Routing/Routed Protocols

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Routing/Routed Protocols

• Part 2

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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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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• To notice when routes in the table are no longer
  valid and remove them from the routing table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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Key Points of Interior Routing Protocols

• 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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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

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No VLSM with RIPv1 or IGRP

• Neither RIPv1 nor IGRP routing protocols has a
  field for subnet information.
• If a router running RIPv1 has a subnet mask of a
  certain value, it assumes that all interfaces
  interfaces within the classful address space have
  the same have the same subnet mask.
• This is called classful routing, and RIP and IGRP
  are both considered classful routing protocols.

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VLSM with RIPv2, EIGRP, OSPF

• Classless routing protocols do support the
  advertisement of subnet information.
• Use VLSM with routing protocols such as RIPv1,
  EIGRP, or OSPF.
• The advantage with this type of network is that
  bunches of IP addresses are saved.

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More Routing Protocol

• Basics

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Static vs. Dynamic Routing

• Static routing occurs when you manually add
  routes in each routers routing table. It will be
  covered in subsequent routing command procedures
  since it is mainly demonstrated through routing
  commands.
• Dynamic routing is when protocols are used to
  find networks and update routing tables on
  routers.

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Interior vs. Exterior Gateway Protocols

• IGPs are used to exchange routing information
  with routers in the same autonomous system (AS).
• An AS is a collection of networks under a common
  administrative domain, which basically means that
  all routers sharing the same routing table
  information are in the same AS.
• EGPs are used to communicate between Ass such as
  in WAN links.

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Administrative Distances

• The administrative distance (AD) is used to rate
  the trustworthiness of routing information
  received on a router from a neighbor router.
• An administrative distance is an integer from 0
  to 255, where 0 is the most trusted and 255 means
  NO traffic will be passed via this route.

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Default Administrative Distances
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The Three Classes of Routing Protocols

• Distance Vector finds the best path to a remote
  network using hop count. (RIP, IGRP)
• Link State (also called shortest-path-first
  protocols) the routers each create three
  separate tables. 1) keeps track of directly
  attached neighbors, 2) topology of network, 3)
  the routing table. (OSPF, IS-IS)
• Hybrid uses aspects of both distance vector and
  link state. (EIGRP)

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Distance Vector Terms

• See board for routing loop example.
• Maximum Hop Count prevents counting to
  infinity.
• RIP maximum hop count 15 (16 infinity)
• IGRP maximum hop count default 100
• Split Horizon reduces incorrect routing
  information and routing overhead by enforcing the
  rule that routing information cannot be sent back
  in the direction from which it was received.

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Distance Vector Terms

• Route Poisoning occurs when an unreachable,
  or infinite, message is distributed when a link
  is down.
• Holddown Timer A holddown prevents regular
  update messages from reinstating a route that is
  going up and down (called flapping).
  Typically, this happens on a serial link that is
  losing connectivity and then coming back up.
  Network might never converge otherwise.

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Holddown Timers

• There are 3 instances when triggered updates will
  reset the holddown timer
• 1) The holddown timer expires
• 2) Another update is received with a better
  metric
• 3) A flush time, which is the time a route would
  be held before being removed, removes the route
  from the routing table when the timer expires.

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Routing Information Protocol (RIP)

• A true distance-vector routing protocol.
• Sends the complete routing table to all active
  interfaces every 30 seconds.
• RIP only uses hop count to determine the best way
  to a remote network max hop count 15, with 16
  being infinite, or unreachable.
• Works well in small networks.
• RIPv1 uses only classful routing.

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Interior Gateway Routing Protocol (IGRP)

• Cisco-proprietary distance-vector routing
  protocol (must use only Cisco routers).
• Classful
• Default max hop count 100.
• Can be used in large networks.
• Uses a different metric than RIP IGRP uses
  bandwidth and delay of line by default. This is
  called a composite metric.
• Reliability, load, and MTU can also be used,
  although they are not by default.

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Open Shortest Path First (OSPF) Basics

• An open standards routing protocol (non-Cisco
  proprietary
• Good for large networks (over 15 routers)
• Uses the Dijkstra algorithm
• Only supports IP routing
• Classless

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OSPF Logic

• Each router discovers its neighbors on each
  interface. The list of neighbors is kept in a
  neighbor table.
• Each router uses a reliable protocol to exchange
  topology information with its neighbors.
• Each router places the learned topology
  information in its topology database.

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OSPF Logic (cont.)

• 4) Each router runs the SPF algorithm against its
  own topology database to calculate the best
  routes to each subnet in the database.
• 5) Each router places the best route to each
  subnet in the IP routing table.

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OSPF

• A link-state protocol uses a neighbor table and a
  topology database in addition to adding routes to
  the routing table.
• OSPF converges more quickly than do
  distance-vector protocols.
• Uses Hello packets to keep in touch with
  neighbors.
• Defined in RFC 2328

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Enhanced Interior Gateway Routing Protocol (EIGRP)

• Cisco proprietary
• Classless
• Uses autonomous system numbers
• A number assigned to a group of routers under
  mutual administration.
• Referred to as a hybrid routing protocol
• Provides support for IP, IPX, and Appletalk
• Best path selection using the Diffusing Update
  Algorithm (DUAL)

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EIGRP

• Before EIGRP routers exchange routes with each
  other, they must become neighbors.
• There are three conditions that must be met for
  neighborship establishment
• Hello or ACK received
• AS numbers match
• Identical metrics

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EIGRP Feature Comparison

• Link-state Features
• Converges quickly
• Discovers neighbors via Hello packets
• Builds topology table
• After learning its neighbors routes, only
  changes to the routing table are propagated.

• Distance-vector Features
• Uses autonomous system number (like IGRP)
• Uses metric based on bandwidth delay
• Advertises entire routing table to new neighbors.

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