Router, Routing

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??????Router, Routing RIPChap 1, 2, 3, 4
5, Semester 2
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Overview

• Routers
• Routing Basics
• Why Routing Protocols are necessary?
• Distance-Vector Routing / Link State Routing
• The Context of Different Routing Protocols
• RIP

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Chap 1Router Basics Routing
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Router Basics
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Router Basics

• Routers are Computers
• OS, CPU, Memory, RAM, Flash (HD), etc
• Routers are at the network center
• Routers determine the best path

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Router Bootup Process
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Show version
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Router Interfaces
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Router OSI 7-layers
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Basic Router Configuration

• Naming the router
• Setting passwords
• Configuring interfaces
• Configuring a banner
• Saving changes on a router
• Verifying basic configuration and router
  operations

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Routing Table (show ip route)
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Routing Table (show ip route)
Next hop address
exit interface
Hop count
Administrative Distance
seconds passed since last update
Destination network
RIP is actually running on this router
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Packet forwarding

• Involves two functions
• Path determination function
• Switching function

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Path Determination

• The process of how the router determines which
  path to use when forwarding a packet.
• To determine the best path, the router searches
  its routing table for a network address that
  matches the packet's destination IP address.

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Routing Table
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One of three path determinations results

• Directly Connected Network
• Remote Network
• No Route Determined
• Is default route configured?

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Switching Function

• The process used by a router to accept a packet
  on one interface and forward it out another
  interface.
• A key responsibility of the switching function is
  to encapsulate packets in the appropriate data
  link frame type for the outgoing data link.

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Best Path

• Determining a router's best path involves the
  evaluation of multiple paths to the same
  destination network and selecting the optimum or
  "shortest" path to reach that network.

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Two commonly used metrics are

• Hop count
• Bandwidth
• Of course there are many others

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Hop counts vs Bandwidth
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Equal Cost Load Balancing

• When a router has multiple paths to a destination
  network and the value of that metric is the same
• equal cost metric ? the router will perform equal
  cost load balancing
• The routing table will contain the single
  destination network but will have multiple exit
  interfaces, one for each equal cost path.

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Unequal cost load balancing

• EIGRP (as well as IGRP) are the only routing
  protocols that can be configured for unequal cost
  load balancing.

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Review

• A day in the life of a packet
• Make sure the life of a packet is in your day
  of networking

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How routers route packets from source to
destination?

• The KEY IS (again) Network Address Routing
  Table

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Router OSI 7-layers
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Chap 2

• Trouble-shooting
• Static route
• CDP Cisco Discovery Protocol

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Router Trouble-Shooting
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show ip route?
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show interface?
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show running-config
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show cdp neighbors detail(we did not mention
this before Details later )
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Verify changes to the routing table with debug
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CDP Cisco Discovery Protocol
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Cisco CDP

• Cisco Discovery Protocol (CDP) is a Layer 2
  protocol that connects lower physical media and
  upper network layer protocols

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Cisco CDP

• When a Cisco device boots up, CDP starts up
  automatically and allows the device to detect
  neighboring devices that are also running CDP.
• It runs over the data link layer and allows two
  systems to learn about each other, even if they
  are using different network layer protocols.

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Cisco CDP

• CDP is used to obtain information about
  neighboring devices, such as
• types of devices connected,
• the router interfaces they are connected to,
• the interfaces used to make the connections,
• the model numbers of the devices.
• CDP is media and protocol independent

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CDP provides the following information about each
CDP neighbor device

• Device identifiers - For example, the configured
  host name of a switch
• Address list - Up to one Network layer address
  for each protocol supported
• Port identifier - The name of the local and
  remote port-in the form of an ASCII character
  string such as ethernet0
• Capabilities list - For example, whether this
  device is a router or a switch
• Platform - The hardware platform of the device
  for example, a Cisco 7200 series router

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show cdp neighbors
How long is this information valid
CDP frame can retrieve a great deal of useful
information about connected neighboring Cisco
devices. This information can be used to create a
network map of the connected devices
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Terms (of different bridges)

• T Transparent Bridge
• Ethernet
• Bridge that learns Switching Table
  transparently
• B Source Route Bridge
• Token Ring
• Developed by IBM

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CDP Version

• CDP Version 2 (CDPv2) is the most recent release
  of the protocol.
• Cisco IOS (Release 12.0(3)T or later) supports
  CDPv2.
• CDP Version 1 (CDPv1) is enabled by default with
  Cisco IOS (Release 10.3 to 12.0(3)T).

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Information exchanged through CDP

• CDP provides information about each CDP neighbor
  device by transmitting type length values (TLVs),
  which are blocks of information embedded in CDP
  advertisements.
• Device TLVs include
• Device ID, Local Interface, Holdtime, Capability,
  Platform, Port ID
• (CDPv2 only) VTP Management Domain Name, Native
  VLAN, Full/Half-Duplex

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CDP Commands
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Show CDP
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Configure cdp(Setting the holdtime Timer)
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Getting Neighbor Information
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Show CDP Entry
Will show neighbors enabled routed protocols
addresses
Version Information
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Show cdp neighbors detail
CDP version 2
Detail adds network address, enabled protocols,
and software version
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Show CDP Traffic
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Show CDP Traffic
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Show CDP Interface
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Show CDP Interface
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Disable CDP
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Static route
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A router can learn about remote networks in one
of two ways

• Manually, from configured static routes
• Automatically, from a dynamic routing protocol

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Static vs Dynamic Routes
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Why use static route?(Stub Network)
Running a routing protocol between R1 and R2 is
a waste of resources
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Configuring static route (1)
192.168.2.0 255.255.255.0 172.17.2.2
R1
R1
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Static route setup using next-hop address(Side
effect Recursive Lookup)
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Configuring static route (2)
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Differences of the above two

• Using Interface
• Default Administrative Distance 0
• the same as a directly connected network
• Using Next-Hop IP address
• Default Administrative Distance 1
• Default Administrative Distance can be changed
• waycross(config)ip route 172.16.3.0
  255.255.255.0 172.16.4.1 130

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Static route setup using exit interface
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Modifying Static Routes
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Verify Static Routeshow ip route
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Does it matter if the next-hop is via
Point-to-Point or Multi-hop link?
Point-to-point WAN
Ethernet
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Difference between Point-to-Point and Multi-hop
link?

• Point-to-Point
• ip route 172.16.3.0 255.255.255.0 172.16.4.1
• ip route 172.16.3.0 255.255.255.0 s0/0? s0/0
  also directs to 172.16.4.1
• Multihop
• ip route 172.16.3.0 255.255.255.0 172.16.4.1
• ip route 172.16.3.0 255.255.255.0 fa0/0? fa0/0
  may link to a number of next-hop
  destinations (multi-access connection)

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And

• Remember ARP?
• which is required for Ethernet to map IP address
  to MAC address

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Now, do you see the difference between these two?

• ip route 172.16.3.0 255.255.255.0 172.16.4.1
• ip route 172.16.3.0 255.255.255.0 fa0/0
• For static route configured using exit interface
• the Router does not know the next-hop IP address
  and therefore it cannot determine the destination
  MAC address for the Ethernet frame.

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Any way to fix it?

• ip route 172.16.3.0 255.255.255.0 fa 0/0
  172.16.4.1Routing table then looks likeS
  172.16.3.0/24 1/0 via 172.16.4.1 FastEthernet0/0

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Summary Static Route
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Route Summarization

• Multiple static routes can be summarized into a
  single static route if
• The destination networks can be summarized into a
  single network address, and
• The multiple static routes all use the same
  exit-interface or next-hop IP address

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Route Summarization - Example
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Configuring a Summary Route
R3(config)no ip route 172.16.1.0 255.255.255.0
serial0/0/1 R3(config)no ip route 172.16.2.0
255.255.255.0 serial0/0/1 R3(config)no ip route
172.16.3.0 255.255.255.0 serial0/0/1
R3(config)ip route 172.16.0.0 255.255.252.0
serial0/0/1
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How does router deal with this
Part of routing table looks like
And, a packet destined for 172.16.1.100, which
matches both routes, arrives
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Most Specific Match

• The routing table lookup process will use the
  most-specific match.
• Because 24 bits match the 172.16.1.0/24 route,
  and only 16 bits of the 172.16.0.0/16 route
  match, the static route with the 24 bit match
  will be used.
• the longest match.
• The packet will then be encapsulated in a Layer 2
  frame and sent via the Serial 0/0/0 interface.

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Default Static Route

• Default static routes are used when a route will
  match all packets.
• When connecting a company's edge router to the
  ISP network.
• A stub router.

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Default Route

• 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

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Default Route
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Configuring default route
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Configure a default route
R1(config)no ip route 172.16.1.0 255.255.255.0
serial 0/0/0 R1(config)no ip route 192.168.1.0
255.255.255.0 serial 0/0/0 R1(config)no ip route
192.168.2.0 255.255.255.0 serial
0/0/0 R1(config) ip route 0.0.0.0 0.0.0.0
serial 0/0/0
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Practice

• Please go through the routing process depicted in
  Section 2.7.1 (Static Routes and Packet
  Forwarding)

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Chap 3Dynamic Routing Protocol
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What is dynamic routing protocol?

• Routing protocols are used to facilitate the
  exchange of routing information between routers.
• Routing protocols allow routers to dynamically
  share information about remote networks and
  automatically add this information to their own
  routing tables.

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Dynamic Routing

• The success of dynamic routing depends on two
  basic router functions
• Maintenance of a routing table
• Timely distribution of knowledge, in the form of
  routing updates, to other routers 

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Dynamic Routes Update
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Components of a routing protocol

• Data structures
• Routing information kept in tables and/or
  databases and stored in RAM.
• Algorithm
• Routing protocols use algorithms for facilitating
  routing information and for best path
  determination.
• Routing protocol messages
• Routing protocols use various types of messages
  to discover neighboring routers, exchange routing
  information, and other tasks to learn and
  maintain accurate information about the network.

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Routing protocol

• A routing protocol defines the set of rules used
  by a router when it communicates with neighboring
  routers.
• For example, a routing protocol describes
• how to send updates
• what knowledge is contained in these updates
• when to send this knowledge
• how to locate recipients of the updates

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Dynamic Routing Protocol Operation

• The router sends and receives routing messages on
  its interfaces.
• The router shares routing messages and routing
  information with other routers that are using the
  same routing protocol.
• Routers exchange routing information to learn
  about remote networks.
• When a router detects a topology change the
  routing protocol can advertise this change to
  other routers.

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Dynamic vs Static Routing
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Dynamic Routing Protocol Classification
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Interior vs Exterior
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Autonomous System??(??)??

• Internet ???????????(??)?????
• IBM ????????? AS
• Hinet (may contain one or many ASs)
• Seednet (so is Seednet)

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Autonomous System

• ?? Autonomous System ?????????? Routing ??
• IGP Interior Gateway Protocol
• ASs ???? Routing ???
• EGP Exterior Gateway Protocol(BGP Border
  Gateway Protocol)

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AS ??? Routing
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Distance-Vector

• A router periodically sends all of its neighbors
  two pieces of information about the destinations
  it knows how to reach
• how far away (it thinks) the destination is
• what direction to use to get to the destination

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Distance-Vector Protocol

• Distance ??
• How many hops away from destination(sometimes,
  distance may be measured in time delay or else)
• Vector ??
• Which way to some destinatione.g., send packet
  to me if you want to reach some destination

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Link-State Protocol

• A router provides information about the topology
  of the network in its immediate vicinity
• links it attaches to
• state (functioning or not) of those links
• The information is then flooded to the network
• Each router computes its best path (bases on the
  above information) to all destinations

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Link-State Protocol

• Which path is best is up to each protocol
• path with the least number of hops
• based on bandwidth, current load on the link,
  administrative weights, or even policy
  information restricting which packets may
  traverse the link (e.g., a given link might not
  be allowed to carry confidential information)

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Distance vector protocols work best in situations

• The network is simple and flat and does not
  require a special hierarchical design.
• The administrators do not have enough knowledge
  to configure and troubleshoot link-state
  protocols.
• Specific types of networks, such as hub-and-spoke
  networks, are being implemented.
• Worst-case convergence times in a network are not
  a concern.

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Hub Spoke
Spoke
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Link-state protocols work best in situations

• The network design is hierarchical, usually
  occurring in large networks.
• The administrators have a good knowledge of the
  implemented link-state routing protocol.
• Fast convergence of the network is crucial.
• Convergence later

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Distance-vector vs Link-state
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DV vs LS

• ???????
• Neighbors vs All
• ????????
• ?? vs ?????
• ???????
• ????? vs ?????? local ??
• ????????
• ??? vs SPF

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Classful Routing
Classful routing protocols do not send subnet
mask information in routing updates.
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Classless Routing
Classless routing protocols include the subnet
mask with the network address in routing
updates.
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Convergence

• Convergence is when all routers' routing tables
  are at a state of consistency.
• The network has converged when all routers have
  complete and accurate information about the
  network.

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Time to Convergence

• Convergence time is the time it takes routers to
  share information, calculate best paths, and
  update their routing tables.
• Fast convergence is a desirable network feature
  because it reduces the period of time in which
  routers would continue to make incorrect/wasteful
  routing decisions.

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Criteria for Path Determination

• When a routing algorithm updates a routing table,
  its primary objective is to determine the best
  information to include in the table.
• Each routing algorithm interprets what is best in
  its own way.
• The algorithm generates a number, called the
  metric value, for each path through the network.
• Typically, the smaller the metric number, the
  better the path

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Criteria for Path Determination

• You can calculate metrics based on a single
  characteristic of a path
• You can also calculate more complex metrics by
  combining several characteristics.

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Most commonly used metrics - 1

• Hop count - A simple metric that counts the
  number of routers a packet must traverse
• Bandwidth - Influences path selection by
  preferring the path with the highest bandwidth
• Load - Considers the traffic utilization of a
  certain link

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Most commonly used metrics - 2

• Delay - Considers the time a packet takes to
  traverse a path
• Reliability - Assesses the probability of a link
  failure, calculated from the interface error
  count or previous link failures
• Cost - A value determined either by the IOS or by
  the network administrator to indicate preference
  for a route. Cost can represent a metric, a
  combination of metrics or a policy.

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The metric for each routing protocol is

• RIP
• Hop count
• IGRP and EIGRP
• Bandwidth, Delay, Reliability, and Load
• IS-IS and OSPF
• Cost
• Cisco's implementation of OSPF uses bandwidth.
• IS-IS is discussed in CCNP.

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Metric Example with RIP
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Load balancing with RIP
The interface pointer cycles through the
interfaces and routes in a deterministic fashion
such as 1-2-3-4-1-2-3-4-1 and so on. Because the
metric for RIP is hop count, no regard is given
to the speed of the links.
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Choose best path from 2 routing tables generated
by 2 routing protocols
How R2 determine the best path to 192.168.6.0
network?
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Administrative Distance
90 vs 120
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Administrative Distance of Various Protocols
The administrative distancecan be modified.
(CCNP)
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Criteria in selecting the best path

• Route with the lowest administrative distance
• Path with the lowest cost or metric to the
  destination
• If, multiple paths with the same administrative
  distance and cost to a destination,
  load-balancing can occur.

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Distance Vector Routing Protocol
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With distance vector routing protocol

• A router does not have the knowledge of the
  entire path to a destination network.
• Instead the router knows only
• The direction or interface in which packets
  should be forwarded and
• The distance or how far it is to the destination
  network

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Distance Vector Routing Protocol

• Periodic Updates
• Neighbors
• Broadcast Updates
• Not necessarily true
• Entire Routing Table Updates

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Advantages Disadvantages of Distance Vector
Protocol
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Routing Protocol Learning Check
Later
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Network Discovery with DVRP
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Dynamic Routing Protocol Evolution
Classification
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Routing Loop
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The loop may be a result of

• Incorrectly configured static routes
• Incorrectly configured route redistribution
  (redistribution is a process of handing the
  routing information from one routing protocol to
  another routing protocol and is discussed in
  CCNP-level courses)
• Inconsistent routing tables not being updated due
  to slow convergence in a changing network
• Incorrectly configured or installed discard
  routes

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A routing loop can create the following
conditions

• Link bandwidth will be used for traffic looping
  back and forth between the routers in a loop.
• A router's CPU will be strained due to looping
  packets.
• A router's CPU will be burdened with useless
  packet forwarding that will negatively impact the
  convergence of the network.
• Routing updates may get lost or not be processed
  in a timely manner. These conditions would
  introduce additional routing loops, making the
  situation even worse.
• Packets may get lost in "black holes."

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Mechanisms to eliminate routing loops

• Defining a maximum metric to prevent count to
  infinity
• Holddown timers
• Split horizon
• Route poisoning or poison reverse
• Triggered updates

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TTL (in IP packet) in effect
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Infinity 16 with RIP
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Holddown Timer
Holddown timers are used to prevent regular
update messages from inappropriately reinstating
a route that may have gone bad. Any other
information for that route containing the same
status, or worse, is ignored for a predetermined
amount of time (the holddown period).
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Route Poisoning
Route poisoning is used to mark the route as
unreachable in a routing update that is sent to
other routers.
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Split Hosizon
The split horizon rule says that a router should
not advertise a network through the interface
from which the update came.
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Split Horizon with Poison Reverse
R3 poisons the metric with a value of 16 and then
sends out a triggered update stating that
10.4.0.0 is unavailable. R2 processes that
update, invalidates the routing entry in its
routing table, and immediately sends a poison
reverse back to R3.
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Triggered Updates
Triggered updates, used in conjunction with route
poisoning, ensure that all routers know of failed
routes before any holddown timers can expire.
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RIPRouting Information Protocol
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Characteristics of RIP
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Configuring RIP
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Verify RIP configuration
Next hop address
exit interface
Hop count
Administrative Distance
seconds passed since last update
Destination network
RIP is actually running on this router
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Verify RIP configuration
Timers
CCNP
Only RIP used
Load balancing route counts
Connected networks
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Debug RIP
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No Router beyond Fa0/0
However, updates keep sendingout through Fa0/0
RIPv1 uses broadcast!
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Stopping Unnecessary RIP Updates(with
passive-interface)
Fa0/0 is not listed here any more
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Automatic Sumarization

• RIP is a classful routing protocol that
  automatically summarizes classful networks across
  major network boundaries.
• ?? Class A, B C

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Automatic Summarization
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Automatic Summarization
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Rules for Automatic Summarization

• If a routing update and the interface on which it
  is received belong to the same major network, the
  subnet mask of the interface is applied to the
  network in the routing update.
• If a routing update and the interface on which it
  is received belong to different major networks,
  the classful subnet mask of the network is
  applied to the network in the routing update.

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Rules in Chinese

• ? routing update ???????? update ??????? ?
  ??????? subnet mask
• ? routing update ???????? update ??????? ? ??
  routing update ???? default subnet mask (? A, B
  C ?)

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Can you figure out the contents of each routers
routing table?
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R2 debug output
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Routing table for R1
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Routing table for R3
WHY??? Look back the rules for summarization
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Advantages of Automatic Summarization

• Smaller routing updates sent and received, which
  uses less bandwidth for routing updates between
  R2 and R3.
• R3 has a single route for the 172.30.0.0/16
  network, regardless of how many subnets there are
  or how it is subnetted. Using a single route
  results in a faster lookup process in the routing
  table for R3.

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Disadvantages of Automatic Summarization
Discontinuous networks
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Disadvantages of Automatic Summarization
Routing Table for R1
OOPS ?R1 does not have any routes to the LANs
attached to R3
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Disadvantages of Automatic Summarization
Routing Table for R3
OOPS ?R3 does not have any routes to the LANs
attached to R1
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Disadvantages of Automatic Summarization
Routing Table for R2
OOPS ?R2 has two equal-cost paths to the
172.30.0.0 network ?R2 will load balance traffic
destined for any subnet of 172.30.0.0. This
means that R1 will get half of the traffic
and R3 will get the other half of the traffic
whether or not the destination of the
traffic is for one of their LANs
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Adding Internet Access to the Network
1. Disable RIP routing for network 192.168.4.0 on
R2. 2. Configure R2 with a static default route
to send default traffic to R3. 3. Completely
disable RIP routing on R3. 4. Configure R3 with
a static route to the 172.30.0.0 subnets.
Stub Network
172.30.3.0/24
172.30.2.0/24
172.30.1.0/24
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Configuration
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Routing Tables
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Default Route Propagation