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IP Overview

IP Overview

The Cisco IOS software supports a variety of routing protocols. The Network Protocols Configuration Guide, Part 1 discusses the following network protocols:

The Network Protocols Configuration Guide, Part 2 discusses the following network protocols:

The Network Protocols Configuration Guide, Part 3 discusses the following network protocols:

Each part of the Network Protocols Configuration Guide has an accompanying Network Protocols Command Reference.

This overview chapter provides a high-level description of IP. For configuration information, refer to the appropriate chapter in this publication.


The Internet Protocol (IP) is a packet-based protocol used to exchange data over computer networks. IP handles addressing, fragmentation, reassembly, and protocol demultiplexing. It is the foundation on which all other IP protocols (collectively referred to as the IP Protocol suite) are built. A network-layer protocol, IP contains addressing and control information that allows data packets to be routed.

The Transmission Control Protocol (TCP) is built upon the IP layer. TCP is a connection-oriented protocol that specifies the format of data and acknowledgments used in the transfer of data. TCP also specifies the procedures that the computers use to ensure that the data arrives correctly. TCP allows multiple applications on a system to communicate concurrently because it handles all demultiplexing of the incoming traffic among the application programs.

Cisco's implementation of IP provides most of the major services contained in the various protocol specifications. Cisco IOS software also provides the TCP and User Datagram Protocol (UDP) services called Echo and Discard, which are described in RFCs 862 and 863, respectively.

Cisco supports both TCP and UDP at the transport layer, for maximum flexibility in services. Cisco also supports all standards for IP broadcasts.

IP Routing Protocols

Cisco's implementation of each of the IP routing protocols is discussed in detail at the beginning of the individual protocol sections throughout the IP Routing chapter in this publication.

IP routing protocols are divided into two classes: Interior Gateway Protocols (IGPs) and Exterior Gateway Protocols (EGPs). The IGPs and EGPs that Cisco supports are listed in the following sections.

Note Many routing protocol specifications refer to routers as gateways, so the word gateway often appears as part of routing protocol names. However, a router usually is defined as a Layer 3 internetworking device, whereas a protocol translation gateway usually is defined as a Layer 7 internetworking device. The reader should understand that regardless of whether a routing protocol name contains the word "gateway," routing protocol activities occur at Layer 3 of the OSI reference model.

Interior Gateway Protocols

Interior protocols are used for routing networks that are under a common network administration. All IP interior gateway protocols must be specified with a list of associated networks before routing activities can begin. A routing process listens to updates from other routers on these networks and broadcasts its own routing information on those same networks. Cisco IOS software supports the following interior routing protocols:

Exterior Gateway Protocols

Exterior protocols are used to exchange routing information between networks that do not share a common administration. IP exterior gateway protocols require the following three sets of information before routing can begin:

The supported exterior gateway protocols are

Router Discovery Protocols

Our routers and access servers also support two router discovery protocols, Gateway Discovery Protocol (GDP) and Internet Control Message Protocol (ICMP) Router Discovery Protocol (IRDP). These allow hosts to locate routers and access servers.

GDP was developed by Cisco and is not an industry standard. Unsupported example GDP clients can be obtained upon request from Cisco. Our IRDP implementation fully conforms to the router discovery protocol outlined in RFC 1256.

Multiple Routing Protocols

You can configure multiple routing protocols in a single router or access server to connect networks that use different routing protocols. You can, for example, run RIP on one subnetted network, IGRP on another subnetted network, and exchange routing information between them in a controlled fashion. The available routing protocols were not designed to interoperate, so each protocol collects different types of information and reacts to topology changes in its own way. For example, RIP uses a hop-count metric and IGRP uses a five-element vector of metric information. If routing information is being exchanged between different networks that use different routing protocols, you can use many configuration options to filter the exchange of routing information.

The Cisco IOS software can handle simultaneous operation of up to 30 dynamic IP routing processes. The combination of routing processes on a router or access server consists of the following protocols (with the limits noted):

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