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  • JNCIA Juniper™ Networks Certified Internet Associate

    Study Guide - Chapter 3

    by Joseph M. Soricelli with John L. Hammond, Galina Diker Pildush, Thomas E. Van Meter, and Todd M. Warble

    This book was originally developed by Juniper Networks Inc. in conjunction with Sybex Inc. It is being offered in electronic format because the original book (ISBN: 0-7821-4071-8) is now out of print. Every effort has been made to remove the original publisher's name and references to the original bound book and its accompanying CD. The original paper book may still be available in used book stores or by contacting, John Wiley & Sons, Publishers. www.wiley.com.

    Copyright © 2003-6 by Juniper Networks Inc. All rights reserved.

    This publication may be used in assisting students to prepare for a Juniper JNCIA exam but Juniper Networks Inc. cannot warrant that use of this publication will ensure passing the relevant exam.

  • Chapter

    3

    Protocol-Independent Routing

    JNCIA EXAM OBJECTIVES COVERED IN THIS CHAPTER:

    Describe configuration options for static and aggregate routes

    Identify the default routing tables and preference values

    Describe the options available for load balancing

  • In this chapter, we discuss the options available for IP routes that are not specific to a particular routing protocol. We first explore routes that are configured locally on the router. Then we investi-

    gate the various routing tables in the JUNOS software and learn how those tables select routes to use for forwarding. Finally, we explore how a Juniper Networks router handles the load bal- ancing of user data packets.

    Generally speaking, functions that are protocol independent affect the router as a whole. They are not specific to a particular routing protocol, such as Open Shortest Path First (OSPF) or Border Gateway Protocol (BGP). Routes that are manually configured on the router are a good example, so let’s start there.

    Configured Routes

    Every network has a requirement for locally configured routes. These routes are not learned through a dynamic routing protocol but are manually entered by you, the administrator. Within the JUNOS software, locally configured routes include the following: �

    Static routes �

    Aggregated routes �

    Generated routes

    In a typical operating environment, you use a dynamic routing protocol to propagate and learn route information. Dynamic protocols offer many benefits, including prevention of rout- ing loops and minimal user intervention. However, there is still a special role for nondynamic routing information. Let’s examine these special situations as we take a look at static routes.

    Static Routes

    A

    static route

    within the JUNOS software is a route to a destination with an assigned next hop. If we want to place the route into the routing table, the next hop must be valid. This means that the router is able to forward packets using the next hop. We discuss options for valid next hops in the section “Next-Hop Options” later in this chapter.

    Static routes are not affected by topology changes or new routing information. They are present in the routing table until you remove them through a configuration change. As a quick

  • Configured Routes

    109

    example, let’s look at Figure 3.1. In this figure, Chardonnay is required to reach the subnets of 192.168.16.0 /24, 192.168.32.0 /24, and 192.168.48.0 /24. These routes are physically located behind the routers of Merlot, Riesling, and Cabernet, respectively. You may decide that a static route on Chardonnay for each destination can provide the connectivity you desire. The interface on Riesling that connects to Chardonnay would be the next hop for the static routes. Since the link between these two routers is the only physical path available to Chardonnay, the use of static routes in this example is perfectly acceptable. Should the link fail, no alternate paths would exist and Chardonnay would lose connectivity to those remote destinations. The problem with this simple example, of course, is scalability. As the network grows, Chardonnay will need to con- tinually update its list of static routes to provide adequate connectivity.

    F I G U R E 3 . 1

    Example of static route use

    The Ideal Routing Protocol?

    If you took a poll of network engineers and asked them for their reaction on building an entire network out of static routes, most would say something along the lines of “Are you out of your mind?” After all, static routes are an administrative hassle to build and maintain. They don’t respond to dynamic changes in the network. Worst of all, they might require you to awake at 3

    AM

    to a page about a connectivity problem due to a misconfigured static route!

    If we forget about reality for a little while, a static routing solution might be the ideal routing protocol. It requires no convergence time and utilizes a minimum of router resources, such as memory and processing time. It supplies you with the ultimate control over packet forwarding. You’ve told each router what to do and they will each perform that function until powered down. If we could only reconfigure the static routes automatically, we would be completely happy.

    Merlot Cabernet

    Riesling Chardonnay

    192.168.16.0/24 192.168.48.0/24

    192.168.32.0/24

  • 110

    Chapter 3 �

    Protocol-Independent Routing

    Using a static route when you have a single physical connection is a valid consideration. Let’s examine Figure 3.2. Shiraz is a border router within Autonomous System 65001 with physical connections to five customer routers. Each of these customers has requested Internet connectivity from AS 65001. With just a single physical connection between each set of routers, a static route solution might be appropriate. Shiraz sets up five static routes, one for each customer. These routes are then advertised by a dynamic routing protocol to the other routers within AS 65001.

    F I G U R E 3 . 2

    Static routes in a service provider

    Each customer router configures a static route for each destination that it wishes to commu- nicate with. This approach has the same limitation we discussed for Chardonnay in Figure 3.1; as the number of reachable destinations grows, the number of static routes also grows. An opti- mal solution to this problem is for the customer to configure a

    default route

    of 0.0.0.0 /0 as its only static route. The default route will provide connectivity to all possible destinations that the customers would ever wish to communicate with.

    Okay, stop dreaming and come back to the real world. As with most things in network engineer- ing, there is a trade-off. The advantages of static routes do not outweigh the disadvantages when it comes to widespread use. You can think of large networks as living, breathing entities. This dynamic nature of a living beast means that a dynamic solution is really the only choice.

    This is not to say that static routes do not have their place—they do! Many Internet service pro- viders (ISPs) use static routes to supply connectivity to single-homed customers. Those cus- tomers, in turn, use static routes to provide connectivity to the Internet.

    AS65001

    Shiraz

    Customer A Customer B Customer C Customer D Customer E

  • Configured Routes

    111

    Now that we have a better understanding of why we’d want to use static routes, let’s learn how to configure them within the JUNOS software.

    Next-Hop Options

    You must configure a valid and usable next hop for a static route before placing the route in the routing table. The JUNOS software provides six different options for a static route next hop:

    Directly connected IP address

    An IP address belonging to a physically connected subnet is often used as the next hop for a static route. The interface connected to the remote router is used to for- ward user packets.

    Remote IP address

    You can also use any known IP address in the network as a next hop. The local router performs a

    recursive lookup

    in the

    inet.0

    routing table to find a physical next hop to the configured address. You enable this functionality by adding the

    resolve

    keyword when defining the IP address.

    reject

    The value

    reject

    is a configured null value. Route lookups that match a static route with a

    reject

    next hop are dropped.

    discard

    The value

    discard

    is also a configured null value. Route lookups matching this next- hop value are also dropped.

    Qualified next hop

    Routes utilizing a

    qualified next hop

    allow you to assign multiple IP address next hops and/or JUNOS software preference values to a single static route. This enables multiple versions of the same prefix to appear in the routing table at the same time. In effect, you end up with a

    floating static route

    . We discuss preference values in the section “JUNOS software Preference Values” later in this chapter.

    Label switched path (LSP)

    In a network configured to use Multiprotocol Label Switching (MPLS), a static route can be assigned an LSP as a next-hop value. All route lookups matching this next hop are forwarded using a label value instead of an IP address. See Chapter