Routing/Routed Protocols

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Routing/Routed Protocols

• Part I

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Routed Protocol Definition

• Routed Protocol used to transmit user data
  (packets) through an internetwork. Routed
  protocols are assigned to an interface and
  determine the method of packet delivery.
• Examples IP, IPX, AppleTalk, DECNet, Banyan
  Vines

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Routing Protocol Definition

• Routing protocol any protocol that defines
  algorithms to be used for updating routing tables
  between routers. Basically, a routing protocol
  determines the path of a packet through an
  internetwork.
• Examples RIP, RIPv2, IGRP, EIGRP, OSPF, IS-IS,
  BGP

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Remember

• A Routed Protocol defines logical addressing.
  Most notable example on the test IP
• A Routing Protocol fills the routing table with
  routing information. Examples on the test RIP,
  IGRP, EIGRP, OSPF, IS-IS

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CCNA Exam Tips -- Routing

• Since IP routing is basically what Cisco routers
  do, this protocol is the backdrop for the whole
  CCNA exam. See prior chapter notes on IP
  Addressing for more info.
• Next, some routing protocols that are strongly
  rumored to be on the CCNA 801 will be outlined.
• But first, some generalities about routing
  protocols

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Main Goals of Routing Protocols

• To fill the routing table with current best,
  loop-free routes
• To notice when routes in the table are no longer
  valid and remove them from the routing table
• To add new routes or replace lost routes
• The time for finding a working route is called
  convergence.

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Two Categories of Routing Protocols

• Exterior Routing Protocols used for use between
  different organizations such as ISPs or ISPs and
  their customers.
• Ex Border Gateway Protocol (BGP)
• Interior Routing Protocols used to distribute
  routing information inside a single organization.
  
• Ex RIP, IGRP, EIGRP, OSPF, IS-IS

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Border Gateway Protocol (BGP)

• The most popular exterior routing protocol the
  only one on the CCNA 801 exam
• ISPs use BGP to exchange routing info between
  themselves and other ISPs and customers.
• BGP advertises only routing info to specifically
  defined peers using TCP.
• BGP does not use a metric like internal routing
  protocols

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Terminology of Interior Routing Protocols

• This is not as painful as it sounds. There are
  only 6 basic concepts.

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TYPE of routing protocol

• Each interior routing protocol can be
  characterized based on the underlying logic used
  by the routing protocol.
• The underlying logic is referred to as the TYPE
  of routing protocol.
• The three types are
• Distance vector
• Link-state
• Hybrid

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Full/partial Update

• Full routing updates entire routing tables are
  sent regularly
• Partial routing updates only a subset of the
  routing table is sent, typically just information
  about changed routes.
• Partial routing updates require less overhead
  than full routing updates.

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Convergence

• Convergence refers to the time required for
  routers to react to changes in the network.

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Metric

• The metric refers to the numeric value that
  describes how good a particular route is.
• The lower the value, the better the route.

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Support for VLSM

• Variable-length subnet masking (VLSM) means that,
  in a single Class A, B, or C network, multiple
  subnet masks can be used.
• The advantage of VLSM is that it enables you to
  vary the size of each subnet, based on the needs
  of that subnet.
• Some routing protocols support VLSM, and some do
  not

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Classless or Classful

• Classless routing protocols transmit the subnet
  mask along with each route in the routing updates
  sent by that protocol.
• Classful routing protocols do not transmit mask
  information.
• Only classless routing protocols support VLSM. To
  say that a routing protocol is classless is to
  say that it supports VLSM.

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Now we apply those terms to some interior routing
protocols.

• .

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Distance Vector Protocols RIP and IGRP

• Distance vector protocols advertise routing
  information by sending messages, called routing
  updates, out the interfaces on a router.
• These updates contain a series of entries, with
  each entry representing a subnet and a metric.
• Failure to receive updates from a neighbor in a
  timely manner results in the removal of the
  routes previously learned from that neighbor.

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Distance Vector Protocols RIP and IGRP

• Routers send periodic full updates and expect to
  receive periodic updates from neighboring
  routers.
• When possible, routers use broadcasts or
  multicasts to send routing updates. This way, all
  neighbors on a LAN can receive the same routing
  information in a single update.
• If a router learns multiple routes to the same
  subnet, the router chooses the best route based
  on the metric.

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Routing Information Protocol (RIP)

• Been around 15 years for use with IP networks.
• Easier to use than some newer routing protocols,
  but severely limited in comparison.

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Basic RIP Summary

• Based on distance vector logic
• Uses hop count for the metric
• Hop count number of routers between two points
• Sends periodic full routing updates every 30
  seconds
• Converges slowly, often taking 3 to 5 minutes
• Does not support VLSM, also making it a classful
  routing protocol

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Interior Gateway Routing Protocol (IGRP)

• IGRP is a Cisco-proprietary IP routing protocol
  created to provide a better distance vector
  protocol.
• The most obvious difference between RIP-1and
  IGRP is the metric.
• IGRP advertises up to five parameters that
  describe the metric for each route, although, by
  default only two are used bandwidth and delay.

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IGRP

• Other three possible parameters used to describe
  IGRP metric can include reliability, load, and
  MTU (maximum transmission unit).
• IGRP calculates the metric based on a
  mathematical formula that you do not really need
  to know for the exam. (Wendell Odom, CCNA INTRO,
  p.415)

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Distance Vector Protocols
Feature RIP (v1) IGRP
Update timer for full routing updates 30 seconds 90 seconds
Metric Hop Count Function of bandwidth and delay (default)
Supports VLSM No No
Infinite-metric value 16 4,294,967,295
Convergence Slow Slow
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Link-State Protocols OSPF and Integrated IS-IS

• The goal of link-state protocols is to fill the
  routing tables with the current best routes.
• Link-state advertises a large amount of
  topological info about the network
• Discovers neighbor routers before exchanging
  routing information.
• A router running a link-state protocol uses more
  memory and more processing cycles than do
  distance vector protocols.

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Link-State Protocols OSPF and Integrated IS-IS

• To figure out the current best routes, a router
  processes the link-state topology database using
  an algorithm called the Dijkstra Shortest Path
  First (SPF) algorithm.
• This info helps link-state protocols avoid loops
  converge quickly.
• Quick convergence often less than 10 seconds.

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Open Shortest Path First (OSPF)

• OSPF is the most popular link-state IP routing
  protocol today.
• Because OSPF does not send full updates on a
  regular short interval (like RIP), the overall
  number of bytes sent for routing information is
  typically smaller.

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Open Shortest Path First (OSPF)

• OSPF uses a concept called cost for the metric.
  Each link is considered to have a cost a routes
  cost is the sum of the cost for each link.
• By default, Cisco derives the cost value for a
  link from the bandwidth.
• OSPF supports VLSM.

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Integrated IS-IS

• OSI defines a network layer protocol called the
  Connectionless Network Protocol (CLNP). It also
  defines a routing protocol a routing protocol
  used to advertise CLNP routes, called
  Intermediate System-to-Intermediate System
  (IS-IS). IS-IS advertises CLNP routes between
  intermediate systems, which is what OSI calls
  routers.

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Integrated IS-IS

• Integrated IS-IS has the capability to advertise
  IP routes as well as CLNP routes.
• most installations could care less about CLNP.
  (Wendell Odom, CCNA INTRO, p.419)
• Supports VLSM

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Balanced Hybrid Protocols Enhanced IGRP

• EIGRP uses features similar to link-state
  protocols, and others similar to distance vector
  protocols, and yet others unlike either of the
  two.
• The internal workings of EIGRP depend on an
  algorithm called the Diffusing Update Algorithm
  (DUAL).
• Requires less processing than the Dijkstra SPF
  algorithm.

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EIGRP Summary

• A balanced hybrid protocol
• Converges in less than 3 seconds
• Discovers neighbors (via Hello packets) before
  sending them information.
• Requires little design effort
• Supports VLSM
• Cisco proprietary
• Metric based on bandwidth delay, scaled by
  multiples of 256.

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Thats IT

• Next week, May 3, to be discussed
• -- Routing Protocol Configuration Commands
• -- Routing Protocol Logic
• -- VLSM