Overview routing and Bridges

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Overview routingand Bridges

• January, 2009

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Routing in different networks

• Small networks
• ISP networks
• Global Internet

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Small networks

• Important goal
• Auto-configuration, or Plug and play
• Easy to manage
• Examples
• Switch (bridge) it is not a router, but also
  provide simple routing and forwarding as a router
• Mobile Ad hoc wireless networks
• On-demand routing
• Geographic routing

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ISP network

• Important goal
• Optimality / Efficiency
• Shortest path routing
• Load balancing
• Examples
• RIP simple distance vector protocol to implement
  shortest path routing
• OSPF link state implementation of shortest path
  routing, link costs configured
• Large ISPs assign link costs according to traffic
  matrix to achieve load balancing

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Global Internet

• Global Internet is the result of connecting many
  ISP networks together, by peering agreements
• Peering agreements are business agreements,
  specifying who should pay who, based on traffic
• ISPs route their traffic based on such peering
  agreements, goal is to minimize costs
• Policy-based routing BGP

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Auto-configuration

• You buy a new telephone, plug it in the wall, and
  it works
• You expect the same with connecting to a network
• Auto-configuration means getting the things you
  need for configuration from the network, using
  some protocol

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Configuring a host

• What does a host need in order to operate?
• IP address (plus network mask)
• Local DNS server
• Router addresses
• Domain name (specially if web server)
• name to address mapping in DNS
• Anything else?
• Many other things, e.g. application settings,
  security features
• Most can operate based on default settings

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Auto configuration

• First try RARP (Reverse Address Resolution
  Protocol)
• RARP server listens for broadcast on LAN
• Given a hosts Mac address, returns its IP
  address
• But
• More things need to be auto-configured
• Need a way of forwarding the request if there is
  no RARP server on the local LAN
• Next came BOOTP, which became DHCP

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Auto configuration cont

• Dynamic Host Configuration Protocol (DHCP)
• RARP has a IP address to MAC address mapping
• DHCP does not use fixed mapping, instead leases
  IP addresses
• Configure local name server, router list as well
• Specially useful for mobile hosts
• -- Plug-and-play, zeroconfig (IETF WG)
• Can we auto-configure the web server as well?

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DHCP cont

• DHCP server returns its own address for future
  use by client
• Things auto-configured
• Clients host address
• Name server to use
• List of routers to use
• Boot server, file
• Other options
• Given leased IP address, client may test it using
  ARP
• Upon expiry, the client can renew the lease

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Which router

• Suppose H1 gets both R1 and R2 from DHCP
• Sends a packet to H3 via R1
• R1 notices R2 is on same LAN
• R1 sends a ICMP redirect message to H1,
  suggesting H1 to use R2 for H3
• H1 needs to get R2s MAC address using ARP
• This way, H1 builds up a small forwarding table
  (cache) locally
• Review question how does H1 talk to H2?

H4
Rest of network
H3
R1
R2
H1 H2
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Recap

• We reviewed different mechanisms for
  auto-configuring the host
• DHCP is the most important one to remember
  (browse RFC1531 for details)
• Read Perlman 11.2.2.4 for review

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Configuration of routers

• What do routers need for IP forwarding?
• Forwarding tables
• Is manual configuration possible? Sometimes, just
  configure default router
• Ultimately Routing protocols are necessary, why?
• Network dynamic links, nodes may be down
• Task too complex, error-prone
• Routing protocols are more complicated

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Routing configuration

• Even with the help of routing algorithm, routers
  are not plug-and-play
• Configure each interface with IP address/mask
• Security parameter for running routing protocols
• Hierarchical routing (for scalability) requires
  different settings for routers
• Policy configurations for inter-domain routing
• In contrast, bridged LANs are essentially
  plug-and-play. Lets take a detour

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What is a bridge?

• Link level store-and-forward device that connects
  two or more LAN (Ethernet segments)
• Bridge isolates collision domains since it
  buffers frames
• Can connect LANs of different types

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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 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

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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

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Forwarding and Filtering rules

• Forwarding 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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One assumption

• The learning algorithm still works even when
  multiple LANs connecting many LANs, provided the
  bridges and LANs do not form a loop!

- When h1 sends a packet to h2, C thinks h1 is to
its left - When B forwards h1s packet, C thinks
h1 is to its right
B
A
h2
h1
C
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Bridges Spanning Tree

• 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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Bridge spanning tree algorithm

• A protocol to find out which interfaces to
  disable, so that there is
• complete connectivity
• no loops
• Each bridge sends a configuration msg to a port
  unless a better one heard on that LAN
• Configuration message contains
• Root id
• Transmitting bridges id
• Cost

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Ranking of configuration msg

• If C1s root id is lower than C2s
• If root ids equal, then C1s cost is lower
• If root ids and costs same, then C1s
  transmitters id is lower than C2s

• A bridges own configuration msg
• Root id is its own id or the lowest heard so far
• Cost is the number of hops from root id (0 if
  self)

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Example

• These are configuration msgs heard on each port
• Root Cost Transmitter
  id
• Port 1 12 9 51
• Port 2 12 8 47
• Port 3 81 0 81
• Port 4 15 3 27
• The bridges own id is 15
• So its configuration msg is (12,9,15)
• This is transmitted on port 1, 3, and 4
• The bridge that wins for each LAN is the
  designated bridge for that LAN

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Result of algorithm

• In steady state
• A single root is elected
• Each bridge knows its shortest distance to the
  root
• only one bridge transmits conf msgs on each LAN
  (the designated bridge) this is the bridge that
  will forward packets from that LAN towards the
  root
• Every bridge determines its port that gives its
  best path from itself to the root that port is
  called a root port
• Only the root ports, and the ports on which
  self has been selected as designated bridge are
  enabled. All other ports are disabled.
• Result Each designated bridge is a parent and
  the other bridges attached to the same LAN are
  its children.
• Since each bridge has only one root port, hence a
  tree

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Example bridge spanning tree
2 1 7
2 1 62
2 2 12
2 2 78
2 2 17
2 2 5
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A little poem by Perlman
I think that I shall never seeA graph more
lovely than a tree. A tree whose crucial
propertyIs loop-free connectivity. A tree which
must be sure to spanSo packets can reach every
LAN. First the Root must be selectedBy ID it is
elected. Least cost paths from Root are tracedIn
the tree these paths are placed. A mesh is made
by folks like me.Then bridges find a spanning
tree.
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Routers vs. Bridges

• Bridges and -
• Bridge (with the spanning tree algorithm) can
  auto-configure through learning
• Topologies are restricted with bridges a
  spanning tree must be built to avoid cycles
• Bridge forwarding does not contain TTL field a
  packet may loop indefinitely if there is a loop
  (spanning tree algorithm must not fail). Perlman
  has a new proposal (called Rbridge) to fix this
  problem

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Routers vs. Bridges

• Routers and -
• arbitrary topologies can be supported, cycling
  is limited by TTL counters (and good routing
  protocols)
• provide firewall protection against broadcast
  storms
• - require IP address configuration (not plug and
  play)
• - require higher processing bandwidth
• bridges do well in small (few hundred hosts)
  while routers used in large networks (thousands
  of hosts)

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References

• There is extensive discussion of bridges in
  Perlman chapter 3.
• The spanning tree algorithm is in Perlman 3.3.
• Some discussion of Router vs Bridge in Perlman
  chapter 17