives Bridge 1s config msg, and forwards
  config msg with Bridge 1 as root to all outgoing
  ports

14
Next, Build Shortest-Path Forwarding Tree to Root
(2)

• When a bridge receives a config message from
  another bridge on same LAN with Bridge 1 as root,
  it stops sending config messages on that port/LAN
  if
• Other bridge is closer to root
• Other bridge is same distance from root, but has
  a lower ID
• Thus, a bridge de-selects itself as the
  designated forwarding bridge for that port/LAN

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Next, Build Shortest-Path Forwarding Tree to Root
(3)

• Bridge 1 floods its config message
• Bridge D is part of a loop, and will receive
  multiple config msgs from Bridge 1
• Bridge D deselects itself from both LANs because
  Bridges 2 3 are closer to root Bridge 1

Bridge 1
Bridge 2
Bridge 3
Bridge D
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Next, Build Shortest-Path Forwarding Tree to Root
(4)

• Bridge 4 is designated forwarding bridge for LAN
  A, since it closer to root than Bridge 3 on LAN A
• Bridge 3 removes itself
• For LAN B, Bridge 2 is designated forwarding
  bridge
• Bridge 3 removes itself

Bridge 4
Bridge 3
LAN A
LAN C
LAN D
LAN E
Bridge1
Bridge 2
17
Topology Change

• Root bridge periodically sends keep-alive
  messages
• If this is not heard locally, then local bridges
  start the spanning-tree algorithm all over again
• Handles the case when root bridge failed
• Handles the case when intermediate bridge failed,
  and the network becomes a
• Partitioned network
• Non-partitioned network

18
Ethernet Switches

• Essentially, the same as bridges, with support
  for many more interfaces
• Still forward frames based on destination address
• Still construct forwarding table based on source
  address
• Special routing fabric to speed frame routing
  from input interface to output interface

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80/20 Rule

• Position a bridge so that
• 80 of traffic on a segment is local
• 20 is forwarded
• Higher throughput, because each LAN has its own
  conversation
• Example place users of Server 1 on same LAN.
  Server 1 could be a file/Web server

Server 2
Server 1
20
Why Not Bridge Ethernet Indefinitely?

• Couldnt really bridge cross-country
• Delay accumulates in each bridge
• Many bridges, due to small segment sizes
• Many different types of LANs, e.g. Token Ring
  and FDDI, with completely different addressing
  schemes

?
Ethernet
Ethernet
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ATM Switching

• Point-to-Point Links Interconnect Switches
• Closer to Internet topology
• Dont connect shared-media segments

Switch C
Host A
Switch B
Host F
Switch E
Switch D
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ATM Switching (2)

• Big difference with Internet routing ATM uses
  virtual circuits to route packets
• Packet switching, but with fixed-length cells
• 48 bytes 5 bytes header
• Why fixed-length cells?
• Optimized hardware in switch can get higher
  throughput
• Why 48 bytes?
• Europe and US couldnt agree, one wanted 64 bytes
  and another 32 bytes, so they split the difference

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ATM Adaptation Layer 3/4

• Due to small packet sizes, need a layer above ATM
  to fragment and reassemble long packets
• ATM Adaptation Layer (AAL) 3/4
• IP packets can be encapsulated in ATM packets IP
  over ATM
• ATM operates as a part of Internet backbone
• Since ATM is network layer protocol, then still
  need link layer SONET, e.g. encapsulate IP over
  ATM over SONET
• too much overhead!

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Virtual Circuit Routing

• Create a virtual circuit path across an
  interconnected mesh of switches
• Each packet is labeled with a virtual circuit ID
  in its header

Switch C
Host A
Switch B
Host F
Switch E
Switch D
25
Virtual Circuit Routing (2)

• Each node chooses an unused VC number on a leg of
  circuit
• Each switch maintains a routing table mapping VC
  on input interface to VC on output interface

Switch C
7
Host A
Switch B
Host F
10
88
Switch E
Switch D
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Virtual Circuit Routing (3)
Switch B Routing Table
Any cell with VCI7 from A Is (1) relabeled with
VCI88 (2) Then routed onto E interface
Incoming Interface Incoming VCI Outgoing Interface Outgoing VCI
From A 7 To E 88

Switch C
7
Host A
Switch B
Host F
10
88
Switch E
Switch D
27
Virtual Circuit Routing (4)
Switch E Routing Table
Any cell with VCI88 from B Is (1) relabeled with
VCI10 (2) Then routed onto F interface
Incoming Interface Incoming VCI Outgoing Interface Outgoing VCI
From B 88 To F 10

VCs have local scope
Switch C
7
Host A
Switch B
Host F
10
88
Switch E
Switch D
28
Setting up VC Routing Tables

• Permanent Virtual Circuits (PVC) are set up by a
  network administrator
• Switched Virtual Circuits (SVC) are set up by
  sending control signals into the network
• Send setup message with dest. address
• Assume for now that switches can determine the
  best outgoing interface to forward a setup packet
  on when the setup packet arrives
• As setup message courses through network, each
  switch picks its incoming VCI (an unused )
• When setup msg reaches destination, send
  acknowledgment back along same path, so each
  upstream switch knows VCI chosen by downstream
  switch