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Last Updated: 6/24/2011 6:08:51 PM
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A Look at Common Routing Protocols

By Erik Rodriguez

This article describes the common interior protocols used by routers. This includes low and high-end routers.

For information on the hardware based portion of routers click here. The interior protocols are used more commonly than the exteriors. Exterior protocols are used mostly by an ISP for backbone connectivity and MAN (metropolitan area network) connections. Do not confuse the two terms "routing protocol" and "routed protocol." They are two different concepts. The list below shows 3 classes of routing protocols:

Interior Protocols Classes
Distance Vector
Link State
  • Distance Vector

  • The distance vector class of routing determines the direction (vector) and distance used to send data over a network. These types of protocols share routing tables with neighboring routers to reflect the topology and status of the network. This method saves system resources on the router because it only broadcasts a portion of the routing table. This is done using complicated algorithms. Distance vector algorithms are also known as the Bellman-Ford algorithms. Distance Vector Protocols include IGRP and RIP. The way I explain distance vector routing is using a highway intersection. Signs point toward a destination and show the distance to the destination. As you driver further, another sign shows the destination, but now the distance to the destination is shorter. If the distance continues to get shorter, you know you're taking the right "route" or path to the destination.
    • IGRP (Interior Gateway Routing Protocol)

    Developed by Cisco, IGRP uses several methods to determine the correct route for traffic. These methods are combined to create something called a "metric." Metrics are sets of numbers based on network delay, bandwidth, reliability, and load. It is not uncommon for a router to hold several metrics. The router can determine which metric to use based on the traffic it needs to route. IGRP is commonly used in medium sized networks.
    • RIP (Routing Information Protocol)

    RIP is similar to IGRP in some ways. It is supported by low-end routers (linksys, D-link, belkin, etc.) commonly used for broadband connections. RIP uses the same method as IGRP, meaning it sends broadcasts of its routing table to neighboring devices. This is done quite often (the default is every 30 seconds). This protocol also uses metrics. However, its metric values are calculated using only the "hop count." RIP will route data based upon the lowest hop count regardless of bandwidth limitations. If a hop count is greater than 15, the data is discarded. This becomes inefficient in large networks that use multiple routers. RIP is a good protocol for small networks.

    RIP comes in version 1 and version 2. Version 2 is completely backward-compatible with version 1. There are numerous improvements in version 2 including added security, larger packet capability, optional multi-casting, and most importantly support for VLSM. RIP version 2 is now considered the standard and is currently used more than RIP version 1.

  • Link State

  • The link-state approach, also known as the Dijkstras algorithm or as "shortest path first" (SPF) are better for large networks. They are "smarter" than distance vector protocols because they maintain a complex database of the network topology and status. This is accomplished using "link state advertisements" (LSA). LSAs are used to "map" out the network. The router uses this map to determine the shortest path for data destinations. The problem with this is that every time a configuration in the network changes, LSAs are broadcasted to and from all routers on the network. This causes a spike in router CPU usage, memory, and network bandwidth.
    • OSPF (Open Shortest Path First)

    Standard protocol supported by high-end routers, OSPF is mostly used in large networks. It is commonly known as the "standard link state protocol." The main problem arises from LSA broadcasts that "flood" the network. Advanced DoS attacks can target internal routers using the OSPF protocol to constantly send requests to routers forcing them to consume large amounts of network bandwidth.

  • Hybrid

  • Also called "balanced hybrids," these protocols use a combination of distance vector and link state properties to function. The idea of developing this type of protocol was to solve to solve the problems encountered with the other two protocols.
    • EIGRP (Enhanced Interior Gateway Protocol)

    Developed by Cisco, EIGRP uses distance vector and link state methods to determine the best path for routing data. It calculates the SPF, uses load balancing, and uses an altered form of LSA broadcasts. Broadcasts are conducted every 90 seconds or whenever the network topology changes. The SPF is calculated using the Diffused Update Algorithm (DUAL).

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