Intro

In this lesson, we explore routing protocols as they relate to the OSI model, including IS-IS, OSPF, IGRP, EIGRP, RIP, and BGP. You can download the script for this video from above or at the end of the video.

Routing protocols defined

There are several routing protocols, but all of them try to achieve efficient communication across multiple network segments. The protocols define the way routers communicate locations of resources to each other, and they enable functionality to route packets over the quickest paths to their destinations.

Topology and Convergence

Two concepts we need to understand when discussing routing are network topology and convergence. Network topology is the layout of the network. It includes all connected devices on all network segments. Complete convergence occurs when all routers know the network topology. This can take time after a change is made. The convergence time is an important consideration for understanding how a change may affect the network operation.

Distance-vector and link-state categories

Routing protocols fall under one or more categories. Let's quickly walk through them. Distance and link-state protocols help determine the most efficient paths to network destinations. Distance-vector uses the number of hops, and link-state protocols use actual speed and cost parameters.

Autonomous systems

Before looking at the next category, gateway protocols, let's define autonomous systems, or AS's. AS's are the largest network division on the internet. Each AS consists of a large network or collection of networks with a unified routing policy. Every device connected to the internet is connected to an AS. An AS can be assigned to large networks, such as an ISP, a large technology enterprise, a university, or a government agency.

Interior gateway and exterior gateway categories

So let's continue with protocol categories. Interior gateway protocols usually exchange information between routers within the same network or AS. External gateway routers communicate with routers in other AS's, enabling cross AS communication.

Classful and classless categories

The final categories are classful and classless protocols. Classful protocols exchange routing data and divide the IPv4 space into five address spaces: A, B, C, and D. The move to classless addressing has relegated classful protocols to the IPv4 compatibility space. Classful protocol topology updates DO NOT include subnet masks. Classless interdomain routing, or CIDR. CIDR is commonly used today. CIDR was created to slow the growth of routing tables and slow IPv4 address space exhaustion. Classless routing protocols support IPv6 and send subnet masks as part of topology updates.

Routing protocol types

Now that we've gotten through all the dry stuff, and are familiar with routing protocol categories, let's look at six protocol types: RIP, IGRP, EIGRP, OSPF, BGP, and IS-IS.

RIP

The routing information protocol, or RIP, is an example of a distance-vector protocol. All routing information is sent to all active interfaces every 30 seconds. The only metric RIP uses to determine routing path is the number of hops, or the number of routers it takes to get from one point to another. In this example, there are two hops over one path and only one over the other. Because of this, RIP will likely select the single-hop path to the destination. The advantages of using RIP include its fitness for small networks where simplicity and configuration and use is preferred. It supports almost all routers, and it does not require an update whenever the topology changes. The disadvantages become more significant as network size increases. They include the possibility of traffic bottlenecks because of the 30-second topology broadcast period. Hop counts tracked are limited to 15, so if you have more than 15 hops to get where you're going, RIP falls far short, and the convergence rate is slow. This causes routing decision changes to lag significantly behind topology changes. There's a variation on how different RIP versions work. RIPv1 is a classful routing protocol, and RIPv2 is classless. A detailed look at how this works can be downloaded from the link shown.

IGRP and EIGRP

The interior gateway routing protocol, or IGRP, was developed by Cisco. It's both a distance-vector and interior-gateway protocol that manages routing within an AS. One of the reasons for IGRP was to eliminate the limitations of RIP, in addition to expanding the hop count far behind beyond 15 to 255. It also uses additional information to determine the most efficient routes, including bandwidth, delay, traffic load, and reliability. EIGRP, or enhanced interior gateway routing protocol, is also a distance-vector protocol that routes within an AS. The difference between these two protocols is the routing technique. IGRP uses classful routing and EIGRP uses classless routing.

OSPF

Open shortest path first, or OSPF, is a link-state protocol and is also a type of interior gateway protocol. Instead of using the number of hops like distance-vector protocols, OSPF determines the most efficient path by looking at bandwidth and calculating route cost. At a high level, this is done by dividing a reference bandwidth, the expected bandwidth based on topology, by each interface bandwidth, as shown.

BGP

The lower the cost, the better. The border gateway protocol, or BGP, is an exterior gateway protocol. It manages routing between AS's. It makes routing decisions based on paths, network policies, and rule sets.

IS-IS

Last but not least is IS-IS. IS-IS is a link-state internal gateway protocol that uses an approach similar to OSPF. The differences between IS-IS and OSPF are in how the protocols divide the network. OSPF divides the network into areas and sends advertisements based on these areas. IS-IS categorizes the routing domain into two layers. An OSPF router can belong to multiple areas, but an IS-IS router can belong to only one. Well, that's it for this lesson. If you have questions, please ask. ...and until next time be careful once you click.