Cisco Systems CCNA 2 7.3 IGRP

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.

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.

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