Related Links Networkers session: "IP Routing Technologies" Introduction to Enhanced IGRP IGRP and Enhanced IGRP Technical Tips Enhanced IGRP Optimizations BGP Technical Tips Open Shortest Path First (OSPF) Design Guide External Link For background on RIP, RIPv2, and OSPF, visit: www.ietf.org and choose "RFCs."

Packet™ Magazine Archives, Fourth Quarter 1998 Know Your IP Routing Protocols Choosing the routing protocol that best matches your intranet needs and then configuring it to reflect your particular network policies comprise a significant part of creating an optimal network. At the core of the rich networking features of Cisco IOS® software is the lifeblood of what makes routers do what they do. Routing protocols are fundamental software components in Layer 3 devices that enable routers to exchange essential "network reachability" data -- or information about which networks can be reached and the associated costs -- as well as information about network conditions. This communication enables the network to take the appropriate action to maintain application performance when conditions shift. Choosing and configuring routing protocols make up a strategic component of network design. The protocols -- with a bit of the network manager's configuration help -- are what empower routers to know about what's going on across the network and to use that information to successfully forward packets to their intended destinations. Routing processes build tables and maps so that each router knows where adjacent routers are located. The tables might reflect the up or down status of network components and communications links, if and where there are congested areas, the speeds of the many network paths, and other fundamental information that aids routers in sending packets down an optimal network path. The more sophisticated routing protocols combine this information into composite routing metrics, such as link reliability, delay, traffic load, and bandwidth, to make dynamic determinations about the optimal path a given traffic flow should take. Cisco's Enhanced Interior Gateway Routing Protocol (Enhanced IGRP), for example, combines this advanced decision-making in Cisco networks with schemes that also reduce the overhead required for interrouter communication. "This optimization aims at a best-of-both-worlds approach to 'smart' Layer 3 networking," says Doug Hantula, Consulting Engineer in Cisco's Office of the CTO. "Historically, the larger and more complex the network became, the more overhead the protocols generated, and this was often an unwelcome tradeoff." Enhanced IGRP can be configured to take into account an organization's preferences about security policies in its decision-making. In order for the routers to know about your organization's preferences, you must tell the routing protocols something about the policies -- after you choose the routing protocol that best fits the needs of your networking environment.

Distance-vector protocols calculate the cost ramifications of load-balancing traffic over communications links of various speeds and costs.

Cisco IOS Software Supports the Range of Protocols At a basic level, there are two kinds of routing protocols -- interior and exterior. Interior protocols are those you implement within an intranet, such as a private corporate network, that falls completely under a single administrative domain. Exterior protocols convey routing information among network domains, such as in the Internet. They are intended to protect one system of networks (a domain) against errors or intentional misrepresentation by other domains. The Internet Engineering Task Force's (IETF's) Border Gateway Protocol, Version 4 (BGP4) is the current standard exterior protocol in use. The bulk of the routing protocols in use, however, are of the interior variety. They include the following, listed in chronological order, each with improvements upon the feature sets of earlier protocols: Routing Information Protocol (RIP), derived from Xerox Corporation's XNS for IP networks Interior Gateway Routing Protocol (IGRP) from Cisco for Cisco IP and OSI networks Open Shortest Path First (OSPF) from the IETF for IP networks Integrated Intermediate System-to-Intermediate System (IS-IS) for OSI and IP networks RIPv2 from the IETF for IP networks Enhanced IGRP from Cisco for multiprotocol Cisco networks Cisco IOS software supports all of these interior protocols to allow users to choose the scheme that is optimal for their environments. Some of a network manager's goals in choosing and configuring an interior protocol include the following: Achieving optimal path selection for traffic Fast convergence -- or the rate at which routers are able to make decisions based on network changes Scalability Minimizing update traffic among devices in order to keep network overhead low Cisco IOS software also supports a function called redistribution, which benefits a network that employs two or more routing protocols -- either by design or by circumstance, such as a corporate merger. "Redistribution allows routing information from one routing protocol to get mapped into another so that two subnetworks running different protocols can exchange basic connectivity information," explains Hantula. "As a network grows, route redistribution allows for migration between protocols and still provides full connectivity during migration," he continues. Complex Networks Need Smart Protocols The basic difference among the various routing protocols lies in the sophistication of their decision-making capabilities (and the breadth of their metric support) and is therefore one of the main factors to consider when choosing a protocol to match the characteristics of your network. Generally, the simpler the network topology is, the simpler the routing protocol you can use. A simpler network tends to conserve network capacity in terms of network overhead and processing cycles on the routers. Simpler networks are generally also easier to configure. Intranets are growing more complex, however, so the more sophisticated routing protocols will likely find homes in a growing number of domains. With the exception of OSPF and IS-IS, the interior protocols described are all known as distance-vector protocols, with varying degrees of decision-making capabilities. Distance-vector protocols, by definition, use metrics (distance) and next-hop data (vectors) to make forwarding decisions. Still, some distance-vector protocols, such as RIP, are simplistic and don't scale well. RIP, for example, employs a simple metric in determining a traffic path: the "hops" -- the number of routers -- that data transmission must traverse to its destination. It makes a black-and-white decision based on the shortest hop count. However, the largest hop count allowed in a RIP network is 15, placing a scalability ceiling on the protocol. If your network is fairly simple in terms of the topology and the number of subnetworks, an early distance-vector protocol such as RIP or IGRP could work fine. If you're running a multivendor network, RIP, its successor RIPv2, IS-IS, and OSPF are interoperable across many vendors' router implementations. OSPF Features Overshadowed by Topology Limits Industry-standard OSPF and IS-IS both are link-state protocols, which determine a traffic path through the network on the basis of whether the communication links that form the shortest path to the destination are active. The open specification of this decision-making is what allows OSPF to be implemented on any vendor's router for consistent decision-making through the network. It also carries a minimum of overhead. The drawback to OSPF and IS-IS, though, is that they place fairly significant constraints on network topology options. They require a hub, or "star," configuration, which in turn requires groupings of subnetworks called areas to be adjacent to a single backbone area. Consequently, all interarea traffic and routing information must traverse this backbone area. "This characteristic limits the size of the backbone area, places overhead and resource burdens on the backbone components, and does not allow for peer backbones in distributed sites," Hantula notes. Enhanced IGRP Blends Best of Link State, Distance-Vector Traits Enhanced IGRP is the most current routing protocol that has been optimized for use in Cisco networks. An implementation aimed at solving the limitations of earlier routing protocols, Enhanced IGRP builds on the fast convergence introduced by its IGRP predecessor. It offers support for multiple Layer 3 protocols -- IP, IPX, and AppleTalk -- and reduces bandwidth utilization by implementing partial and incremental updates for interrouter communications. Only when a topology change occurs do routers exchange updates, sending the changes only, rather than automatically exchanging complete routing tables at regular intervals. In addition, Enhanced IGRP supports variable-length subnet masks (VLSMs), allowing subnets to be different sizes. "With RIP and IGRP, the size of all subnets had to be the same everywhere in a network, which wasted IP addresses," Hantula explains. "A 255-address subnet with only two users on it still used up 255 IP addresses." Enhanced IGRP is backward-compatible with IGRP to protect Cisco customers' investments. Both protocols use the same compound metric. In a mixed IGRP/Enhanced IGRP network, routes are redistributed automatically by default. Users can change this default with a configuration command that does not require the routing processes to restart. Configuration Tip: Keep It Simple! "Choosing a single routing protocol for use throughout the network is generally a good idea," notes Hantula. This is because a consistent implementation of interrouter communication and capabilities leads to consistent network path determination and performance. However, in some cases, such as in a multivendor network, islands of dissimilar routing protocols emerge. In these cases, network managers can use the IOS™ redistribution capability and configure OSPF, RIPv2, or RIP for consistent intervendor router communications. Hantula suggests that when configuring the routing protocol, network managers choose advanced features carefully to avoid unnecessarily complicating the operation of protocols. For example, all routing protocols run on timers. If network managers tweak the timers to speed up the frequency at which the protocols perform their functions or to reduce overhead, they must make sure that they do it consistently across every router in the network. "If you forget one, adjacent routers will misunderstand each other and delete routing information for that router," Hantula explains. In addition, making the protocols work faster increases the consumption of processing resources on the routers and uses more bandwidth. Overall, your routing protocol strategy should balance choosing a protocol that successfully handles the type of network you are running with simplicity of installation and minimal overhead. From there, it is up to the network manager to remain alert when making configuration changes to optimize performance so as to ensure that routers remain in continual and consistent communication.

Doug Hantula, Consulting Engineer to Cisco Systems' Office of the CTO, presented the "IP Routing Technologies" Networkers session. A seven-year veteran of Cisco, his current areas of specialization are Layer 3 designs, network reliability, and Web surfing. To contact him, send e-mail to dhantula@cisco.com.

Top of Page Table of Contents Posted: Tue Jun 22 10:20:00 PDT 1999 Copyright © 1998 Cisco Systems, Inc.