CCNA 2 Module 6

1
CCNA 2 Module 6

• Routing and Routing Protocols

"Watch your thoughts they become your words.
Watch your words they become your actions. Watch
your actions they become your habits. Watch your
habits they become your character. Watch your
character for it will become your destiny. -
Frank Outlaw
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CCNA 2 Module 6 Objectives

• By the end of this module you should be able to
• Explain the significance of static routing
• Configure static and default routes
• Verify and troubleshoot static and default routes
  
• Identify the classes of routing protocols
• Identify distance vector routing protocols
• Identify link-state routing protocols
• Describe the basic characteristics of common
  routing protocols
• Identify interior gateway protocols
• Identify exterior gateway protocols
• Enable Routing Information Protocol (RIP) on a
  router

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Introducing Routing

• Routing is the process used to forward packets
  toward the destination network
• A router makes decisions based upon the
  destination IP address of a packet
• Devices along the way use the destination IP
  address to point the packet in the correct
  direction so it arrives at its destination
• Routers must learn the direction to remote
  networks
• With dynamic routing, this information is learned
  from other routers
• With static routing, the administrator configures
  information about remote networks manually
• Since static routes are configured manually, any
  topology changes requires the network
  administrator to reconfigure the routes

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Static Route Operation

• Static route operations can be divided into three
  parts
• Network administrator configures the route
• Router installs the route in the routing table
• Packets are routed using the static route
• Administrator must configure the static route on
  the router using the ip route command.

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Administrative Distance

• Administrative distance is an optional parameter
  that measures of the reliability of a route
• A lower value for the administrative distance
  indicates the more reliable route
• Default administrative distance when using
  next-hop address is 1, while the default
  administrative distance when using the outgoing
  interface is 0
• If an administrative distance other than the
  default is desired, a value between 0 and 255 is
  entered after the next-hop or outgoing interface
  as follows

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AD in Static Routes

• If the router cannot reach the outgoing interface
  used in the route, the route will not be
  installed in the routing table, meaning that if
  that interface is down, then the route is not
  placed in the routing table
• Sometimes static routes are used for backup
  purposes
• A static route can be configured on a router that
  will only be used when the dynamically learned
  route has failed
• A wise back up route
• Set the administrative distance higher than that
  of the dynamic routing protocol being used

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Examples of Static Routes
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Configuring Static Routes

• Use the following steps to configure static
  routes
• Determine all desired destination networks, their
  subnet masks, and their gateways. A gateway can
  be either a local interface or a next hop address
  that leads to the desired destination
• Enter global configuration mode
• Type the ip route command with a destination
  address and subnet mask followed by their
  corresponding gateway from Step one. Including an
  administrative distance is optional
• Repeat Step three for as many destination
  networks as were defined in Step one
• Exit global configuration mode
• Save the active configuration to NVRAM by using
  the copy running-config startup-config command

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More Examples of Static Route Configurations
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Configuring Default Routes

• Default routes are used to route packets with
  destinations that do not match any of the other
  routes in the routing table
• Routers are typically configured with a default
  route for Internet-bound traffic, since it is
  often impractical and unnecessary to maintain
  routes to all networks in the Internet.
• A default route is actually a special static
  route that uses this format
• ip route 0.0.0.0 0.0.0.0 next-hop-address
  outgoing interface
• The 0.0.0.0 mask, when logically ANDed to the
  destination IP address of the packet to be
  routed, will always yield the network 0.0.0.0.
• If the packet does not match a more specific
  route in the routing table, it will be routed to
  the 0.0.0.0 network.
• Use the following steps to configure default
  routes
• Enter global configuration mode.
• Type the ip route command with 0.0.0.0 for the
  destination network address and 0.0.0.0 for the
  subnet mask. The gateway for the default route
  can be either the local router interface that
  connects to the outside networks or the IP
  address of the next-hop router. In most cases, it
  is preferred that the IP address of the next hop
  router is specified.
• Exit global configuration mode.
• Save the active configuration to NVRAM by using
  the copy running-config startup-config command.

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More Configuring Default Routes

• Steps to configure default routes
• Enter global configuration mode.
• Type the ip route command with 0.0.0.0 for the
  destination network address and 0.0.0.0 for the
  subnet mask. The gateway for the default route
  can be either the local router interface that
  connects to the outside networks or the IP
  address of the next-hop router. In most cases, it
  is preferred that the IP address of the next hop
  router is specified.
• Exit global configuration mode.
• Save the active configuration to NVRAM by using
  the copy running-config startup-config command.

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Default Route Examples
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Verifying Static Route Configuration

• Verify static routes to insure they are present
  in the routing table and working properly
• show running-config displays the active
  configuration to verify the static route was
  entered correctly
• show ip route displays any static route present
  in the routing table
• Use the following steps to verify static route
  configuration
• In privileged mode enter the command show
  running-config to view the active configuration.
• Verify that the static route has been correctly
  entered. If the route is not correct, it will be
  necessary to go back into global configuration
  mode to remove the incorrect static route and
  enter the correct one.
• Enter the command show ip route.
• Verify that the route that was configured is in
  the routing table.

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Introduction to Routing Protocols

• Routing and routed protocols are different in
  both function and task
• Routing protocol is the communication used
  between routers
• Routing protocols allow one router to share
  information with other routers regarding the
  networks it knows about as well as its proximity
  to other routers
• This information is used to build and maintain a
  routing table. 
• Examples of routing protocols are
• Routing Information Protocol (RIP) (ver. 1 2)
• Interior Gateway Routing Protocol (IGRP) a
  proprietary legacy
• Enhanced Interior Gateway Routing Protocol
  (EIGRP)
• Open Shortest Path First (OSPF)
• Routed protocols (TCP/IP or IPX/SPX) are used to
  direct user traffic
• Routed protocol provides enough information in
  its network layer address to allow a packet to be
  forwarded from one host to another based on the
  addressing scheme

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Autonomous Systems

• An autonomous system (AS) is a collection of
  networks under a common administration sharing a
  common routing strategy.
• To the outside world, an AS is viewed as a single
  entity.
• The AS may be run by one or more operators while
  presenting a consistent view of routing to the
  external world
• The American Registry of Internet Numbers (ARIN),
  a service provider, or an administrator assigns
  an identifying number to each AS
• This autonomous system number is a 16 bit number.
  
• Routing protocols, such as Ciscos IGRP EIGRP
  require a unique, autonomous system number

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Routing Protocol Purpose Autonomous Systems

• Routers use routing protocols to manage
  information received from other routers,
  information learned from the configuration of its
  own interfaces, along with manually configured
  routes.
• The routing protocol learns all available routes,
  places the best routes into the routing table,
  and removes routes when they are no longer valid.
  
• The router uses the information in the routing
  table to forward routed protocol packets.
• The routing algorithm is fundamental to dynamic
  routing.
• Whenever the topology of a network changes
  because of growth, reconfiguration, or failure,
  the network knowledgebase must also change.
• The network knowledgebase needs to reflect an
  accurate consistent view of the new topology.
• When all routers in an internetwork are operating
  with the same knowledge, the internetwork is said
  to have converged.
• Fast convergence is desirable because it reduces
  the period of time in which routers would
  continue to make incorrect routing decisions.
• Autonomous systems (AS) provide the division of
  the global internetwork into smaller and more
  manageable networks.
• Each AS has its own set of rules and policies and
  an AS number that will uniquely distinguish it
  from other autonomous systems throughout the
  world.

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Identifying the Classes of Routing Protocols

• Most routing algorithms can be classified into
  one of two categories
• distance vector
• link-state
• The distance vector routing approach determines
  the direction (vector) and distance to any link
  in the internetwork.
• The link-state approach, also called shortest
  path first, recreates the exact topology of the
  entire internetwork.

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Distance Vector Routing Protocol Features

• Distance vector routing algorithms pass periodic
  copies of a routing table from router to router.
• These regular updates between routers communicate
  topology changes.
• Distance vector based routing algorithms are also
  known as Bellman-Ford algorithms.
• Each router receives a routing table from its
  directly connected neighbor routers.
• The algorithm eventually accumulates network
  distances so that it can maintain a database of
  network topology information.
• However, distance vector algorithms do not allow
  a router to know the exact topology of an
  internetwork as each router only sees its
  neighbor routers.
• Each router that uses distance vector routing
  begins by identifying its own neighbors.
• The interface that leads to each directly
  connected network is shown as having a distance
  of 0.
• As the distance vector network discovery process
  proceeds, routers discover the best path to
  destination networks based on the information
  they receive from each neighbor.
• Each of the other network entries in the routing
  table has an accumulated distance vector to show
  how far away that network is in a given
  direction.
• Routing table updates occur when the topology
  changes.
• As with the network discovery process, topology
  change updates proceed step-by-step from router
  to router.
• Distance vector algorithms call for each router
  to send its entire routing table to each of its
  adjacent neighbors.
• The routing tables include information about the
  total path cost as defined by its metric and the
  logical address of the first router on the path
  to each network contained in the table.

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Distance Vector Topology Changes
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Link-State Routing Protocol Features

• Link-state algorithms (Dijkstras algorithm) or
  SPF (shortest path first) algorithms
• Link-state routing algorithms maintain a complex
  database of topology information
• A link-state routing algorithm maintains full
  knowledge of distant routers and how they
  interconnect
• Link-state routing uses
• Link-state advertisements (LSAs) A link-state
  advertisement (LSA) is a small packet of routing
  information that is sent between routers
• Topological database A topological database is
  a collection of information gathered from LSAs
• SPF algorithm The shortest path first (SPF)
  algorithm is a calculation performed on the
  database resulting in the SPF tree
• Routing tables A list of the known paths and
  interfaces

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Network Discovery Processes for Link-State Routing

• LSAs are flooded between routers starting with
  directly connected networks for which they have
  direct information.
• Routera in parallel with the others constructs a
  topological database consisting of all the
  exchanged LSAs
• SPF algorithm computes network reachability
• The router constructs this logical topology as a
  tree, with itself as the root, consisting of all
  possible paths to each network in the link-state
  protocol internetwork
• The router lists the best paths and the
  interfaces to these destination networks in the
  routing table
• The router that first becomes aware of a
  link-state topology change forwards the
  information so that all other routers can use it
  for updates.
• This involves sending common routing information
  to all routers in the internetwork.
• To achieve convergence, each router keeps track
  of its neighbor routers, the router name,
  interface status, and the cost of the link to the
  neighbor.
• The router constructs an LSA packet that lists
  this information along with new neighbors,
  changes in link costs, and links that are no
  longer valid.
• The LSA packet is then sent out so that all other
  routers receive it.
• When the router receives an LSA, the database is
  updated with the most recent information and
  computes a map of the internetwork using the
  accumulated data and calculates the shortest path
  to other networks using the SPF algorithm.
• Each time an LSA packet causes a change to the
  link-state database, SPF recalculates the best
  paths and updates the routing table.

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Link-State Concerns

• There are three main link-state concerns
• Processor overhead
• Memory requirements
• Bandwidth Consumption
• Routers running link-state protocols require more
  memory and perform more processing than distance
  vector routing protocols.
• Routers must have sufficient memory to be able to
  hold all the information from the various
  databases, the topology tree, and the routing
  table.
• Initial link-state packet flooding consumes
  bandwidth and during the initial discovery
  process, all routers using link-state routing
  protocols send LSA packets to all other routers.
• This action floods the internetwork and
  temporarily reduces bandwidth available for
  routed traffic carrying user data.
• After this initial flooding, link-state routing
  protocols generally require only minimal
  bandwidth to send infrequent or event triggered
  LSA packets reflecting topology changes.

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Path Determination

• A router determines the path of a packet from one
  data link to another, using two functions
• Path determination occurs at the network layer
• The path determination function enables a router
  to evaluate the paths to a destination and to
  establish the preferred handling of a packet
• The routing table determines the best path and
  forwards the packet using the switching function
• The switching function is the internal process
  used by a router to accept a packet on one
  interface and forward it to a second interface on
  the same router
• The switching function of the router encapsulates
  packets in the appropriate frame type for the
  next data link

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Routing Configuration

• Enabling an IP routing protocol on a router
  involves the setting of both global and routing
  parameters.
• Global tasks include selecting a routing
  protocol, such as RIP, IGRP, EIGRP or OSPF.
• The major task in the routing configuration mode
  is to indicate IP network numbers.
• Dynamic routing uses broadcasts and multicasts to
  communicate with other routers.
• The routing metric helps routers find the best
  path to each network or subnet.
• The router command starts a routing process.
• The network command is required because it
  enables the routing process to determine which
  interfaces participate in the sending and
  receiving of routing updates.
• An example of a routing configuration is
• GAD(config)router ripGAD(config-router)network
  172.16.0.0
• The network numbers are based on the network
  class addresses, not subnet addresses or
  individual host addresses.

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Examples of Enabling Routing Protocols
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Routing Protocols

• At the Internet layer of the TCP/IP suite of
  protocols, a router can use an IP routing
  protocol to accomplish routing through the
  implementation of a specific routing algorithm.
• Examples of IP routing protocols include
• RIP A distance vector interior routing protocol
  
• IGRP Cisco's distance vector interior routing
  protocol
• OSPF A link-state interior routing protocol
• EIGRP Ciscos advanced distance vector interior
  routing protocol
• BGP A distance vector exterior routing protocol

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Routing Protocols - RIP

• Routing Information Protocol (RIP) was originally
  specified in RFC 1058.
• Its key characteristics include the following
• It is a distance vector routing protocol.
• Hop count is used as the metric for path
  selection.
• If the hop count is greater than 15, the packet
  is discarded.
• Routing updates are broadcast every 30 seconds,
  by default.

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Routing Protocols - IGRP

• Interior Gateway Routing Protocol (IGRP) is a
  Cisco proprietary protocol
• IGRP key design characteristics
• A distance vector routing protocol
• Bandwidth, load, delay and reliability are used
  to create a composite metric
• Routing updates are broadcast every 90 seconds,
  by default
• Replaced by EIGRP

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Routing Protocols - OSPF

• Open Shortest Path First (OSPF) is a
  nonproprietary link-state routing protocol
• OSPF key characteristics
• link-state routing protocol.
• Open standard routing protocol described in RFC
  2328
• Uses the SPF algorithm to calculate the lowest
  cost to a destination
• Routing updates are flooded as topology changes
  occur rapid convergence
• Wide implementation

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Routing Protocols - EIGRP

• Cisco proprietary enhanced distance vector
  routing protocol
• EIGRP key characteristics
• Enhanced distance vector routing protocol.
• Uses load balancing.
• Uses a combination of distance vector and
  link-state features.
• Uses Diffused Update Algorithm (DUAL) to
  calculate the shortest path.
• Routing updates are broadcast every 90 seconds or
  as triggered by topology changes.

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Routing Protocols - BGP

• Border Gateway Protocol (BGP) is an exterior
  routing protocol (WAN)
• BGP key characteristics
• Distance vector exterior routing protocol.
• Used between ISPs or ISPs and clients
• Used to route Internet traffic between autonomous
  systems

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Autonomous Systems and IGP versus EGP

• Interior routing protocols are for use in a
  network under the control of a single
  organization
• Design criteria for an interior routing protocol
  require it to find the best path through the
  network, meaning the metric and how that metric
  is used is the most important element in an
  interior routing protocol
• Exterior routing protocol is for use between two
  different networks under the control of two
  different organizations
• Typically used between ISPs or between a company
  and an ISP
• IP exterior gateway protocols require the
  following three sets of information before
  routing can begin
• List of neighbor routers with which to exchange
  routing information
• List of networks to advertise as directly
  reachable
• Autonomous system number of the local router
• Exterior routing protocols must isolate
  autonomous systems
• This autonomous system number is a 16-bit number
• Routing protocols such as Ciscos IGRP and EIGRP
  require a unique, autonomous system number

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Interior and Exterior Routing Protocols
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Distance Vector - Summary

• Distance vector algorithms (AKA Bellman-Ford
  algorithms) requires routers to send all or part
  of routing table only to its neighbors
• Distance vector algorithms perform routing
  decisions based upon information provided by
  neighboring routers
• Distance vector protocols use fewer resources but
  may suffer from slow convergence and may use
  metrics that do not scale well to larger systems
• The algorithms involve passing copies of a
  complete routing table from router to router on a
  periodic basis
• This type of routing protocol requires that each
  router simply inform its neighbors of its routing
  table
• For each network path, the receiving routers pick
  the neighbor advertising the lowest cost and then
  adds this entry into their routing table for
  re-advertisement
• RIP/IGRP are common distance vector routing
  protocols

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Link-State - Summary

• Link-state algorithms (also known as shortest
  path first algorithms) flood routing information
  to all routers in the internetwork that creates a
  map of the entire network.
• Each router sends packets to all its neighbors
  and these packets contain descriptions of the
  network or networks to which the router is
  linked.
• The routers assemble all the information into a
  complete view of the internetwork topology to
  calculate the shortest path to all known sites on
  the network.
• It then generates routing tables showing the best
  path for any destination on the network.
• Once converged, link state protocols use small
  update packets, which contain only changes rather
  than copies of the entire routing table.
• Update packets are passed across the network in
  event-triggered updates, so convergence is fast.
  
• Because they converge more quickly than distance
  vector protocols, link-state algorithms are less
  prone to routing loops.
• Link-state protocols are also less prone to
  routing errors, but they use more system
  resources, however, are generally more scalable
  than distance-vector protocols.
• When a network link changes state, a notification
  called a link-state advertisement (LSA) is
  flooded throughout the network.
• All the routers note the change and adjust their
  routes accordingly.
• OSPF and IS-IS are examples of link-state routing
  protocols.

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Summary

• You should now be able to
• Explain the significance of static routing
• Configure static and default routes
• Verify and troubleshoot static and default routes
  
• Identify the classes of routing protocols
• Identify distance vector routing protocols
• Identify link-state routing protocols
• Describe the characteristics of common routing
  protocols
• Identify interior gateway protocols
• Identify exterior gateway protocols
• Enable Routing Information Protocol (RIP) on a
  router
• Any Questions?