CCNA Explorer 2 Introduction to Dynamic Routing Protocols

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Introduction and Advantages

Perspective and Background


Network discovery and routing table maintenance

The purpose of a routing protocol includes:

  • Discovery of remote networks
  • Maintaining up-to-date routing information
  • Choosing the best path to destination networks
  • Ability to find a new best path if the current path is no longer available


Dynamic vs Static Routing


  • RIP - A distance vector interior routing protocol
  • IGRP - The distance vector interior routing developed by Cisco (deprecated from 12.2 IOS and later)
  • OSPF - A link-state interior routing protocol
  • IS-IS - A link-state interior routing protocol
  • EIGRP - The advanced distance vector interior routing protocol developed by Cisco
  • BGP - A path vector exterior routing protocol

Classifying Dynamic Routing Protocols


IGP vs. EGP Routing Protocols

Distance Vector and Link State

Distance Vector bruger for det meste Bellman-Ford algoritmen.
Distance vector protocols work best in situations where:

  • 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.
Distance vector

Link-state protocols work best in situations where:

  • 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.

Classful and Classless

Classful vs. Classless




Purpose of a Metric


Metrics and Routing Protocols

The metric for each routing protocol is:

  • RIP: Hop count - Best path is chosen by the route with the lowest hop count.
  • IGRP and EIGRP: Bandwidth, Delay, Reliability, and Load - Best path is chosen by the route with the smallest composite metric value calculated from these multiple parameters. By default, only bandwidth and delay are used.
  • IS-IS and OSPF: Cost - Best path is chosen by the route with the lowest cost. . Cisco's implementation of OSPF uses bandwidth. IS-IS is discussed in CCNP.

Metric used in IP routing protocols:

  • 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
  • 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.
Show ip Route

Load Balancing

Equal Cost Load Balancing
Equal Cost Load Balancing Topology

Administrativ Distance

Purpose of Administrative Distance

Administrativ Distance
Administrativ Distance

Cisco implementation

Valid for IPv4 and IPv6

Default Administrative Distance (metric)
Route Source Distance
Connected Interface 0
Static Route out an Interface 0
Static Route to a next hop 1
EIGRP summary route 5
External BGP 20
Internal EIGRP 90
IGRP 100
OSPF 110
IS-IS 115
RIP (Version 1 og 2) 120
EGP 140
ODR (On Denmand Routing) 160
External EIGRP 170
Internal BGP 200
Ukendt source 255

Static Routes

Static Routes