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Interdomain Routing Basics


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Interdomain Routing Basics



Overview of Routers and Routing



Segregating the World into Administrations



Border Gateway Protocol Version 4



UPDATE Message and Routing Information



Looking Ahead



Frequently Asked Questions




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Internet Routing Architectures (CISCO)

From: Internet Routing Architectures (CISCO)
Author: Bassam Halabi
Publisher: Cisco Press (53)
More Information

4. Interdomain Routing Basics

This chapter covers the following key topics:

  • Overview of IGPs

    A brief consideration of interior gateway protocols as a point of contrast for this chapter's more in-depth consideration of exterior gateway protocols.

  • Autonomous Systems

    An autonomous system is a set of routers sharing the same routing policies. Various configurations for autonomous systems are possible, depending on how many exit points to outside networks are desired and whether the system should permit through traffic.

  • How BGP Works

    An overview of how the Border Gateway Protocol (version 4) operates, including its message header format, and how and what it negotiates with neighboring routers. The formats and purposes of BGP's four main message types—OPEN, NOTIFICATION, KEEPALIVE, and UPDATE—are covered.

The Internet is a collection of autonomous systems that define the administrative authority and the routing policies of different organizations. Autonomous systems run Interior Gateway Protocols (IGPs), such as RIP, IGRP, EIGRP, OSPF, and ISIS, within their boundaries and interconnect via an Exterior Gateway Protocol (EGP) called the Border Gateway Protocol (BGP).

Routers are devices that direct traffic between hosts. Routers build routing tables that contain their collected information on all the best paths to all the destinations they know how to reach. They both announce and receive route information to and from other routers. This information goes into the routing tables.

Overview of Routers and Routing

Routers develop a hop-by-hop mechanism by keeping track of “next hop” information that enables a data packet to find its destination through the network. A router that does not have a direct physical connection to the destination checks its routing table and forwards the packet to another next hop router that is closer to that destination. The process repeats until the traffic finds its way through the network to its final destination.

EGPs, such as BGP, were introduced because IGPs do not scale in networks that go beyond the enterprise level. IGPs were never designed for the purpose of global internetworking because they do not have the necessary hooks to segregate enterprises into different administrations that are technically and politically independent from one another. This chapter touches upon basic IGP functionality and then explains the specifics of BGP.

Figure 4-1 describes three routers, RTA, RTB, and RTC, connecting three local area networks,,, and, via serial links. Each serial link is repesented by its own network number, which results in three additional networks,,, and Each network has a metric associated with it indicating the level of overhead (cost) of transmitting traffic on that particular link. The link between RTA and RTB, for example, has a cost of 2,000, much higher than the cost of 60 of the link between RTA and RTC. In practice, the link between RTA and RTB is a 56 Kbps link with much bigger delays than the T1 link between RTA and RTC and the T1 link between RTC and RTB combined.

Routers RTA, RTB, and RTC would exchange network information via some interior gateway protocol and build their respective IP routing tables. Figure 4-1 shows examples of RTA's IP routing table for two different scenarios; the routers are exchanging routing information via RIP in one scenario and OSPF in another.

As an example of how traffic is passed between end stations, if host is trying to reach host, it will first send the traffic to RTA. RTA will look in its IP routing table for any network that matches this destination and would find that network is reachable via next hop (RTC) out on Serial line 2 (S2). RTC would receive the traffic and would try to look for the destination in its IP routing table (not shown). RTC would discover that the host is directly connected to its Ethernet 0 interface (E0) and would send the traffic to

Figure 4-1. Basic routing behavior.

In the preceding example, the routing is the same whether RTA is using the RIP or OSPF scenario. RIP and OSPF, however, fall into different categories of IGP protocols, namely distance vector protocols and link state protocols, respectively. For a different routing example in figure 4-1, the results might be different depending on whether you are looking at the RIP or OSPF scenario. It is useful at this point to consider characteristics of both IGP protocol categories, to see how protocols generally have evolved to meet increasingly sophisticated routing demands.

Distance Vector Protocols

Distance vector protocols such as RIP version 1 were mainly designed for small network topologies. The term distance vector derives from the fact that the protocol includes in its routing updates a vector of distances (hop counts). By using hop counts, distance vector protocols do not factor into the routing equation the overhead of sending information over a particular link. Low-speed links are treated equally or sometimes preferred over a high-speed link, depending on the calculated hop count in reaching a destination. This would lead to suboptimal and inefficient routing behaviors.

Consider, for example, the RTA routing tables shown in figure 4-1. In the RIP case, RTA has listed the direct link between RTA and RTB to reach network RTA prefers this link because it requires just one hop via RTB versus two hops via RTC and then RTB. But the preferred route is inefficient because the total cost of the routing path via RTC and then RTB (60 + 60 = 120) is much less than the cost of crossing the RTA-RTB link (2,000).

Another issue with hop counts is the count to infinity restriction: distance vector protocols have a finite limit of hops (15) after which a route is considered unreachable. This would restrict the propagation of routing updates and would cause problems for large networks.

The reliance on hop counts is one deficiency of distance vector protocols; another deficiency is the way that the routing information gets exchanged. Distance vector algorithms work on the concept that routers exchange all the network numbers they can reach via periodic broadcasts of the entire routing table. In large networks, the routing table exchanged between routers becomes very large and very hard to maintain, leading to slower convergence.

Convergence refers to the point in time at which the entire network becomes updated to the fact that a particular route has appeared or disappeared. Distance vector protocols work on the basis of periodic updates and hold-down timers: If a route is not received in a certain amount of time, the route goes into a hold-down state and gets aged out of the routing table. The hold-down and aging process translates into minutes in convergence time before the whole network detects that a route has disappeared. The delay between a route's becoming unavailable and its aging out of the routing tables can result in routing loops and black holes.

Another major drawback of distance vector protocols is their classfull nature and their lack of support of Variable Length Subnet Masks or CIDR. Distance vector protocols do not exchange mask information in their routing updates. A router that receives a routing update on a certain interface will apply to this update its locally defined subnet mask. This would lead to confusion, in case the interface belongs to a network that is variably subnetted, and a misinterpretation of the received routing update.

Finally, distance vector networks are considered to be flat. They present a lack of hierarchy, which translates into a lack of aggregation. This flat nature has made distance vector protocols incapable of scaling to larger and more efficient enterprise networks.

RIP version 2 has added support for VLSM and CIDR, but it still carries most of the other deficiencies that its predecessor, RIP version 1, has.

Link State Protocols

Link state protocols, such as the Open Shortest Path First (OSPF) [1] and Intermediate System-to-Intermediate System (ISIS) [2], are more advanced routing protocols that have addressed the deficiencies of distance vector protocols. Link state protocols work on the basis that routers exchange information elements, called link states, which carry information about links and nodes. This means that routers running link state protocols do not exchange routing tables. Each router inside a domain will have enough bits and pieces of the big puzzle that it can run a shortest path algorithm and build its own routing table.

Following are some of the benefits that link state protocols provide over distance vector protocols:

  • No hop count—No limits on the number of hops a route can take. Link state protocols work on the basis of metrics rather than hop counts.

    As an example of a link state protocol's reliance on metrics rather than hop count, turn again to the RTA routing tables shown in figure 4-1. In the OSPF case, RTA has picked the optimal path to reach RTB by factoring in the cost of the links. Its routing table lists the next hop of (RTC) to reach (RTB). This is in contrast to the RIP scenario, which resulted in a suboptimal path.

  • Bandwidth representation—Link bandwidth and delays are factored in when calculating the shortest path to a certain destination. This leads to better load-balancing based on actual link cost rather than hop count.

  • Better convergence—Link and node changes are flooded into the domain via link state updates. All routers in the domain will immediately update their routing tables.

  • Support for VLSM and CIDR—Link state protocols exchange mask information as part of the information elements that are flooded in the domain. As a result, networks with variable length masks can be easily identified.

  • Better hierarchy—Whereas distance vector networks are flat networks, link state protocols divide the domain into different levels and areas. This hierarchical approach provides better control over network instabilities and a better mechanism to summarize routing updates across areas, specifically, by lumping multiple contiguous routing updates into supersets of routing updates called aggregates.

Even though link state algorithms have provided better routing scalability, which enables them to be used in bigger and more complex topologies, they still should be restricted to interior routing. Link state protocols, by themselves, cannot provide a global connectivity solution required for Internet interdomain routing. In very large networks and in case of route fluctuation caused by link instabilities, link state retransmission and recomputation will become too large for any router to handle.


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