September 3, 2010

IP Multicast for the Intimidated: A Primer

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ip multicastLet me ask you this: What is it about IP multicast that gives most people panic attacks?

For me, IP Multicast was scary and different than what I was used to; it is a little counterintuitive when you are used to dealing with standard IP unicast and broadcast concepts. Plus, there are a lot of new terms and concepts that come with IP multicast, and that can be overwhelming.

I’m fortunate: I happen to work in an environment where IP multicast is an integral part of a software suite that was developed in-house. The fact that IP multicast is so important to a product we support really forced me to have to learn and understand IP multicast, and specifically on Cisco infrastructure.

Since this article is designed to be a gentle introduction, I will concentrate on defining some crucial IP multicast terminology and some very general concepts of multicast.

Definition: IP Multicast

The Cisco IP Multicast Glossary has this definition of IP multicast:

  • A routing technique that allows IP traffic to be sent from one source or multiple sources and delivered to multiple destinations. Instead of sending multiple packets to each destination, a single packet is sent to a group of destinations, known as a multicast group, which is defined by a single IP destination group address.

In multicast, our source hosts, also called senders, send out a group of packets (sometimes called a stream) to a multicast group IP address. Multicast IP addresses are Class D addresses (224.0.0.0/4); there are many IPs in this range that are reserved for administrative (routing, etc) functions, but there are also IPs set aside that are like RFC 1918 addresses in the unicast world, meaning you can use them on your internal network  and not on the Internet. Different hosts on a network within your administrative control (your LAN) can subscribe to the group IP to receive the content that is being sent from the source host.  Unlike IP broadcast, which is ‘one-to-all’ whether a host wishes to receive traffic or not, IP multicast protocols allow hosts to selectively receive the traffic they want, and hosts that don’t want that traffic don’t get it.

Definition: IGMP

IGMP, the Internet Group Management Protocol (originally specified in RFC 1112), is a standards-based protocol that provides a protocol for hosts on a LAN to tell their local router that they want to receive certain IP multicast group traffic. The point of IGMP was and is to allow hosts to dynamically register to receive the desired multicast traffic.

There are three versions of IGMP, appropriately named version 1, 2 and 3. Each version has characteristics that I will attempt to cover in future articles. The key point to remember here is that IGMP is how a host on your LAN tells its router that it wants to receive a certain multicast group.

Definition: PIM

Protocol Independent Multicast, PIM, is the multicast routing protocol that allows existing IP networks to route IP multicast, regardless of what unicast routing protocol is in use. You could be using EIGRP, OSPF, ISIS, even iBGP; PIM was designed to work over any existing IP routing protocol (thus the name ‘Protocol Independent’). PIM is designed to use the existing routing tables as provided by the unicast routing protocol to make its multicast routing decisions.

PIM was defined by the Internet Engineering Task Force and is an open standard. Since I have not worked with PIM on any platform other than Cisco, I won’t attempt to cover differences in implementation.

PIM has two typical modes of operation: Sparse and Dense. It’s difficult to really explain these well in a few words, so I will only say this: Dense mode floods traffic out until other routers tell it to stop, while Sparse mode only sends traffic when specifically requested. It’s an oversimplification, but think of Dense=more, Sparse=less.

Key Points Summary

We know that IGMP deals with multicast-enabled hosts on a LAN communicating with their router. IGMP=Host and Router. There are three versions of the IGMP protocol, with v2 being most prevalent, and v3 up and coming.

PIM deals with router-to-router communication. PIM relies on the underlying routing protocol (OSPF, EIGRP, etc) to be able to make its multicast routing happen. PIM=Router-to-Router.

Tune in Next Time

That is all for this installment. The next article will deal with IGMP and IGMP snooping more in-depth. After that, we’ll venture up the OSI stack and have a detailed look at PIM, SM and DM respectively.

For the most comprehensive IP Multicast for Cisco reference, look to Beau Williamson’s ‘Developing IP Multicast Networks, Volume 1’ from Cisco Press. It is absolutely required reading for the aspiring CCIE. If you have Cisco CCO access, locate the ‘Internetworking Technology Handbook’ and read the IP Multicast Chapter.

Understanding Network Routing Protocols

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network routing protocolsNetwork Routing Protocols - IGRP, EIGRP, OSPF, ISIS, BGP

By Shaun Hummel

Overview

The purpose of routing protocols is to learn of available routes that exist on the enterprise network, build routing tables and make routing decisions. Some of the most common routing protocols include RIP, IGRP, EIGRP, OSPF, IS-IS and BGP. There are two primary routing protocol types although many different routing protocols defined with those two types. Link state and distance vector protocols comprise the primary types. Distance vector protocols advertise their routing table to all directly connected neighbors at regular frequent intervals using a lot of bandwidth and are slow to converge. When a route becomes unavailable, all router tables must be updated with that new information. The problem is with each router having to advertise that new information to its neighbors, it takes a long time for all routers to have a current accurate view of the network. Distance vector protocols use fixed length subnet masks which aren't scalable. Link state protocols advertise routing updates only when they occur which uses bandwidth more effectively. Routers don't advertise the routing table which makes convergence faster. The routing protocol will flood the network with link state advertisements to all neighbor routers per area in an attempt to converge the network with new route information. The incremental change is all that is advertised to all routers as a multicast LSA update. They use variable length subnet masks, which are scalable and use addressing more efficiently.

Interior Gateway Routing Protocol (IGRP)

Interior Gateway Routing Protocol is a distance vector routing protocol developed by Cisco systems for routing multiple protocols across small and medium sized Cisco networks. It is proprietary which requires that you use Cisco routers. This contrasts with IP RIP and IPX RIP, which are designed for multi-vendor networks. IGRP will route IP, IPX, Decnet and AppleTalk which makes it very versatile for clients running many different protocols. It is somewhat more scalable than RIP since it supports a hop count of 100, only advertises every 90 seconds and uses a composite of five different metrics to select a best path destination. Note that since IGRP advertises less frequently, it uses less bandwidth than RIP but converges much slower since it is 90 seconds before IGRP routers are aware of network topology changes. IGRP does recognize assignment of different autonomous systems and automatically summarizes at network class boundaries. As well there is the option to load balance traffic across equal or unequal metric cost paths.

Characteristics

· Distance Vector

· Routes IP, IPX, Decnet, Appletalk

· Routing Table Advertisements Every 90 Seconds

· Metric: Bandwidth, Delay, Reliability, Load, MTU Size

· Hop Count: 100

· Fixed Length Subnet Masks

· Summarization on Network Class Address

· Load Balancing Across 6 Equal or Unequal Cost Paths ( IOS 11.0 )

· Metric Calculation = destination path minimum BW * Delay (usec)

· Split Horizon

· Timers: Invalid Timer (270 sec), Flush Timer (630 sec), Holddown Timer (280 sec)

Enhanced Interior Gateway Routing Protocol (EIGRP)

Enhanced Interior Gateway Routing Protocol is a hybrid routing protocol developed by Cisco systems for routing many protocols across an enterprise Cisco network. It has characteristics of both distance vector routing protocols and link state routing protocols. It is proprietary which requires that you use Cisco routers. EIGRP will route the same protocols that IGRP routes (IP, IPX, Decnet and Appletalk) and use the same composite metrics as IGRP to select a best path destination. As well there is the option to load balance traffic across equal or unequal metric cost paths. Summarization is automatic at a network class address however it can be configured to summarize at subnet boundaries as well. Redistribution between IGRP and EIGRP is automatic as well. There is support for a hop count of 255 and variable length subnet masks.

Continue reading »

Adding A Third Router to Our Virtual Network Using GNS3

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In this lesson we expand our virtual network that we've built using GNS3. We examine our current network that consists of two emulated Cisco 2621 routers. We then add a third Cisco 2621 router to our virtual network.

We review some topics from the previous lesson and expand on the use of GNS3 to add a router to out virtual network. My goal is that by the end of this lesson you are comfortable enough with using GNS3 that you are able to build your own virtual networks.




For more lessons like these check out the members area where you can download tools, Visio stencils, plus check out some more advanced videos. Membership is free so check it out!

Installing GNS3 Router Simulator

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dns3 router simulatorGNS3 is a routing simulator and allows you to simulate Cisco routers, switches and firewalls as well as simulate Juniper routers too. In this post we are going to download and install GNS3.

To begin we need to download the software - a simple Google search takes us to http://www.gns3.net/download to be able to download the software. It is available for Windows, MacOS and Linux. For this demonstration we will install the Windows version.

The biggest trick to getting GNS3 running is also downloading the IOS image needed for the simulator to work. You can download IOS or JunOS images from Cisco or Juniper's website. Some support platforms are the Cisco 2600 and 3600 series routers.

Check out this video as I run through the entire process and getting GNS3 installing and running on my laptop.



In our next video we will be build out our virtual network a bit more to include a third routing and start talking about routing.

What is a Router Simulator

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Router Simulators are a great way to create and test new network topologies without the expense of lots of hardware. Many people use router simulators to help them practice and study for upcoming certification tests and many people are able to pass these certifications because of the convenience of a router simulator.

GNS3 is one such router simulator. Its is an open source project that is available for download to run on your Mac, Windows or Linux computer. With GNS3 you can simulate Cisco routers, switches, PIX firewalls and ASA firewalls. You can also simulate Juniper devices and connect your virtual network to a real live network. The is great if you want to expand an existing lab and don't have the equipment to test a particular new network design.

Check out the video below as I go over exactly what GNS3 is. Also coming up in the next video in this series, I will show you how to install GNS3 on your desktop and show you how to get this cool router simulator working so you can create your own virtual network on your desktop or laptop.

What is GNS3?

Coming up Next... Installing GNS3
In our next video, available free in our members area, we will show you how easy it is to install and setup GSN3 and create your very own virtual network.