SPECIAL REPORT: Understanding IP ROUTERS
Nov 1, 2006 12:00 PM, BY CIPRIAN POPOVICIU
Data plane — forwarding
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). 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.
Router architecture
With the rapid adoption of IP, more is required of IP networks and IP routers. Large amounts of traffic must be switched with minimal packet loss, and time-sensitive applications require packet delivery with minimal delay and jitter. These requirements demand high-performance router architectures that leverage powerful processors or the implementation of forwarding functions into hardware. Figure 4 compares software and hardware router architectures.
A router's position within the network dictates its required capabilities. Core routers must forward large amounts of traffic, a capability that can be implemented in hardware, while edge routers must support a rich set of features and functions that might not be suited for full hardware implementations. Their price and flexibility ultimately dictates the router selection for specific roles within a network.
Advanced router features
IP has outgrown its original scope of simply transporting data between two end points. It is now used to deliver a wide variety of services, each service requiring advanced functionalities and feature support by the IP routers. For example:
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Voice, audio and video services require a certain QoS to be enforced. Thus, routers support a set of congestion avoidance, congestion management and resource management mechanisms that enable them to treat IP packets based on the service requirements.
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Content delivery and collaborative services are supported in a scalable manner by enabling the IP networks and their routers to optimally multicast packets from a source to a set of listeners.
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Traffic control and security concerns require routers to be capable of filtering traffic based on certain parameters and to make more complex forwarding decisions than simply looking up the packet destination address in the forwarding table.
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The operation of today's networks requires routers to support various additional control and management protocols.
These functions — some integrated in hardware while others handled exclusively by the CPU — stand witness to the extraordinary evolution of the IP router from its original, basic IP switching role to its current critical role in supporting complex services.
Ciprian Popoviciu, PhD, CCIE, is a technical leader within the Networked Solutions Integration Test Engineering (NSITE) group at Cisco Systems.
Ciprian is an author of “Deploying IPv6 Networks,” a comprehensive guide to IPv6 concepts, service implementation and existing interoperability in IPv4 environments. It's available from Cisco Press at www.ciscopress.com/title/1587052105.
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