Routing and Routing Protocols

1
Routing and Routing Protocols

• Module 6
• CCNA 2 Version 3.0

2
Introduction to Routing

• Routing - the process that a router uses to
  forward packets toward the destination network
• A router makes decisions based upon the
  destination IP address of a packet
• Routers must learn the direction to remote
  networks
• Dynamic routing - info is learned from other
  routers
• Static routing - a network admin configures
  information about remote networks manually

3
Static Route Operation

• Static route operations can be divided into these
  three parts
• Network administrator configures the route
• Router installs the route in the routing table
• Packets are routed using the static route
• Two commands to configure a static route

Specify Outgoing Interface
Specify Next Hop Address
4
(No Transcript)
5
(No Transcript)
6
Static Routing andAdministrative Distance

• Only difference between choose outgoing interface
  and next-hop address is the administrative
  distance
• an optional parameter that gives a measure of the
  reliability of the route
• Lower is better (more reliable)
• A route with a lower administrative distance will
  be installed before an identical route with a
  higher administrative distance
• Default administrative distance when using
  next-hop address is 1
• Default administrative distance when using the
  outgoing interface is 0 (same as a directly
  connected network)
• So which is more reliable next-hop or outgoing
  interface?

7
Static Routing andAdministrative Distance

• Administrative distance can be assigned any value
  between 0 and 255 using the following command
• waycross(config)ip route 172.16.3.0
  255.255.255.0 172.16.4.1 130
• Routers learn about routes to a network in 3 ways
• Info is gathered from its own config regarding
  directly connected networks
• Other routers forward info. about known NWs
• Routes are entered manually by a network admin

8
More on Static Routing

• If an interface that is used in configuring a
  static route is down, the route will not be
  placed in the routing table (network may be
  unreachable)
• 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
• To do this, set the administrative distance of
  the static route higher than that of the dynamic
  routing protocol being used

9
Static Routing Default Route Forwarding

• Default routes - 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 (routers cant
  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

10
Verifying Static Router Configuration Operation

• show running-config
• used to view the active configuration in RAM to
  verify that the static route was entered
  correctly
• show ip route
• used to make sure that the static route is
  present in the routing table (indicated with an
  s)

11
Routed vs. Routing Protocols

• Examples of routed protocols are
• Internet Protocol (IP)
• Internetwork Packet Exchange (IPX)
• 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)

12

• Routing protocol
• Used to build and maintain a routing table
• Tables contain learned NWs and associated ports
  for those NWs
• Learns all available routes
• Places the best routes into the routing table
• Removes routes when they are no longer valid
• Protocol is used to manage info recd from other
  routers, info. learned from the config of its own
  interfaces, and manually configured routes

13

• Routed protocol
• Used to direct traffic
• Provides enough info to allow packets to be
  forwarded from one host to another based on the
  addressing scheme

14
Autonomous System

• 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
• Presents 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, require
  assignment of a unique, autonomous system number
  during configuration

15
Autonomous System

• Exterior gateway protocols are used between
  autonomous systems

16
More on Autonomous Systems

• The goal of a routing protocol is to build and
  maintain the routing table
• Routers use routing protocols and routing
  algorithms to manage information received from
  other routers
• Convergence - when all routers in an internetwork
  are operating with the same knowledge (all have
  same routing table info)
• Fast convergence is desirable because it reduces
  the period of time in which routers would
  continue to make incorrect routing decisions
• Fast convergence depends on dynamic routing
  protocols and routing algorithms to choose the
  best path to enter in the routing table
• Autonomous systems (AS) divide the global
  internetwork into smaller and more manageable
  networks

17
Routing Protocols

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

18
Distance-Vector Routing Protocols

• Distance-vector routing protocols pass regular
  updates that communicate topology changes to
  directly connected routers
• Also known as Bellman-Ford algorithms
• Do not allow a router to know the exact topology
  of an internetwork

19
Distance-Vector Routing Protocols

• 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
• Topology change updates proceed step-by-step from
  router to router
• Convergence happens once all routers are aware of
  the topology change

20
Distance Vector

• Analogy signs found at a highway intersection
• Sign points towards a destination and indicates
  the distance
• Further down the road, another sign points toward
  the destination, but now the distance is short
• As long as the distance is shorter, the traffic
  is following the best path

21
Link-State Routing

• Link-state Routing Algorithms
• are also known as Dijkstras algorithm or as SPF
  (shortest path first) algorithms
• maintain a complex database of topology
  information
• maintain full knowledge of distant routers and
  how they interconnect unlike distance vectors
  which have nonspecific info about distant NWs and
  no knowledge of distant routers

22
More on Link-State Routing

• Link-state routing uses
• Link-state advertisements (LSAs) 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 calculation performed on the
  database resulting in the SPF tree (used to
  calculate the shortest path to other networks)
• Routing tables A list of the known paths and
  interfaces
• Each router develops its own map of the entire NW
• Routers send triggered updates when changes in NW
  occur
• The router that first becomes aware of a topology
  change forwards the info so that all other
  routers can use it for updates

23
More on Link-State Routing

• Link-state concerns
• Processor overhead
• Memory requirements
• Bandwidth Consumption
• LS require more memory
• Initial LS packets flooding consumes BW
• After flooding, LS generally require only minimal
  BW to send infrequent topology changes

24
Path Determination

• A router determines the path of a packet from one
  data link to another, using two basic functions
• A path determination function
• Path determination occurs at the network layer
• A switching function
• 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
• Key responsibility includes encapsulating packets
  in the appropriate frame type for the next data
  link

25
Routing Configuration

• An example of a routing configuration is
• For RIP
• GADconfig t
• GAD(config)router rip
• GAD(config-router)network 172.16.0.0
• For IGRP
• GADconfig t
• GAD(config)router igrp 100
• GAD(config-router)network 172.16.0.0
• NOTE The network numbers are based on the
  network class addresses, not subnet addresses or
  individual host addresses
• Major network addresses are limited to Class A,
  B, and C network numbers

Need to specify NWs to tell router which
attached NWs participate in routing updates
26
Examining a Routing Table
How Route was Learned
Next Hop to Destination Network
Interface through which to forward packet
Administrative Distance
Destination Networks
Metric (Hop Count)
27
Routing Protocols Overview
28
RIP (Routing Information Protocol)

• Key characteristics include the following
• It is a distance vector routing protocol.
• Hop count is used as the metric for path
  selection.
• Maximum hop count is 15 (then packet is
  discarded)
• Routing updates are broadcast every 30 seconds,
  by default.

29
IGRP (Interior GatewayRouting Protocol)

• 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.

30
OSPF (Open Shortest Path First)

• 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

31
EIGRP (Enhanced Interior Gateway 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 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.

32
BGP (Border Gateway Protocol)

• Border Gateway Protocol (BGP) is an exterior
  routing protocol
• The key characteristics of BGP are as follows
• It is a distance vector exterior routing
  protocol.
• Used between ISPs or ISPs and clients
• Used to route Internet traffic between autonomous
  systems

33
Distance-Vector vs. Link-State Routing

• Advantages of Link-State over Distance-Vector
• Less prone to routing loops routing errors
• More scalable
• More reliable
• Easier to debug
• Less Bandwidth intensive
• Disadvantages of Link-State Routing
• Uses more system resources
• Can be more expensive to implement and support

34
Routing and Routing Protocols
THE END

• Module 6
• CCNA 2 Version 3.0