Foundation Summary

Study Material For Cisco Ccna Ccnp And Ccie Students

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The "Foundation Summary" section of each chapter lists the most important facts from the chapter. Although this section does not list every fact from the chapter that will be on your CCNA exam, a well-prepared CCNA candidate should know, at a minimum, all the details in each "Foundation Summary" section before going to take the exam.

All routing protocols have several general goals, as summarized in the following list:

■ To dynamically learn and fill the routing table with a route to all subnets in the network.

■ If more than one route to a subnet is available, to place the best route in the routing table.

■ To notice when routes in the table are no longer valid, and to remove those routes from the routing table.

■ If a route is removed from the routing table and another route through another neighboring router is available, to add the route to the routing table. (Many people view this goal and the preceding one as a single goal.)

■ To add new routes, or to replace lost routes with the best currently available route, as quickly as possible. The time between losing the route and finding a working replacement route is called convergence time.

■ To prevent routing loops.

The following list summarizes a few very important terms related to routing and routing protocols:

■ A routing protocol fills the routing table with routing information. Examples include RIP and IGRP.

■ A routed protocol is a protocol with OSI Layer 3 characteristics that define logical addressing and routing. The packets defined by the network layer (Layer 3) portion of these protocols can be routed. Examples of protocols include IP and IPX.

■ The term routing type has been used in other Cisco courses, so you also should know this term. It refers to the type of routing protocol, such as link-state or distance vector.

Table 14-8 lists some of the major comparison points between interior routing protocols.

Table 14-8 Major Comparison Points Between Interior Routing Protocols

Point of Comparison

Description

Type of routing protocol

Each interior routing protocol covered in this chapter can be characterized based on the underlying logic used by the routing protocol. This underlying logic often is referred to as the type of routing protocol. The three types are distance vector, link state, and hybrid.

Full/partial updates

Some interior routing protocols send their entire routing tables regularly, which is called full routing updates. Other routing protocols send only a subset of the routing table in updates, typically just the information about any changed routes. This is called partial routing updates. Partial updates require less overhead in the network.

Convergence

Convergence refers to the time required for routers to react to changes (for example, link failures and router failures) in the network, removing bad routes and adding new, better routes so that the current best routes are in all the routers' routing tables.

Metric

The numeric value that describes how good a particular route is. The lower the value is, the better the route is. Some metrics provide a more realistic perspective on which routes are truly the best routes.

Support for VLSM

Variable-length subnet masking (VLSM) means that, in a single Class A, B, or C network, multiple subnet masks can be used. The advantage of VLSM is that it enables you to vary the size of each subnet, based on the needs of that subnet. For instance, a point-to-point serial link needs only two IP addresses, so a subnet mask of 255.255.255.252, which allows only two valid IP addresses, meets the requirements but does not waste IP addresses. A mask allowing a much larger number of IP addresses then can be used on each LAN-based subnet. Some routing protocols support VLSM, and some do not.

Classless or classful

Classless routing protocols transmit the subnet mask along with each route in the routing updates sent by that protocol. Classful routing protocols do not transmit mask information. So, only classful routing protocols support VLSM. To say that a routing protocol is classless is to say that it does support VLSM, and vice versa.

Table 14-9 summarizes the most important points of comparison between the interior routing protocols.

Table 14-9 Interior IP Routing Protocols Compared—Summary

Routing Protocol

Metric

Convergence Speed

Supports VLSM, and Is a Classless Routing Protocol

Period for Full routing Updates

RIP-1

Hop count

Slow

No

30 seconds

RIP-2

Hop count

Slow

Yes

30 seconds

IGRP

Calculated based on constraining bandwidth and cumulative delay

Slow

No

90 seconds

EIGRP

Same as IGRP, except multiplied by 256

Very fast

Yes

N/A

OSPF

Cost, as derived from bandwidth by default

Fast

Yes

N/A

Integrated IS-IS

Metric

Fast

Yes

N/A

Table 14-10 outlines some of the key comparison points between RIP and IGRP. Table 14-10 RIP and IGRP Feature Comparison

Feature

RIP (Default)

IGRP (Default)

Update timer

30 seconds

90 seconds

Metric

Hop count

Function of bandwidth and delay (the default). Can include reliability, load, and MTU.

Hold-down timer

180

280

Flash (triggered) updates

Yes

Yes

Mask sent in update

No

No

Infinite-metric value

16

4,294,967,295

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