Operation of IGRP

From a high-altitude view, IGRP shares many operational characteristics with RIP. It is a classful distance vector protocol that periodically broadcasts its entire routing table—with the exception of routes suppressed by split horizon—to all its neighbors. Like RIP, IGRP broadcasts a request packet out all IGRP-enabled interfaces upon startup and performs a sanity check on received updates to verify that the source address of the packet belongs to the same subnet on which the update was received.111 New update entries with reachable metrics are placed in the routing table, and an entry replaces an older entry to the same destination only if the metric is smaller. Split horizon with poisoned reverse, triggered updates, and holddown timers are used for stability; IGRP summarizes addresses at network boundaries.

111 This sanity check can be disabled With the command no validate-update-source.

Unlike RIP, which is accessed via UDP, the IGRP process is accessed directly from the IP layer as protocol 9.

IGRP also utilizes the concept of autonomous systems. Recall from Chapter 4,"Dynamic Routing Protocols," that an autonomous system can be defined either as a routing domain or as a process domain. An IGRP autonomous system is an IGRP process domain—a set of routers whose common routing protocol is an IGRP process.

By defining and tracking multiple autonomous systems, IGRP allows the establishment of multiple process domains within an IGP environment, isolating the communications within one domain from the communications within another domain. Traffic between the domains can then be closely regulated by redistribution (Chapter 11, "Route Redistribution" ) and route filtering (Chapter 13, "Route Filtering" ).

Figure 6.1 illustrates the contrast between process domains and routing domains. Here two autonomous systems (ASs) are defined: AS 10 and AS 40. These systems are routing domains-a set of routers running one or more IGPs under a common administration. They communicate via an Exterior Gateway Protocol (in this case, Border Gateway Protocol, or BGP).

Figure 6.1. An autonomous system number may specify a routing domain, which is a group of routers running one or more IGP processes under a single administrative domain. An autonomous system number may also specify a process domain, which is a group of routers sharing routing information by means of a single routing process.

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Figure 6.1. An autonomous system number may specify a routing domain, which is a group of routers running one or more IGP processes under a single administrative domain. An autonomous system number may also specify a process domain, which is a group of routers sharing routing information by means of a single routing process.

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Within AS 10 are two IGRP process domains: IGRP 20 and IGRP 30. Under IGRP, the 20 and 30 are defined as autonomous system numbers. In this context the numbers serve to distinguish two routing processes within the same routing domain. IGRP 20 and IGRP 30 communicate via the single router connected to both domains. This router runs both IGRP processes and redistributes between them. The configuration section of this chapter includes a case study demonstrating the configuration of multiple IGRP process domains.

Within its updates, IGRP classifies route entries into one of three categories: interior routes, system routes, and exterior routes.

An interior route is a path to a subnet of the network address of the data link on which the update is being broadcast. In other words, a subnet advertised as an interior route is "local" to the major network to which the advertising router and the receiving router are commonly connected .

A system route is a path to a network address, which has been summarized by a network boundary router.

An exterior route is a path to a network that has been flagged as a default network. A default network is an address to which a router will send any packet that cannot be matched to a more specific destination.121

Default networks and their configuration are covered in Chapter 12, "Default Routes and On-Demand Routing."

121 Classifying a default network as an external route is unique to IGRP and EIGRP. Open protocols such as RIP and OSPF advertise default networks with the address 0.0.0.0.

Figure 6.2 shows how IGRP uses these three categories. The routers LeHand and Tully are connected to subnet 192.168.2.64/26, so major network 192.168.2.0 is considered the "local" network shared by those two routers. LeHand is also attached to 192.168.2.192/26, which is another subnet of the network connecting the two routers. Therefore, LeHand advertises that subnet to Tully as an internal route.

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