Interior Gateway Routing Protocol

Background

The Interior Gateway Routing Protocol (IGRP) is a routing protocol that was developed in the mid-1980s by Cisco Systems, Inc. Cisco's principal goal in creating IGRP was to provide a robust protocol for routing within an autonomous system (AS).

In the mid-1980s, the most popular intra-AS routing protocol was the Routing Information Protocol (RIP). Although RIP was quite useful for routing within small-to moderate-sized, relatively homogeneous internetworks, its limits were being pushed by network growth. In particular, RIP's small hop-count limit (16) restricted the size of internetworks, and its single metric (hop count) did not allow for much routing flexibility in complex environments. The popularity of Cisco routers and the robustness of IGRP have encouraged many organizations with large internetworks to replace RIP with IGRP.

Cisco's initial IGRP implementation worked in Internet Protocol (IP) networks. IGRP was designed to run in any network environment, however, and Cisco soon ported it to run in OSI Connectionless-Network Protocol (CLNP) networks. Cisco developed Enhanced IGRP in the early 1990s to improve the operating efficiency of IGRP. This chapter discusses IGRP's basic design and implementation. Enhanced IGRP is discussed in "Enhanced IGRP."

IGRP Protocol Characteristics

IGRP is a distance-vector interior gateway protocol (IGP). Distance-vector routing protocols call for each router to send all or a portion of its routing table in a routing-update message at regular intervals to each of its neighboring routers. As routing information proliferates through the network, routers can calculate distances to all nodes within the internetwork.

Distance-vector routing protocols are often contrasted with link-state routing protocols, which send local connection information to all nodes in the internetwork. For a discussion of Open Shortest Path First (OSPF) and Intermediate System-to-Intermediate System (IS-IS), two popular link-state routing algorithms, see "Open Shortest Path First (OSPF)," and "Open System Interconnection (OSI) Protocols," respectively.

IGRP uses a combination (vector) of metrics. Internetwork delay, bandwidth, reliability, and load are all factored into the routing decision. Network administrators can set the weighting factors for each of these metrics. IGRP uses either the administrator-set or the default weightings to automatically calculate optimal routes.

IGRP provides a wide range for its metrics. Reliability and load, for example, can take on any value between 1 and 255; bandwidth can take on values reflecting speeds from 1,200 bps to 10 gigabits per second, while delay can take on any value from 1 to 2 to the 24th power. Wide metric ranges allow satisfactory metric setting in internetworks with widely varying performance characteristics. Most importantly, the metric components are combined in a user-definable algorithm. As a result, network administrators can influence route selection in an intuitive fashion.

To provide additional flexibility, IGRP permits multipath routing. Dual equal-bandwidth lines can run a single stream of traffic in round-robin fashion, with automatic switchover to the second line if one line goes down. Also, multiple paths can be used even if the metrics for the paths are different. If, for example, one path is three times better than another because its metric is three times lower, the better path will be used three times as often. Only routes with metrics that are within a certain range of the best route are used as multiple paths.

Stability Features

IGRP provides a number of features that are designed to enhance its stability. These include hold-downs, split horizons, and poison-reverse updates.

Hold-downs are used to prevent regular update messages from inappropriately reinstating a route that might have gone bad. When a router goes down, neighboring routers detect this via the lack of regularly scheduled update messages. These routers then calculate new routes and send routing update messages to inform their neighbors of the route change. This activity begins a wave of triggered updates that filter through the network. These triggered updates do not instantly arrive at every network device, so it is therefore possible for a device that has yet to be informed of a network failure to send a regular update message (indicating that a route that has just gone down is still good) to a device that has just been notified of the network failure. In this case, the latter device would contain (and potentially advertise) incorrect routing information. Hold-downs tell routers to hold down any changes that might affect routes for some period of time. The hold-down period usually is calculated to be just greater than the period of time necessary to update the entire network with a routing change.

Split horizons derive from the premise that it is never useful to send information about a route back in the direction from which it came. Figure 38-1 illustrates the split-horizon rule. Router 1 (R1) initially advertises that it has a route to Network A. There is no reason for Router 2 (R2) to include this route in its update back to R1 because R1 is closer to Network A. The split-horizon rule says that R2 should strike this route from any updates it sends to R1. The split-horizon rule helps prevent routing loops. Consider, for example, the case where R1's interface to Network A goes down. to R1horizons, R2 continues to inform R1 that it can get to Network A (through R1). If R1 does not have sufficient intelligence, it actually might pick up R2's route as an alternative to its failed direct connection, causing a routing loop. Although hold-downs should prevent this, split horizons are implemented in IGRP because they provide extra algorithm stability.

Figure 38-1: The split horizons rule helps protect against routing loops.

Split horizons should prevent routing loops between adjacent routers, but poison-reverse updates are necessary to defeat larger routing loops. Increases in routing metrics generally indicate routing loops. Poison-reverse updates then are sent to remove the route and place it in hold-down. In Cisco's implementation of IGRP, poison-reverse updates are sent if a route metric has increased by a factor of 1.1 or greater.

Timers

IGRP maintains a number of timers and variables containing time intervals. These include an update timer, an invalid timer, a hold-time period, and a f lush timer. The update timer specifies how frequently routing update messages should be sent. The IGRP default for this variable is 90 seconds. The invalid timer specifies how long a router should wait, in the absence of routing-update messages about a specific route before declaring that route invalid. The IGRP default for this variable is three times the update period. The hold-time variable specifies the hold-down period. The IGRP default for this variable is three times the update timer period plus 10 seconds. Finally, the flush timer indicates how much time should pass before a route should be flushed from the routing table. The IGRP default is seven times the routing update period.

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