Cisco Systems CCNA 2 manual Igrp, Load balancing across multiple paths

This does not consider the bandwidth of each link. So while load balancing may allow packets to travel multiple paths to reach a destination, huge bandwidth differences among equal-cost paths could actually slow throughput.

7.2.9 Load balancing across multiple paths

A router may have multiple paths to a given destination network. If these paths have different metrics the router will use the route with the best metric to forward packets. If multiple routes have the same metric associated with them, the router will use load balancing to spread out the traffic that is forwarded to a particular network. This helps reduce traffic on a given route to speed up communications. Load balancing is enabled by default on routers that use RIP and IGRP. With the exception of BGP, IP routing protocols route to four parallel routes by default. The administrator also has the option of load balancing on a per-packet or per-destination basis. A per-destination basis implies that all packets headed for a particular host on the network during a given communication session will take the same path.

Students should be comfortable with the term “round-robin” load balancing. This means that packets will be equally shared between the equal paths. This is done by alternating the packet output between the interfaces for each of the paths. The students should also understand that this does not equally balance the traffic between the paths. This is because the packets are of various sizes. So even though the same number of packets will be forwarded out of the interfaces, the amount of traffic will vary.

7.2.10 Integrating static routes with RIP

Static routes are user-defined routes that force packets to take a specific path. These are usually used when a dynamic route cannot be built, the overhead of dynamic routing is not desirable, or if another route for fault tolerance is desired. A static route can be configured on the router with the ip route command and removed with the no ip route command. These routes can then be redistributed or shared through the dynamic routing protocol with the redistribute static command.

7.3 IGRP

Essential Labs:	7.3.5 and 7.3.6
Optional Labs:	7.3.8
Core TIs:	All
Optional TIs:	none

Course-Level Claim: Students can configure, verify, analyze, and troubleshoot simple distance vector routing protocols.

Certification-Level Claim: Students can troubleshoot and configure routing protocols based on user requirements.

Hands-on skills: none

86 - 238 CCNA 2: Routers and Routing Basics v3.1 Instructor Guide – Module 7 Copyright ♥ 2004, Cisco Systems, Inc.