Reading

Autonomous System
Cisco BGP Overview
BGP RFC (Skim only!)

Tasks

This lab is focused on configuring and managing routes between administrative domains (also referred to as Autonomous Systems or AS's). The protocol that is used for inter-domain routing is the Border Gateway Protocol (BGP). The version of BGP that is currently used in the Internet is v4. Every AS/network that is connected to the Internet must have a node (router) that participates in BGP (a BGP speaker). These nodes announce which networks (IP address ranges specified using the CIDR "slash notation") are contained within or can be accessed through an AS, and likewise use the information transmitted by other BGP speakers to determine how to send packets to destination networks. For example, the BGP speaker at UW-Madison would announce to the other BGP speakers it is connected to (via BGP sessions) the IP address ranges for the active networks on the UW-Madison campus. If UW-Madison only had a single upstream provider through which it gained access to the rest of the Internet (and therefore only a single BGP session), it would be referred to as a "stub network". Since UW-Madison has multiple upstream providers (and therefore multiple BGP session), it is referred to as a "multi-homed network". A third category is "transit networks". These are networks that are not only the sources/destinations of traffic, but also connectivity between networks. Transit networks are typically larger Internet Service Providers that maintain many BGP sessions with their customers (i.e., the networks who pay them for upstream connectivity) and their peers (i.e., the other large networks that they connect to but neither pays the other).

You will gain experience with inter-domain router configuration and management by conducting several different experiments. Note that configuration and management of inter-domain routes actually has two components: the first are the activities external to a network (covered by EGRP or Exterior Gateway Routing Protocol) and the second are the activities internal to a network (covered by IGRP or Interior Gateway Routing Protocol). All of the inter-domain information is transmitted via EGRP, while the directives about external networks to the intra-domain infrastructures within an AS are transmitted via IGRP. There are many references available on the web and in textbooks for BGP, EGRP and IGRP. As mentioned earlier, inter-domain routes in BGP are established by passing path vectors between networks. In addition to networks, attributes and local preferences are transmitted. Ideally, this local preference information is meant to help establish effective routes, but the lack of standards and the need for flexibility actually complicate this process significantly.

You will begin lab #5 by learning how to configure an end host to behave like a BGP router. While this can be useful by itself (e.g., end hosts are sometimes used for certain BGP measurement related functions), it is particularly important in the WAIL environment to enable studies of larger groups of BGP speakers. There are several different share-ware routing packages that include BGP capability. The package we will use in this lab is called Quagga.

You will also conduct a set of experiments on system configurations in WAIL that include Cisco IP routers that have BGP capability. Like lab #4, you will use Cisco Internetwork Operating System (IOS and its command line interface (CLI) to configure and assess the BGP routes that are established in this lab.

Tools

In addition to the recommended readings above, you will need to familiarize yourself with the Quagga soft-router that is available in Linux and other platforms. Information can be found at:

Quagga BGP module

An important capability in BGP is to limit or filter the information that is propagated. This is especially important for large service providers who periodically receive erroneous updates from their customers that might cause large traffic swings in the network (so called black holes). Information on route filtering can be found at:

ip as-path access-list

Finally, a list of the IOS commands for configuring BGP on Cisco routers can be found at:

route-map

Topology

You will build a network that has the following topology for this lab:

The NS file for this topology can be found at:

NS file or Routerless NS file

Questions

Please enter the answers to these questions into your lab notebooks before you start the lab.

1. What is path vector routing and why is it used in BGP?

2. Compare path vectors to distance vectors in terms of efficiency and functionality.

3. How does Local and Transit traffic differ?

4. What is path exploration in BGP?

5. What is a BGP speaker? How do they relate to an AS? What types of information do they exchange?

6. Give a reason why a non-symmetric route between two nodes might be desired.