[Cisco Networking Academy] Del Mar College
ITNW 1392-Beginning Router Configuration
Instructor:Michael P. Harris

Semester 2, Lesson 13 Notes:

Router configuration and Routing Protocols: RIP and IGRP

"IGP" (Interior Gateway Protocol) - Internet protocol used to exchange routing information within an autonomous system. Examples of common IGPs include IGRP, OSPF, and RIP.

"RIP" (Routing Information Protocol) - IGP supplied with UNIX BSD systems. The most common IGP in the Internet. RIP uses hop count as a routing metric.

"IGRP" (Interior Gateway Routing Protocol) - IGP developed by Cisco to address the problems associated with routing in large, heterogeneous networks.

Each 'interface' on the router must be configured with a unique 'IP address' and 'subnet mask'. Information that relates an IP address to a router is called the "routing table".

Routers learn paths to destinations three different ways:
(Note: The "ip route" command is used to set up 'static routes'. The "ip default-network" command is used to set up 'default routes')

(1)"Static routes" - Manually defined by the system administrator as the only path to the destination (stub network). Route that is explicitly configured and entered into the routing table. Static routes take precedence over routes chosen by dynamic routing protocols. Useful for controlling security and reducing traffic. The "ip route" command is used to setup a static route.

Router(config)#   ip route network [mask] {address/interface) [distance]

network		Destination 'network' or subnet mask
		Subnet mask

address		IP address of next hop router

interface	Name of interface to use to get to 
		destination network

distance	The administrative distance

"Administrative distance" - a rating of the trustworthiness of a routing information source expressed as a numeric value from 0 to 255. The higher the number, the lower the trustworthiness rating.

A 'static route' allows manual configuration of the routing table. No dynamic changes to this table entry will occur as long as the path is active. A static route may reflect some special knowledge of the networking situation known to the network administrator. Manually entered 'administrative distance' values for static routes are usually low numbers.

Routing updates are not sent on a link if only defined by a 'static route', thereby conserving bandwidth.

(2)"Default routes" - Manually defined by the system administrator as the path to take when no route to the destination is known. Routing table entry that is used to direct frames for which a next hop is not explicitly listed in the routing table. 'Default routes' are configured using the 'ip default route' command, while at the 'Router (config)# prompt.

Router(config)#   ip default-network network-number
Network-number: IP network number or subnet number defined as the default.

(3)"Dynamic routing" - Router learns of paths to destinations by receiving periodic updates from other routers. Routing that adjusts auotmatically to network topology or traffic changes. Also called adaptive routing. Dynamic routing uses 'broadcasts' and 'multicasts' to communicate with other routers. The "router" command starts a routing process.

Router(config)#   router protocol [number/word]
protocol     RIP, IGRP, OSPF, or Enhanced IGRP.
number       autonomous-number for IGRP

An "Autonomous System" (AS) is a group of routers under a common administration, An 'autonomous system' consists of touters, run by one or more operators, that present a consistent view of routing to the external world. As numbers are used to prevent updates from different Autonomous Systems being propagated amongst each other.

The "Network Information Center" (NIC) assigns a unique 'autonomous system' to enterprises. This autonomous system is a 16-bit number. A routing protocol, such as Cisco's "Interior Gateway Routing Protocol" (IGRP) requires that you specify this unique, assigned autonomous system number in your configuration.

Router(config-router)#   network network-number


Such as autonomous system, which is used with those protocols that require an autonomous system, such as IGRP.


Specifies a directly connected network.

The "network" command is required because it allows the routing process to determine which interfaces will participate in the sending and receiving of routing updates. The "network number" must be based on the NIC network numbers, not subnet numbers or individual addresses. The network command assigns a NIC-based address to which the router is directly connected. The routing process will associate interfaces with the proper addresses and will begin packet processing on the specified networks.

"Exterior routing protocols" are used to communicate between 'autonomous systems'. "Interior routing protocols" are used to communicate within a single Autonomous System.

At the "Internet layer" of the TCP/IP suite of protocols, a router can use the IP routing protocol to accomplish routing through the implementation of a specific routing algorithm. Examples of the IP routing protocols include:


A 'distance vector' routing protocol.


Cisco's 'distance vector' routing protocol.


A 'link-state' routing protocol.

Enhanced IGRP:

A 'balanced hybrid' routing protocol.

The selection of IP as a routing protocol involves the setting of both global and interface parameters.

Global configuration tasks:

  1. Select a routing protocol, RIP or IGRP.
  2. Assign IP network numbers without specifying subnet values.

Interface configuration task:

  1. assign network/subnet addresses and the appropriate subnet mask.

RIP (Routing Information Protocol) is primarily concerned with the 'hop count'. Key characteristics of RIP include the following:

  • It is a distance vector routing protocol.
  • Hop count is used as the metric for path selection.
  • The maximum allowable hop count is 15.
  • Routing updates are broadcast every 30 seconds by default.

IGRP (Interior Gateway Routing Protocol) is a distance vector routing protocol developed by Cisco. IGRP sends routing updates at 90-second intervals that advertise networks for a particular autonomous system. The following are some key characteristics of IGRP:

  • IGRP is a distance vector routing protocol.
  • A composite metric (including bandwidth, delay, load, reliability, and MTU) is used for path selection.
  • It is versatile, flexable, and scalable, allowing for more complex topologies and large networks.
  • Routing updates are broadcast every 90 seconds.
  • Speed is the primary concern of IGRP.

"MTU" (Maximum transmission unit) - Maximum packet size, in bytes, that a particular interface can handle.

"show ip protocol" - (1) Command that displays values about routing timers and network information associated with the entire router. (2) Command that displays parameters, filters, and network information about the entire router. Use this information to indentify a router that is suspected of delivering bad routing information.

"show ip route" - Command that displays the contents of the IP routing table.
The 'routing table' contains entries for all known networks and subnetworks and contains a 'code' that indicates how that information was learned.

"show ip interface" - Command that displays the status and global parameters associated with an interface.

The Cisco IOS software automatically enters a directly connected route in the routing table if the 'interface' is one through which software can send and receive packets. Such an 'interface' is marked 'up'. If the interface is unusable, it is removed from the routing table. Removing the entry allows implementation of backup routes, if they exist.

"debug ip rip" - Command that displays RIP routing updates as they are sent and received.

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