Institute of Technology, Sligo Dept of Computing Routing & IGRP Semester 3, Chapter 5

Table of Contents Network Layer Basics Routing & Routed Protocols IP Routing Protocols IGRP Operation

Institute of Technology, Sligo Dept of Computing Network Layer Basics

Path Determination  A router’s main responsibility when routing packets is its path determination function.  Path determination is the process of making a forwarding decision? (At what layer?)  What do routers use to make a forwarding decision? What metric counts the number of routers the packet goes through?  What are the two parts of a routed protocol?  Which part do routers use to determine the path?  Once the router has determined the path, what function allows it to forward it out the correct interface?

Router’s Steps  Router operation has four basic steps: 1.Open the frame received on the interface and read the MAC address. 2.If the MAC resides on the same subnet, discard the frame. If the frame is an ARP request for a destination on a different subnet, send a RARP. All other broadcasts are discarded. If the MAC specifies a host on another subnet, open the packet. 3.Read the IP address and look it up in the routing table. Use the routing protocol(s) configured to determine the best path. 4.Re-encapsulate the packet in the correct frame format with the next-hop’s data-link layer address and switch the frame out that interface

Institute of Technology, Sligo Dept of Computing Routed & Routing Protocols

Routed Protocols  Routed protocols are moved across the network by routing protocols.  Each routed protocol has its own layer 3 packet header format with its own unique hierarchical addressing scheme.  What are some examples of routed protocols?

Routing Protocols  Routing protocols are used by routers to communicate with each other about various paths to the destination.  Each routing protocol has its own set of benefits and drawbacks.  What are some examples of routing protocols?

Multiprotocol Routing  Routers are capable of using multiple routed and routing protocols at the same time.  Through the use of subinterfaces, a router can even use different routed protocols on the same interface. (See Graphic)

Routed & Routing Review IPIPXApple InteriorExterior Dist-VLink-SHybridEGPBGP RIPIGRPOSPFEIGRP

Institute of Technology, Sligo Dept of Computing IP Routing Protocols

Primary Goals of Routing Protocols  Optimal Route—pick the best route  Efficiency—minimal use of bandwidth and router processor resources  Rapid Convergence—the faster, the better. Some are quicker at converging than others.  Flexibility—can handle a wide variety of situations such as high usage and failed routes

Interior vs. Exterior  Interior Routing protocols operated within an Autonomous System. What are some examples?  Exterior Routing protocols operated between an Autonomous System. What are some examples?

Interior Routing Protocols  Distance-Vector—passes periodic copies of its routing tables to its neighbor routers. How often for RIP? IGRP?  Link-State—broadcasts or multicasts topology changes when they occur. Also called SPF routing. SPF stands for….?  Hybrid—a distance-vector routing protocol that does not send periodic copies of its routing table. Instead, it sends topology change updates. What protocol is a hybrid?

Dynamic v. Static  Static routing is not a protocol. The net. admin. statically configures a route using the ip route command.  Dynamic routing refers to the use of a routing protocol to determine the path. In dynamic routing…  Routes are dynamically adjusted as the topology changes.  Accurate routing tables and timely updates are crucial to avoid routing loops and unacceptable delay.  A lack of convergence (all routers with the same information) is the main reason for inaccurate routing tables and routing loops.  Routing loops are solved through hold down timers, split horizons, and poison reverse routes. Describe each of these solutions.

Configuring Routing Protocols  Two steps:  Under global configuration mode…  Enable the routing protocol  Router(config)# router protocol [process- id]  [process-id] is the Autonomous System number for IGRP and EIGRP. For OSPF, it has a different meaning.  In router sub configuration mode…  Add the directly connected network numbers  Router(config-router)# network A.B.C.D.  A.B.C.D. is the network address (i.e )

Institute of Technology, Sligo Dept of Computing IGRP Operation

IGRP Metrics  IGRP is a Cisco proprietary routing protocol with an AD of 100. What’s meant by proprietary?  IGRP provides a wide range of configurable metrics. Net. Admin. can set values for…  Bandwidth--choose the fastest route. If two equal paths, then load balance  Delay--chose the route with the least amount of cumulative interface delay along the path  The following are not normally used because they cause more frequent routing table calculations.  Load--similar to delay, based upon total bits per second currently being transmitted (throughput)  Reliability--based on keepalives, chose the link that stays up the most  MTU--choose the link with the largest MTU  Hop Count--can be as high as 255  By default, IGRP uses bandwidth and delay at equal cost.

Configuring IGRP  Two steps:  Under global configuration mode…  Enable IGRP routing  Router(config)# router igrp 100  100 is the Autonomous System number.  In router subconfiguration mode…  Add the directly connected network numbers  Router(config-router)# network