Year 2 - Chapter 5/Cisco 3 - Module 5 Routing Protocols: IGRP

Objectives  Describe the routing functions of the network layer and how these functions relate to path determination in a router  Describe routed and routing protocols  Describe interior and exterior protocols  Describe routing protocol characteristics and configuration  Describe IGRP features, operation, and configuration tasks

Network Layer Basics  Network Layer Path Determination  Routing Tables  The Network Layer Communication Path  Addressing: The Network and the Host

The Goals of Routing Protocols  The Optimal (Best) Route: The best route depends on the metrics and metric weightings used to make the calculation.  Simplicity and Efficiency: Efficiency is particularly important when the routing protocol must run on a device with limited physical resources.  Robustness: The best routing protocols are often those that have withstood the test of time and proven stable under a variety of network conditions.  Rapid Convergence  Flexibility: Routing protocols can be programmed to adapt to changes in network bandwidth, router queue size, network delay, and other variables.

Routing Table

Path Determination

Network and Host Addressing

Path Selection and Packet Switching

Static Versus Dynamic Routes

Complex Metrics

Addresses and Packet Forwarding

Routed Versus Routing Protocol

Multiprotocol Routing

Exterior Versus Interior Protocols

Three Classes of Routing Protocols  Distance Vector  Link State  Hybrid

Issues in Choosing a Routing Protocol  Network size and complexity  Network traffic levels  Security needs  Reliability needs  Network delay characteristics  Organizational policies  Organizational acceptance of change

Distance-Vector Routing Basics

Distance-Vector Topology Changes

Understanding IGRP Operation  Interior, System, and Exterior IGRP Routes  Creating the IGRP Routing Process  Enhancing IGRP Stability  IGRP Metric Information  IGRP Updates  Maximum Hop Count

IGRP Overview

IGRP Key Information  Cisco proprietary protocol Requires all Cisco devices.  Distance-vector routing protocol  Periodic updates every 90 seconds Full route table exchanged. Declares a route inaccessible after three missed update periods (270 seconds). Route removed after seven update periods (630 seconds).  Metric based on bandwidth and delay by default Optional: Can include reliability, load, and MTU.  Large network diameter – 100 hops (routers) Optional: Up to 255 possible.

Interior and Exterior Routes Internet ISP Exterior route Interior routes An AS is a collection of networks under common administration sharing a common routing strategy. Typically this would be a company, but a large company could create multiple ASs. Typically ISPs and clients do not share routing information or protocols. Exterior routes are typically a default route to the ISP and static route(s) to the AS. All routers running IGRP or Enhanced IGRP (EIGRP) and using the same AS number will exchange route information.

IGRP Configuration

Configuring IGRP Example Router(config)#router igrp 100 Router(config-router)#network Router(config-router)#network Router(config-router)#network If you want the bandwidth on a serial link to be interpreted by IGRP as anything but 1.54 Mbps (T1 – default), you must use the optional bandwidth command. This command gives IGRP a value to use but does not alter the actual bandwidth. The following example sets the bandwidth to 256 Kbps: Router(config)#int s0 Router(config-if)#ip address Router(config-if)#bandwidth 256 Router(config-if)#no shutdown

The show ip route Command Router#show 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, * - candidate U - per-user static route, o - ODR Gateway of last resort is not set C /24 is directly connected, Serial1 I /24 [100/8576] via , 00:01:06, Serial1 I /24 [100/10476] via , 00:01:06, Serial1 C /24 is directly connected, Ethernet0 MetricAdministrative DistanceSourceInterface Received On

The show ip protocols Command Router#show ip protocols Routing Protocol is "igrp 100" Sending updates every 90 seconds, next due in 69 seconds Invalid after 270 seconds, hold down 280, flushed after 630 Outgoing update filter list for all interfaces is not set Incoming update filter list for all interfaces is not set Default networks flagged in outgoing updates Default networks accepted from incoming updates IGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0 IGRP maximum hopcount 100 IGRP maximum metric variance 1 Redistributing: igrp 100 Routing for Networks: Routing Information Sources: Gateway Distance Last Update Distance: (default is 100) Administrative Distance

The debug igrp transactions Command Router#debug ip igrp transactions IGRP protocol debugging is on Router# IGRP: received update from invalid source on Ethernet0 IGRP: received update from on Serial1 network , metric 8576 (neighbor 1100) network , metric (neighbor 8476) IGRP: sending update to via Ethernet0 ( ) network , metric=8476 network , metric=8576 network , metric=10476 IGRP: sending update to via Serial1 ( ) network , metric=1100 Router#undebug all IGRP event debugging is off

IGRP Solutions for Routing Loops IGRP features to enhance stability include : Hold-down timers  If Router 1 tells Router 2 that Network A is unavailable and Router 2 sets a hold-down timer, it will not accept information about Network A from other routers until the timer expires. Split horizon  Router 2 will not send routes to Router 1 that it learned about from Router 1 (such as Network A). Poison reverse updates  Typically used with hold-down timers.  If Network A goes down and Router 1 drops it from its routing updates, it will take 3 cycles (270 seconds) for Router 2 to suspect a problem and 7 cycles (630 seconds) to drop the route.  Alternative: Router 1 sends Router 2 an update on Network A with the hop count set to 101 – infinity (unreachable).

Adjust Configurable IGRP Metrics  (config-router)# timers basic update Invalid holddown flush  (config-router)# no timers basic  (config-router)# meteric maximum-hops 2  (config-if) IP split-horizon  Unequal cost load balancing using the variance command: (config-router)variance #  An IGRP router will only accept routes equal to the local best metric for the destination multiplied by the variance value.  (config-if)# no IP route-cache.