An internal gateway protocol (IGP) manages routing of network traffic within an autonomous system (AS) with multiple routers. Distance vector protocols calculate shortest paths based on hop count, while link state protocols share more information and converge faster. EIGRP combines both methods and considers bandwidth and speed.
An internal gateway protocol (IGP) is a method for network administrators to manage the routing of computer network traffic from one part of a controlled network to another. An internal gateway protocol is only necessary if there are multiple routers that must be traversed for traffic to bypass the network. In cases where an IGP is needed, the network is referred to as an autonomous system (AS). The IGP, therefore, is responsible for making sure that all routers in the AS know how to move traffic through each other to their destinations. This is different from an external gateway protocol, which is responsible for directing traffic to or from a particular AS.
An internal gateway protocol is considered a dynamic routing protocol due to its ability to automatically update routing information for each router. Compared to a static method, where the administrator is required to manually update each router, an IGP is significantly more useful for a larger router network; the static method is best for smaller or single router networks. There are several types of internal gateway protocols that fall into a couple of general classifications.
A distance vector routing protocol is based on an algorithm where each router in the AS calculates the shortest path to a destination by counting the number of other routers that the data must traverse to reach a destination. Routers will send messages to each other to trace a path where every other router encountered counts as a “hop” along the path. The path with the fewest hops is then known to the router as the preferred path for data packets. If a router on this path goes offline, the router looks for the next lowest hop count path, and so on.
One drawback is that internal gateway protocols based on distance vector routing can have problems with delay. Whenever a new router is added or removed from the AS, all routers must converge again to determine the shortest path. The delay occurs because routers wait three minutes before abandoning the preferred path and starting the convergence process by looking for a new path. Distance vector IGP routing doesn’t even know if a link to a particular router is faster than another and just relies on the number of hops between them as the ideal route.
The other type of internal gateway protocol is the link state method. In a link-state protocol, each router in the AS shares a little more information. As each router talks to another, they build a database containing information about the other routers in the AS, including how fast communication between the routers should occur. The database is then processed in each router and routing tables are processed. With a link-state IGP, the AS is able to undergo rapid changes and is able to quickly redirect to various other routers should a route become unavailable; link state convergence in a routing protocol occurs in seconds, rather than minutes.
Link-state internal gateway protocols also have a disadvantage in that they tend to use more processing resources than their distance vector cousins. The routers in the AS do a lot of calculations on the fly during convergence, as well as collect and hold a good deal of information, so they tend to use more processor power and memory. If a network using the link state method suffers frequent removal or addition of routers, this can be costly, as convergence quickly floods the AS routers with new information. As a workaround, routers are separated into hierarchies where only routers within a particular group converge with each other. A backbone router, called an area border router (ABR), then converges with the other ABRs to complete convergence across the AS.
Something of a mix of the two types is the Enhanced Interior Gateway Routing Protocol (EIGRP). While EIGRP is proprietary to Cisco Systems routers, it takes both methodologies into consideration. In an EIRGP AS, routers store as many possible routes to a destination and use the best route first, unless that route becomes unavailable. At that point, the router immediately falls back to the secondary path. In addition to the number of hops calculated in a path, EIGRP also stores information about bandwidth and speed between hops.
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