Introduction to Routing (Free Preview)

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Routers forward packets by looking at the destination IP address of a packet and then looking up the routers’ routing table for a matching entry to that destination IP network.

Routers need a mechanism to learn about IP destination networks. There are two ways a router can learn information about destination IP networks.

  1. Static: network engineer statically configures destination IP network information
  2. Dynamic: dynamically learn about destination IP networks based on information received in routing updates from neighbour routers.

Some of the reasons to implement dynamic routing protocols

  1. Compared to static routing, dynamic routing protocols require less administrative overhead.
  2. Discovery of remote networks
  3. Maintaining up-to-date routing information
  4. Choosing the best path to a destination
  5. Detects topology changes and dynamically exchange routing updates to install new paths to networks
  6. Maintain redundant path information

 

Interior Gateway Protocols Exterior

Gateway

Protocols

Distance Vector Link-State Path-Vector
IPv4 RIPv2 EIGRP OSPF v2 IS-IS BGP-4
IPv6 RIP ng EIGRP for IPv6 OSPF v3 IS-IS for IPv6 BGP-MP

Table Routing Protocol Classification

*RIPv1 (legacy) is a classful distance vector routing protocol.

*IGRP (legacy) is a classful distance vector routing protocol.

 

Autonomous System

A very good definition of an Autonomous System can be found in an IETF document, RFC 4271 [4] that describes BGPv4:

Autonomous System is a set of routers under a single technical administration, using an interior gateway protocol (IGP) and common metrics to determine how to route packets within the AS, and using an inter-AS routing protocol to determine how to route packets to other ASs.

Since this classic definition was developed, it is now very common for a single AS to use several IGPs and sometimes several sets of metrics within an AS.

According to above definition, even when multiple IGPs and metrics are used, the administration of an AS appears to other ASs to have a single interior routing plan and presents a consistent picture of what destination networks are reachable through it.

 

Interior Gateway Protocols

Used for routing within an Autonomous System.

Exterior Gateway Protocols

Used for routing between Autonomous Systems.

Distance Vector Routing Protocols

  • There are two characteristics found in the advertisements of routes for this type of routing protocols.
    • Distance (Metric): How far is it to the destination network? Uses metrics such as the hop count, bandwidth, delay etc.
    • Vector (Direction / next-hop): If I want to get to destination network X in which direction should I forward the packets? In other words, what is the routers’ egress interface the router must use to get to the particular destination network it wants to reach.
  • Does not know the entire path (Have no knowledge of the entire topology) to a remote network.
  • The router only knows the Distance and the Metric to a destination IP network. Relies on the neighbouring routers along the path to deliver the packets.

 

Link-State Routing Protocols.

  • Has a complete view of the entire network.
  • Uses link-state information to create a topology map and select the best path to a destination.
  • Does not send periodic updates.
  • After the network, has converged, only topology changes trigger routing updates.

Classful Routing Protocols

  • Unlike classless routing protocols, classful routing protocols do not send the subnet mask information in routing updates (RIP v1).
  • No support for variable length subnet masks(VLSM)
  • Creates issues in discontinuous networks.
    • Discontinuous Network: Subnets from the same classful major network is separated by a different classful network ID (network address)

Classless Routing Protocols

  • Supports VLSM and CIDR (classless interdomain routing)
  • Carries the subnet mask information in routing updates.
  • All IPv6 routing protocols are classless.

 

Comparing Routing Protocols

Distance Vector Link State
RIPv1 RIPv2 EIGRP OSPF IS=IS
Speed of convergence Slow Slow Fast Fast Fast
Scalability – Size of network Small Small Large Large Large
VLSm Support No Yes Yes Yes Yes
Router Resource

Usage

Low Low Medium High High
Implementation

&

Maintenance

Simple Simple Complex Complex Complex

Table Comparison of Routing Protocols

 

Routing Protocol Metrics

  • When a router learns more than one path to a destination network it needs to select the best path out of them to install in to its routing table. This is accomplished using the metric of a route.
  • The metric is used to determine the overall “cost” of a path from the source to destination.
  • For example, RIP uses hop count as the metric. On the other hand, EIGRP uses Bandwidth, Delay, Reliability, Load and MTU along the path to determine its metric.

 

 

 

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