6.8 Hierarchical Routing Architectures

Any Routing Architecture that may support flat architectural model (i.e. a nonhierarchical architectural model) cannot be scalable as the number of routers in the internetworks continues to grow beyond a particular value (Banerjee 2004). The principle behind the Hierarchical Routing Architectures lies in partitioning the given large internetworks into multiple levels of hierarchy.

8.1 The Interior Gateway Protocol (IGP)

The term IGP (Interior Gateway Protocol) is used to describe any routing protocol operating as a separate routing domain within an AS (Autonomous System). IGPs learn about routes to networks that are internal to the AS, hence the name Interior (Osterloh 2002).
Within an organization's network there may be one or more routing protocols (IGPs) keeping track of the routes to subnets within the AS. Routers running a single IGP (routing protocol) only share route information with other routers running the same routing protocol. Routers running more than one IGP, like RIP and OSPF, are participants in two separate routing domains. These routers are referred to as border routers, that is, they sit on the border between two IGP routing domains. Some examples of IGP protocols are: RIP, OSPF, and IGRP (Osterloh 2002). IGPs are responsible for building and maintaining route information within a single Autonomous System (see Figure 2).

6.8 Hierarchical Routing Architectures

8.2 The Interior Gateway Routing Protocol (IGRP)

Interior Gateway Routing Protocol (IGRP) was developed in the mid-1980s by Cisco Systems, Inc. Cisco motivations to create such an internal gateway protocol (Osterloh 2002):

It would be more robust than RIP for routing within an autonomous system (AS) or routing domain.

Allow for more flexible path selection on networks with more complex topologies, which might have links with

different bandwidth capacity and delay characteristics.

This protocol must be capable of distinguishing between a 10Mbps Ethernet connection and a 64Kbps serial

link, rather than determining best path based solely on a single metric (hops).

Each AS is a routing domain or a collection of networks under common administration that share a common routing strategy (IGRP). An IGRP AS is typically given a unique number that identifies all members of that routing domain. RIP is a very simple protocol that relies on a single metric value based on distance (measured by hop count) to determine best path between source and destination through a network. A known example of RIP's limitations is distance a datagram can travel. RIP has a hop count limitation of 15 nodes, in essence capping the diameter of a given network. Any network with a diameter greater than 15 (16 or more hops) is considered "unreachable." This maximum distance limitation restricts the growth and expansion of network size (Osterloh 2002).

6.8 Hierarchical Routing Architectures

8.3 The Exterior Gateway Protocol (EGP)

Exterior Gateway Protocols, such as BGP (Border Gateway Protocol), are designed to serve as a conduit for communication between autonomous systems. BGP is the most popular inter-autonomous system (or Inter-AS) routing protocol used throughout the Internet community. EGP protocols connect separate AS' together providing a transit path between and through these AS' to facilitate traffic forwarding across the Internet. EGP protocols only recognize the AS within the routing hierarchy, ignoring the IGPs. Border routers from different AS' typically run some type of IGP over the interfaces within their home AS and BGP or some other EGP over the external interface used to connect this AS to a remote AS (Osterloh 2002).

8.4 The Border Gateway Protocol (BGP)

Border Gateway Protocol (BGP) is one of the most popular routing protocols in use today. BGP is a very robust protocol that has become the backbone of the Internet. The main purpose of BGP is to advertise a network's presence (and structures) to other BGP routers on the Internet (more specifically the routers of an ISP). BGP routes data between Autonomous Systems. Each system (logically) has no inherent knowledge of any other system in the world. Because no system knows the topology (or for that matter the existence of) any other system, exchanging data between systems can be very difficult (Osterloh 2002).
Although BGP is best known for its capability to route between dissimilar ASs, it is actually comprised of two separate protocols. The EBGP (External Border Gateway Protocol) is used to route data between Autonomous Systems, and the IBGP, or Internal Border Gateway Protocol, is used to route data within a particular AS (Osterloh 2002).