SPECIAL REPORT: Understanding IP ROUTERS
Aug 1, 2006 12:00 PM, BY CIPRIAN POPOVICIU
Today’s broadcast facilities increasingly rely on IP routing technology. The Cisco Catalyst 6513 and two PIX 535 fi rewalls in Lifetime Network’s post-production technical area ensure a secure on-air environment. Photo by Andy Washnik, CORPICOM.
Internet Protocol (IP) has emerged over the past decade as the most deployed data communications transport protocol. To a certain extent, IP's unanimous adoption is due to its simple nature and open standards development. However, the main reason for its unforeseen success is its packet-based and connectionless mode of operation.
With IP, data of any type is placed inside a packet, which is stamped with a destination and a source address. The packet is then left at the mercy of an IP network, which is supposed to somehow get it to its destination.
IP networks represent a collection of devices that have the sole purpose of moving packets from the source to the destination. (See Figure 1.) These highly customized computing devices are called IP routers, and they are interconnected via multiple links that terminate on their interfaces. There are two primary operational planes for a router. The control plane creates and maintains a map that allows the router to make the best forwarding decisions. The data plane forwards IP packets from the ingress to the egress interface.
Control plane — routing
All routers in an IP network must collaborate in order to make sure that each one of them is capable of identifying the best paths and the best backup paths to reach the destinations. In order to perform this function, routers will communicate with their neighbors or routers from an entire domain of the network, exchanging relevant information regarding the state of the network and the reachability of each IP address. Based on this information, each router calculates the best path to all known IP destinations. IP routing protocols identify the mechanism by which routers communicate with each other and the algorithm used to calculate the best routes.
There are multiple routing protocols defined for IP. Those used within an area of the network or an administrative domain are called Interior Gateway Protocols (IGPs). Examples of IGPs include Routing Information Protocol (RIP), InterGateway Routing Protocol (IGRP), Enhanced InterGateway Routing Protocol (EIGRP), Open Shortest Path First (OSPF) and Intermediate System to Intermediate System (ISIS).
Figure 1. A representation of the connectionless and packet-based forwarding nature of IP
communications
Click image to enlarge.
The routing protocols used to route between domains or networks are called Exterior Gateway Protocols (EGPs). The representative example of EGP is Border Gateway Protocol (BGP).
The IGPs are also classified based on their principle of operation. OSPF and ISIS are called link-state protocols, and they maintain a view or state of the entire network. RIP, IGRP and EIGRP are called distance-vector protocols, and they rely on their neighbors to make routing decisions. Each routing protocol has its own benefits and deficiencies, and each is best suited for a certain network environment.
The outcome is a routing table indicating what next hop a packet needs to be sent to in order to be delivered optimally to a particular IP address or to reach destinations within an IP subnet. (See Figure 2.) This data is dynamically maintained. If a path becomes impaired or nonoptimal, the routing protocol will update the routing table.
Data plane — forwarding
Figure 2. A representation of routing control plane operation
Click image to enlarge.
A router has multiple interfaces, often of different media types such as Ethernet, Packet Over SONET (POS), Asynchronous Transfer Mode (ATM) and Integrated Services Digital Network (ISDN), etc. IP packets are delivered to an interface encapsulated in an envelope specific to that interface type.
In the process of forwarding the packet, routers must first unwrap the media-specific information, analyze the header for integrity, if necessary, and extract the relevant IP information, primarily the destination IP address. The router will then use the knowledge learned via the control plane (the routing table, which is mapped into a forwarding table) to switch the IP packet to the interface identified for the optimal path. The packet is then wrapped up in the frame specific to that interface's media type, and it is sent to the neighboring router.
The basic concepts of the forwarding process are presented in Figure 3. In reality, some parameters of the IP packet itself will have to be slightly manipulated in the process of switching, in which case the packet must be rewritten before it is encapsulated into the media-specific frame. While a centralized CPU performs the control plane functions, a CPU can do the data plane forwarding, or it can be done with the help of dedicated hardware.
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