1.IGRP Design Requirements

Routing protocols, such as IGRP, determine the best path to send routable protocols, such as IP and IPX, through a network.

The Washington School District network needs to handle multiprotocol routing. The district requires that both TCP/IP and IPX routing protocols be handled over the network.

The network should use stable routing, and no routing loops should occur. 
The network should quickly respond to changes in the network topology. 
The network should have low overhead. IGRP itself should not use more bandwidth than is actually needed for its task. 
The network design should take into account error rates and level of traffic on different paths.

AS number consistency is a design issue. The same number must be used throughout the Washington School District network. The AS is assigned a 16-bit number by the Internet Assigned Numbers Authority

2. Advertised Networks & Effect Of Dynamic Routing Protocol On District:

Effects Of A Dynamic Routing Protocol On Performance And Maintenance Of The Entire School District:

Routers are capable of multi-protocol routing. This capability allows a router to deliver packets from several routed protocols, such as TCP/IP and IPX, over the same links.

IGRP is a distance vector routing protocol. The distance vector routing approach determines the direction and distance to any link in the network.

IGRP uses a combination of metrics such as network delay, bandwidth, reliability, and load to make routing decisions. The network administrator can determine the settings for each of these metrics, or let IGRP use the default settings of bandwidth and delay to automatically calculate best routes.

A router running IGRP sends a routing update message every 90 seconds by default. A route is declared inaccessible if the router does not receive a routing update from the connected router within three update periods (270 seconds). If the router does not receive a routing update message from that router for that route within seven update periods, the route is removed from the routing table.

IGRP has a number of features that increases it's stability and prevent routing loops: Holddowns, Split Horizons and Poison Reverse Updates.

Holddowns prevent regular update messages from reinstating a route that may have gone bad. The holddown period is usually calculated to be just greater than the period of time necessary to update the entire network with a routing change.

Split Horizons: Split horizon prevents routing loops by stopping routers from advertising routes along the same links that they learned of a route.

Poison Reverse Updates: Poison reverse updates prevent larger routing loops occurring. Increases in routing metrics generally indicate routing loops. Poison reverse updates are then sent to remove the route and place it in holddown. A router poisons the route by sending an update with a metric of infinity to a router that originally advertised a route to a network.

3. Autonomous System Number: 100

Autonomous system numbers are assigned by the Internet Assigned Numbers Authority. An application should be made by the district office for an autonomous system number. Any number used in configurations are for illustrative purposes only and should not be used during the network implementation. An Autonomous System Number is required as part of the IGRP routing protocol. The same number must be used across the entire network.

4. Router Command Sequence

Router(config)# router igrp 100
Router(config-router)#timers basic 15 45 0 60
Router(config-router)#network 100.131.0.0
Router(config-router)#no metric holddown
Router(config-router)#metric maximum hop 50


5. Routing Update Process

Dynamic routing protocols adjust to changing network circumstances. They do this by analysing incoming routing update messages. If a message indicates that a network change has occurred, the routing software recalculates routes and sends out new routing update messages. These messages permeate the network, prompting routers to recalculate their routing tables accordingly.

Routing table updates will be broadcast every 15 seconds.

The effect of updates on bandwidth utilization will be very minimal as the typical bandwidth for the serial lines will be 1600 Kilobytes.

Routing update datagrams are up to 1500 bytes in length allowing up to 104 entries per datagram. If more entries are needed, additional datagrams will be sent.

6. Metric Settings

Maximum Hops: 50
Holddown timer: 0 seconds
Invalid Time: 45 seconds
Update Timer: 15 seconds
Flush Time: 60

Serial Interface Bandwidth Setting: 1600 Kilobytes