CCNA 2 Module 6

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CCNA 2 Module 6

• Routing and Routing Protocols

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Routing

• Routing is moving data from one network to
  another based on the Layer 3 address.
• Routers look at the destination address in the
  layer 3 header and forward the data out the
  appropriate interface.
• Network numbers and their associated interfaces
  are kept in the routers routing table.

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Different Types of Routes

• There are basically three types of routes
• Static Routes
• Dynamic Routes
• Default Routes

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Static Routes

• Static routes must be manually entered by the
  network administrator.
• Static routes are configured according to the
  destination (remote) network number, subnet mask
  and next hop address or the exiting interface of
  the local router.
• Static routes are configured from global
  configuration mode and are represented by an S -
  in the routing table.

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Static Routes

• Because static routes must be configured
  manually, any network topology changes require
  the network administrator to add and delete
  static routes to account for the changes.
• Because of the extra administrative requirements,
  static routing does not have the scalability of
  dynamic routing.

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Static Route Configuration
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Static Route-Outgoing Interface
Gateway exiting local interface
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Static Route-Next Hop Address
Gateway next hop address
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Administrative Distance

• The administrative distance is an optional
  parameter that gives a measure of the reliability
  of the route.
• A lower value for the administrative distance
  indicates the more reliable route.
• Default Admin Dist 1 and Max is 255
• A static route to a dest. network with a higher
  admin distance then the routing protocol for the
  same network can be used as a backup route. This
  is called a Floating Static Route

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Administrative Distance
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Static 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.
• Static default routes are configured exactly like
  any other static route except the destination
  network and subnet mask are 0.0.0.0
• This is called the quad zero route.
• Router(config) ip route 0.0.0.0 0.0.0.0
  172.16.0.1

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Dynamic Routing

• Dynamic routing is achieved by use of a routing
  protocol
• A routing protocol allows one router to share
  information with other routers regarding the
  networks it knows about and their distance from
  that router (metric).
• The information a router gets from another router
  is used to build and maintain a routing table.

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Routing Protocols

• Examples of routing protocols are
• Routing Information Protocol (RIP)
• Interior Gateway Routing Protocol (IGRP)
• Enhanced Interior Gateway Routing Protocol
  (EIGRP)
• Open Shortest Path First (OSPF)
• Border Gateway Protocol (BGP)

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Routing Protocols

• Routing protocols can be broken into different
  categories according to whether they are
• External or Internal to the Autonomous System
• Distance Vector or Link-State
• Classful or Classless

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Interior and Exterior Protocols

• A routing protocol can be used to route traffic
  within an Autonomous System (AS), in which case
  the routing protocol would be interior.
• A routing protocol can also be used to route
  traffic from one AS to another, in which case it
  would be exterior.
• Interior and exterior refer to internal to the AS
  or external to the AS.

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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 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.

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Interior and Exterior Protocols

• Interior Gateway Protocols
• RIP
• IGRP and EIGRP
• OSPF
• Exterior Gateway Protocols
• BGP

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Distance Vector and Link-State

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

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Routing Metrics

• Routers decide how close a network is to itself
  by using a routing metric.
• Routing metrics can be calculated according to
• Hop count
• Bandwidth
• Reliability
• Load
• Delay

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Routing Metric
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Distance Vector Concepts

• Distance vector routing algorithms pass periodic
  copies of a routing table (updates) from router
  to router.
• Each router receives the entire routing table
  from its directly connected neighbor routers.
• This same step-by-step process occurs in all
  directions between neighbor routers until all
  routers have a complete routing table of the
  entire internetwork.
• Once this happens, the network has converged.

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Distance Vector Updates
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Routing Information Protocol

• 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 (Max hop count 15)
• Routing updates are broadcast every 30 seconds,
  by default.
• RIP is a classful routing protocol

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Interior Gateway Routing Protocol

• Interior Gateway Routing Protocol (IGRP) is a
  proprietary protocol developed by Cisco. Some of
  the IGRP key design characteristics emphasize the
  following
• It is 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.
• IGRP is a classful routing protocol

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Link-State Concepts

• Link-state algorithms are also known as Dijkstras
  algorithm or as 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 routers exchange Link State
  Advertisements (LSAs) to update network
  reachability.

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OSPF Route Calculation
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Link-State Routing

• 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 for only the best routes to a
  destination.

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Open Shortest Path First

• Open Shortest Path First (OSPF) is a
  nonproprietary link-state routing protocol. The
  key characteristics of OSPF are as follows
• It is a 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 to a multicast address.
• Update packets are passed across the network in
  event-triggered updates, so convergence is fast.
• OSPF is a classless routing protocol.

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Enhanced IGRP - Hybrid

• EIGRP is a Cisco proprietary enhanced distance
  vector routing protocol. The key characteristics
  of EIGRP are as follows
• It is an enhanced distance vector routing
  protocol.
• Uses load balancing.
• Uses a combination of distance vector (metric)
  and link-state features (updates).
• Uses Diffused Update Algorithm (DUAL) to
  calculate the shortest path.
• Routing updates are triggered by topology
  changes.
• EIGRP is a classless routing protocol.

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Classful and Classless

• Classful routing protocols such as RIP and IGRP
  do not support classless addresses or Variable
  Length Subnet Masks (VLSM) unless all subnet
  masks are the same!
• Classful routing protocols can only route for
  specific subnets, if all subnet masks are the
  same!
• Classless routing protocols (RIPv2, EIGRP, OSPF)
  support classless address and VLSM by sending
  subnet mask information as well as network
  information as part of the routing update.
• Classless protocols support routing between
  individual subnets of different length subnet
  masks.