Bridges, Routers and Switches Part One

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Bridges, Routers and SwitchesPart One

• Three things are certain
• Death, taxes, and lost data.
• Guess which has occurred.
• Haiku error message

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Bridging

• Bridges connect two independent LANs to form
  internetworks

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Types of Bridges

• Transparent
• Source Routing
• Transnational

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Bridges

• Each LAN is referred to as a Network Segment

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Bridge Ports

• Bridge Ports connect the Bridge to the Network
• Consist of a Network Interface (Ethernet, Token
  Ring, FDDI)

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Bridges Form One Logical Network
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Bridging and ISO Model

• Bridges function at at the Media Access (MAC)
  layer

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Bridging and ISO Model

• Bridges see the network at the MAC layer
• Bridges make forwarding decisions based on MAC
  (Physical) addresses

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Bridging and ISO Model

• Bridges have no knowledge of paths between
  address
• Bridges are transparent to higher level protocol
• Bridges are protocol independent, the same bridge
  can pass IP, IPX, DecNet traffic

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Transparent Bridges

• Usually used on Ethernet networks
• Called Transparent because nodes are not aware of
  its presence
• Defined by IEEE 802.1 standard
• Forwards frames between networks
• Self learning

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Transparent Bridges

• Maintains a data base of all known node address
  on each port

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Transparent Bridges

• Transparent Bridges are self learning
• Examine every frame which passes a port
• Determines the source address
• Compares each address to its data base
• If address is not in data base then the bridge
  adds the address

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Transparent Bridges
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Transparent Bridge Frame Forwarding

• Examines Destination address
• If address is on the same LAN as the incoming
  port, ignores the frame
• If Not
• Looks up destination address in its Data Base
• If Address is found
• Forward out the appropriate Port
• If Address is not found
• Forwards out all Ports

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Spanning Tree Algorithm
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Spanning Tree Algorithm
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Spanning Tree Algorithm
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Spanning Tree Algorithm
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Spanning Tree Algorithm

• Eliminates all active loops in the network by
  creating a set of paths with only one path
  between each node
• Bridges negotiate which paths are to remain open
  and which paths will be blocked
• If a path becomes inaccessible, the bridges will
  reconfigure the network, creating a new spanning
  tree and new set of paths

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Source Route Bridging

• Only used in Token Ring environments

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Source Route Bridging

• NOTE Despite its name Source Routing is
  Bridging and takes place at the MAC layer

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Source Route Bridging

• Node maintain their own routing table
• When a frame is transmitted to a node on another
  ring, the packet must contain the route or path
• Node discovers the route by sending out an
  Explorer Frame with the address of the
  destination Node
• Depending on the protocols used the node may send
  an All Route Exploder Frame or a Singe Route
  Explorer Frame

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Source Route Bridging

• The route explorer frame records the Bridge
  Number and Ring Number of all bridges and
  networks crossed
• When it reaches the destination node, the
  destination node sets a flag and transmits the
  frame back to the sending node
• The Route Explorer Frame now has the route to the
  destination station
• If two Routes return to the sending node, it
  usually uses the first route received

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Source Route Bridging
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Source Route Bridging

• Source Route Bridges can connect rings running at
  different speeds

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Whats Wrong with Source Routing?

• Takes processing resources of end nodes
• Each Ring and Bridge must be assigned a Ring
  Number and Bridge Number - Any error in assigning
  these numbers causes fatal errors
• Fatal errors happen often

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Source Route Bridging
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Source Route Bridging

• Allows loops
• How many paths though this network?

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Source Route Bridging

• What else is wrong with Source Routing?
• What is good about Source Routing?

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Source Route and Transparent Bridges in Same
Network

• Usually not very reliable - may work in some
  simple networks
• Source Routed traffic can only cross Source Route
  Bridges

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Transnational Bridges

• Connect two dissimilar topologies

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Advantages of Bridging

• Easy to install and configure
• Inexpensive
• Transparent to Network
• Automatically adapt to Network Changes
• Bridge unroutable protocols

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Disadvantages of Bridges

• Can not take advantage of redundant paths in the
  network
• Can not prevent Broadcast Storms
• Because bridges form a single logical network the
  network becomes large and therefore difficult to
  manage

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Routers What makes a Protocol Routable

• To be routable a protocol must assign addresses
  so that the address contain Network and Node
  address
• Routable Protocols include
• IP
• IPX
• DecNet
• Vines
• Non-Routable Protocols
• NetBios

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Routers

• Function of a Router is to efficiently forward
  packets between networks
• Very often more than one path exists, a router
  must chose the best path
• Routers function at the Network layer of the OSI
  Model, routing decisions are made using the
  address assigned by a Network layer protocol such
  as IP

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Routers

• Since routers function at the Network layer
  routers are protocol dependent devices
• Must support IP, IPX, AppleTalk, DecNet, etc.

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Routers

• Multiple Protocols
• Most routers can route a verity of protocols
• The ability to route multiple protocols is
  implemented in software

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Routers
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Routers

• Routers can connect dissimilar LAN topologies
• Routers can connect LANs to the WAN

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Routers - Why use Routers

• Segment Networks
• Congestion Control
• Source Quench technique - Router can request a
  source to stop transmitting until congestion is
  relieved

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Routers

• Bridges examine each frame on the network segment
• Routers only receive packets addressed to them
• Packets are addressed to routers from
  workstations or other routers

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Routers Node Configuration
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Routers Routing Tables

• Routers maintain a Routing Table
• The Routing Table instructs the router where (out
  which interface) to send the packet
• Packets are routed one hop at a time

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Routers Routing Table
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Routers - Multiple Routes
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Routers Multiple Routes
Routing Table Router 1 Target Net Next Hop
Router Metric Net4 Attached
0 Net 1 Router 2
1 Net 1 Router 3
2 Net 3 Router 3
1 Net 3 Router 2
2 Net 2 Router 3
1 Net2 Router 2 2

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Routers Metric Values

• Chose route with lowest Metric value
• Metric value can represent
• Hops
• Transmission Delay
• Administrative Considerations (Cost)
• Line Capacity

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Routers - Backup Links
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Routers - Routing Protocol

• Routing Tables are dynamically created by the
  router
• Router communicate with other routers to learn
  all paths through the network
• This communication consists of all or portions of
  each routers routing table

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Routers - Routing Protocol

• Routers use these Routing Tables to build a map
  of the network topology
• The time it takes for all routers in a network to
  exchange routing tables is called Convergence
• Router to Router communication is implemented
  according to Routing Protocols

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Routers - Routing Protocol Design Goals

• Ideally Routing Protocols would meet all these
  criteria
• Be able to find the optimal path
• Simple enough algorithm to require a minimum of
  processing power and overhead
• Robust enough to function when the unexpected
  happens, such as hardware failures
• Rapid convergence - failure to converge quickly
  can cause (temporary) network outages
• There a numerous Routing Protocol, each has its
  strengths and weaknesses

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Routers - Routing Protocols

• Two types of Routing Protocols
• Distance-Vector Algorithm
• Link-State Algorithm

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Routers - Distance-Vector Algorithm

• Each router calculates the metric value of the
  links to all its neighbors
• Each router then sends a broadcast message
  containing its entire routing table to its
  neighboring routers
• The receiving station then uses its neighbors
  routing table to calculate a new routing table
• The router then broadcasts its new routing table
  to its neighbors

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Routers - Distance-Vector Algorithm

• Routers use the metric values received to
  calculate the shortest path through the network
• The routers do not know how the network segments
  are connected
• This algorithm results in slow convergence
• The exchange of Routing Tables uses a high
  amount of bandwidth

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Routers - Link State Algorithm Protocols

• Each router broadcasts the portion of its Routing
  Table which describes its links to its
  neighboring routers
• This description includes links and metric values
• From this information routers can determine the
  entire network topology - That is they know how
  all the segments are connected together

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Routers - Link State Algorithm Protocols

• Router use their knowledge of the network
  topology to calculate the optimum path though the
  network
• When a router detects any changes in network
  topology it broadcasts an update to other routers
• Converge faster than Distance Vector protocols
• More CPU/memory intensive than Distance Vector
  Protocols

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Routers
Interior Routers route within an
Autonomous System Run Interior Routing
Protocols Boarder Gateway Routers route between
Autonomous Systems Run Boarder Routing Protocols
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Routers - Routing Information Protocol - RIP

• Distance Vector protocol
• Interior Gateway Protocol
• One of the original Routing Protocols - Very
  widely used

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Routers - Routing Information Protocol

• Sends routing updates at regular intervals
• Uses a Hop Count for metric value
• Each Hop usually adds one Hop to the metric value
• Uses IP address as next hop
• There is limit of 15 hops between source and
  destination
• Prevents routing loops

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Routers - Interior-Gateway Routing Protocol IGRP

• Developed by Cisco Systems
• Distance Vector
• Interior routing protocol
• Uses a combination of Inter-network delay,
  bandwidth, reliability and load to determine the
  best path
• A metric value is assigned to each of these
  factors
• The weight of each factor is user definable

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Routers - Interior-Gateway Routing Protocol

• Permits Multi-path routing
• Lines of equal-bandwidth will be used in equal
  amounts
• Lines with unequal bandwidth will be used in
  proportion to its bandwidth, that is, a line
  with twice the bandwidth will be given twice the
  amount of traffic
• Only routes with metrics within a specified range
  of the best route will be used as multiple routes

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Routers - Interior-Gateway Routing Protocol
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Routers - Interior-Gateway Routing Protocol

• Routing updates are sent at regular intervals
• When a router goes down its neighbors are able to
  detect this by their failure to receive a routing
  update
• The router then recalculates it routing table
  based on this information - all routes though
  that router are now invalid
• The router then sends out its routing table at
  the scheduled interval

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Routers - Interior-Gateway Routing Protocol

• Network Convergence takes time, during this
  interval a router which is not aware of the
  invalid routes will broadcast its routing table
  containing the now invalid routes, this table can
  find its way to a router which has already
  received the data about the downed router. This
  router now replaces its current routing table
  with an outdated one.

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Routers - Interior-Gateway Routing Protocol

• Hold Down Time
• Hold down times are used to prevent a router from
  using a route which may have gone down
• Routers hold down any changes which may affect
  routes for a specified period of time
• Hold down times are usually calculated to be
  greater than the network convergence time

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Routers - Interior-Gateway Routing Protocol
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Routers - Interior-Gateway Routing Protocol

• Split Horizon
• Routers never send information about a route back
  in the same direction it came from
• There is no reason for Sue to send Harry
  information about Bob because Harry is closer to
  Bob

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Routers - Boarder Gateway Protocol BGP

• Routes between autonomous systems
• Only exchanges information with other BGP routers
• When a BGP router first joins the network routers
  exchange their entire routing table after that
  they exchange incremental information
• Responsible for exchanging information about the
  ability to reach other networks