Distance Vector Routing Protocols

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Distance Vector Routing Protocols

• PJC CCNA Semester 2 Ver. 3.0
• by
• William Kelly

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Distance Vector Routing Protocols

• Intro to Distance Vector Protocols
• Load Balancing
• Routing Loops and their solution
• Examining Routing Tables
• Administrative Distance
• Gateway of Last Resort
• Integrating Static Routes with RIP
• Basics of RIP and IGRP
• Troubleshooting Routing Protocols

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Distance Vector Protocols Intro.

• The two important Distance vector routing
  protocols are RIP and IGRP
• The ENTIRE routing table is sent periodically to
  neighboring routers
• A topology change or a periodic update sends
  routing tables to neighbors

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Load Balancing

• The maximum paths range from 1 to 6
• By default most IP routing protocols install 4
  routes in parallel
• Static Routes always install 6 routes
• Rip can only load balance paths that have same
  number of hops
• IGRP can load balance up to 6 unequal paths
• The maximum-paths maximum command allows the
  number of parallel paths used to load balance to
  be changed in configuration mode

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Load Balancing (switching)

• process switching (packets)
• The router alternates paths on a per packet basis
• fast switching (per destination)
• All packets in the packet stream bound for a
  specific host take the same path
• Packets bound for a second host on the same
  destination network would all take an alternate
  path

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What is a Routing Loop?
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How to Solve Routing Loops

• Count to Infinity
• Split Horizon
• Route Poisoning
• Triggered Updates
• Hold Down Timers

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Count to Infinity

• Distance Vector Protocols define Infinity as a
  specific number
• Looping continues until Infinity (16 for RIP) is
  reached
• When infinity is reached the Network is
  considered unreachable
• In our example the loop would continue until the
  county reached 16 and then network 1 would be
  marked unreachable

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Split Horizon

• A routing loop occurs when incorrect information
  is sent to a router that just sent out correct
  information
• The solution is to avoid sending information back
  in the direction it came
• In our example Routers A and E would never have
  received Cs bad info and thus no routing loop
  would have occurred.

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Route Poisoning

• Route poisoning is used to overcome loops in
  large networks by setting the hop count to one
  more than the maximum
• The poison reverse rule states Once you learn of
  a route through an interface, advertise it as
  unreachable back through that same interface.
• Route poisoning is essentially the same as poison
  reverse split horizon

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Triggered Updates

• By sending updates faster than the default update
  convergence occurs must faster
• A topology change quickly propagates through a
  network
• Triggered updates used with route poisoning marks
  a down network as unreachable quickly through
  the network

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Hold Down Timers

1. When a router receives information that a
   previously accessible route is inaccessible it
   starts a hold down timer and marks the route as
   inaccessible
2. If an update is received indicating the route is
   back up before the hold down timer expires then
   the route is marked accessible again
3. If an update is received from a different
   neighbor with a better metric then the route is
   marked accessible and the hold down time is
   removed
4. If an update is received from a different
   neighbor with a poorer metric before the hold
   down timer expires then the update is IGNORED

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Examining Routing Tables

• Show ip route command
• How did I receive a route?
• What is the metric?
• What is the administrative distance?
• Is the route directly connected?
• What is the output interface to get to a route?

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Examining Routing Tables

• Vistash ip route
• Codes C - connected, S - static, I - IGRP, R -
  RIP, M - mobile, B - BGP
• D - EIGRP, EX - EIGRP external, O - OSPF,
  IA - OSPF inter area
• N1 - OSPF NSSA external type 1, N2 - OSPF
  NSSA external type 2
• E1 - OSPF external type 1, E2 - OSPF
  external type 2, E EGP
• i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS
  level-2, ia - IS-IS inter area
• - candidate default, U - per-user static
  route, o ODR
• P - periodic downloaded static route
• Gateway of last resort is not set
• 172.16.0.0/24 is subnetted, 3 subnets
• C 172.16.8.0 is directly connected,
  FastEthernet0/0
• R 172.16.5.0 120/1 via 172.16.7.1,
  000002, Serial0/0
• C 172.16.7.0 is directly connected,
  Serial0/0

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Administrative Distance

• Administrative distance is a default value
  assigned to each routing protocol that will favor
  the best route from one protocol over another one
• Distances for common protocols are

Directly Connected 0
Static Routes 1
IGRP 100
OSPF 110
RIP 120
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Gateway of Last Resort

• Routers try to keep routing tables as small as
  possible
• A router may not be able to match a destination
  network with an entry in its routing table
• Default routes can be entered statically or
  learned dynamically
• ip default network x.x.x.x establishes a default
  route in networks using dynamic routing
• Any router set with default network x.x.x.x that
  has a route to x.x.x.x flags x.x.x.x as a
  candidate default route
• A default route can be statically set by saying
• ip route 0.0.0.0 0.0.0.0 next hop ip or exit
  interface
• If no path to the destination network is found
  in the routing table then the quad zero default
  is used

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Integrating Static Routes with RIP

• If your static route was not defined with a
  network command then it is not distributed unless
  you use the redistribute static command
• ip route dest. mask ip/interface admin_distance
• Packets bound for specific destination networks
  can be forced to follow a certain path
• Using a higher administrative distance can
  provide a backup path in case of main link
  failure

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RIP Basics

• To configure use route rip, then network x.x.x.x
• The metric is hop count
• A hop count of 16 is infinity
• Period updates are sent every 30 seconds
• It is a distance vector protocol
• The entire routing table is sent during updates
• The administrative distance is 120
• The hold down timer default is 180 seconds

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IGRP Basics

• To configure use route igrp as-number, then
  network x.x.x.x
• The metrics are bandwidth, delay, load, and
  reliability
• Period updates are sent every 90 seconds
• The hold down time is 3x the update timer or 90
  secs. x 3 10 seconds 280 seconds.
• It is a distance vector protocol
• The entire routing table is sent during updates
• The administrative distance is 100
• Scalable for very large networks

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Troubleshooting Routing Protocols

• Is the routing protocol set?
• show ip protocols
• Is the route in the routing table?
• show ip route
• Are the interfaces configured?
• show running-config
• Am I using the same version of RIP throughout my
  network?
• ping, traceroute, debug