In the two decades since their invention, the heterogeneity of networks
has expanded further with the deployment of Ethernet, Token Ring, Fiber
Distributed Data Interface (FDDI), X.25, Frame Relay, Switched Multimegabit
Data Service (SMDS), Integrated Services Digital Network (ISDN), and most
recently, Asynchronous Transfer Mode (ATM). The Internet protocols are the best
proven approach to internetworking this diverse range of LAN and WAN
The Internet Protocol suite includes not only lower-level
specifications, such as Transmission Control Protocol (TCP) and Internet
Protocol (IP), but specifications for such common applications as electronic
mail, terminal emulation, and file transfer. Figure
1 shows the TCP/IP protocol suite in relation to the OSI Reference
model. Figure 2 shows some of the important
Internet protocols and their relationship to the OSI Reference Model. For
information on the OSI Reference model and the role of each layer, please refer
to the document Internetworking Basics.
The Internet protocols are the most widely implemented multivendor
protocol suite in use today. Support for at least part of the Internet Protocol
suite is available from virtually every computer vendor.
This section describes technical aspects of TCP, IP, related protocols,
and the environments in which these protocols operate. Because the primary
focus of this document is routing (a layer 3 function), the discussion of TCP
(a layer 4 protocol) will be relatively brief.
TCP is a connection-oriented transport protocol that sends data as an
unstructured stream of bytes. By using sequence numbers and acknowledgment
messages, TCP can provide a sending node with delivery information about
packets transmitted to a destination node. Where data has been lost in transit
from source to destination, TCP can retransmit the data until either a timeout
condition is reached or until successful delivery has been achieved. TCP can
also recognize duplicate messages and will discard them appropriately. If the
sending computer is transmitting too fast for the receiving computer, TCP can
employ flow control mechanisms to slow data transfer. TCP can also communicates
delivery information to the upper-layer protocols and applications it supports.
All these characteristics makes TCP an end-to-end reliable transport protocol.
TCP is specified in RFC 793
Figure 1 – TCP/IP Protocol Suite in Relation to the OSI Reference
Figure 2 – Important Internet Protocols in Relation to the OSI
Refer to the
section of Internet
Protocols for more information.
IP is the primary Layer 3 protocol in the Internet suite. In addition
to internetwork routing, IP provides error reporting and fragmentation and
reassembly of information units called datagrams for transmission over networks
with different maximum data unit sizes. IP represents the heart of the Internet
Note: The term IP in the section refers to IPv4 unless otherwise stated
IP addresses are globally unique, 32-bit numbers assigned by the
Network Information Center. Globally unique addresses permit IP networks
anywhere in the world to communicate with each other.
An IP address is divided into two parts. The first part designates the
network address while the second part designates the host address.
The IP address space is divided into different network classes. Class A
networks are intended mainly for use with a few very large networks, because
they provide only 8 bits for the network address field. Class B networks
allocate 16 bits, and Class C networks allocate 24 bits for the network address
field. Class C networks only provide 8 bits for the host field, however, so the
number of hosts per network may be a limiting factor. In all three cases, the
left most bit(s) indicate the network class. IP addresses are written in dotted
decimal format; for example, 126.96.36.199. Figure 3
shows the address formats for Class A, B, and C IP networks.
Figure 3 – Address Formats for Class A, B, and C IP
IP networks also can be divided into smaller units called subnetworks
or "subnets." Subnets provide extra flexibility for the network administrator.
For example, assume that a network has been assigned a Class A address and all
the nodes on the network use a Class A address. Further assume that the dotted
decimal representation of this network's address is 188.8.131.52. (All zeros in the
host field of an address specify the entire network.) The administrator can
subdivide the network using subnetting. This is done by "borrowing" bits from
the host portion of the address and using them as a subnet field, as depicted
in Figure 4.
Figure 4 – "Borrowing" Bits
If the network administrator has chosen to use 8 bits of subnetting,
the second octet of a Class A IP address provides the subnet number. In our
example, address 184.108.40.206 refers to network 34, subnet 1; address 220.127.116.11
refers to network 34, subnet 2, and so on.
The number of bits that can be borrowed for the subnet address varies.
To specify how many bits are used to represent the network and the subnet
portion of the address, IP provides subnet masks. Subnet masks use the same
format and representation technique as IP addresses. Subnet masks have ones in
all bits except those that specify the host field. For example, the subnet mask
that specifies 8 bits of subnetting for Class A address 18.104.22.168 is
255.255.0.0. The subnet mask that specifies 16 bits of subnetting for Class A
address 22.214.171.124 is 255.255.255.0. Both of these subnet masks are pictured in
Figure 5. Subnet masks can be passed through a
network on demand so that new nodes can learn how many bits of subnetting are
being used on their network.
Figure 5 – Subnet Masks
Traditionally, all subnets of the same network number used the same
subnet mask. In other words, a network manager would choose an eight-bit mask
for all subnets in the network. This strategy is easy to manage for both
network administrators and routing protocols. However, this practice wastes
address space in some networks. Some subnets have many hosts and some have only
a few, but each consumes an entire subnet number. Serial lines are the most
extreme example, because each has only two hosts that can be connected via a
serial line subnet.
As IP subnets have grown, administrators have looked for ways to use
their address space more efficiently. One of the techniques that has resulted
is called Variable Length Subnet Masks (VLSM). With VLSM, a network
administrator can use a long mask on networks with few hosts and a short mask
on subnets with many hosts. However, this technique is more complex than making
them all one size, and addresses must be assigned carefully.
Of course in order to use VLSM, a network administrator must use a
routing protocol that supports it. Cisco routers support VLSM with Open
Shortest Path First (OSPF), Integrated Intermediate System to Intermediate
System (Integrated IS-IS), Enhanced Interior Gateway Routing Protocol (Enhanced
IGRP), and static routing. Refer to
Addressing and Subnetting for New Users for more information about IP
addressing and subnetting.
On some media, such as IEEE 802 LANs, IP addresses are dynamically
discovered through the use of two other members of the Internet protocol suite:
Address Resolution Protocol (ARP) and Reverse Address Resolution Protocol
(RARP). ARP uses broadcast messages to determine the hardware (MAC layer)
address corresponding to a particular network-layer address. ARP is
sufficiently generic to allow use of IP with virtually any type of underlying
media access mechanism. RARP uses broadcast messages to determine the
network-layer address associated with a particular hardware address. RARP is
especially important to diskless nodes, for which network-layer addresses
usually are unknown at boot time.
An "internet" is a group of interconnected networks. The Internet, on
the other hand, is the collection of networks that permits communication
between most research institutions, universities, and many other organizations
around the world. Routers within the Internet are organized hierarchically.
Some routers are used to move information through one particular group of
networks under the same administrative authority and control. (Such an entity
is called an autonomous system.) Routers used for information exchange within
autonomous systems are called interior routers, and they use a variety of
interior gateway protocols (IGPs) to accomplish this end. Routers that move
information between autonomous systems are called exterior routers; they use
the Exterior Gateway Protocol (EGP) or Border Gateway Protocol (BGP).
Figure 6 shows the Internet
Figure 6 – Representation of the Internet
Routing protocols used with IP are dynamic in nature. Dynamic routing
requires the software in the routing devices to calculate routes. Dynamic
routing algorithms adapt to changes in the network and automatically select the
best routes. In contrast with dynamic routing, static routing calls for routes
to be established by the network administrator. Static routes do not change
until the network administrator changes them.
IP routing tables consist of destination address/next hop pairs. This
sample routing table from a Cisco router shows that the first entry is
interpreted as meaning "to get to network 126.96.36.199 (subnet 1 on network 34),
the next stop is the node at address 188.8.131.52":
R6-2500# show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route
Gateway of last resort is not set
184.108.40.206/16 is subnetted, 1 subnets
O 220.127.116.11 [110/65] via 18.104.22.168, 00:00:51, Serial0
22.214.171.124/24 is subnetted, 1 subnets
C 126.96.36.199 is directly connected, Serial0
As we have seen, IP routing specifies that IP datagrams travel through
an internetwork one router hop at a time. The entire route is not known at the
outset of the journey. Instead, at each stop, the next router hop is determined
by matching the destination address within the datagram with an entry in the
current node's routing table. Each node's involvement in the routing process
consists only of forwarding packets based on internal information. IP does not
provide for error reporting back to the source when routing anomalies occur.
This task is left to another Internet protocol—the Internet Control Message
ICMP performs a number of tasks within an IP internetwork. In addition
to the principal reason for which it was created (reporting routing failures
back to the source), ICMP provides a method for testing node reachability
across an internet (the ICMP Echo and Reply messages), a method for increasing
routing efficiency (the ICMP Redirect message), a method for informing sources
that a datagram has exceeded its allocated time to exist within an internet
(the ICMP Time Exceeded message), and other helpful messages. All in all, ICMP
is an integral part of any IP implementation, particularly those that run in
routers. See the Related Information
Related Cisco Support Community Discussions
this document for more information on ICMP.
Interior Routing Protocols (IGPs) operate within autonomous systems.
The following sections provide brief descriptions of several IGPs that are
currently popular in TCP/IP networks. For additional information on these
protocols, please refer to the links in the Related Information section
A discussion of routing protocols within an IP environment must begin
with the Routing Information Protocol (RIP). RIP was developed by Xerox
Corporation in the early 1980s for use in Xerox Network Systems (XNS) networks.
Today, many PC networks use routing protocols based on RIP.
RIP works well in small environments but has serious limitations when
used in larger internetworks. For example, RIP limits the number of router hops
between any two hosts in an internet to 16. RIP is also slow to converge,
meaning that it takes a relatively long time for network changes to become
known to all routers. Finally, RIP determines the best path through an internet
by looking only at the number of hops between the two end nodes. This technique
ignores differences in line speed, line utilization, and all other metrics,
many of which can be important factors in choosing the best path between two
nodes. For this reason, many companies with large internetworks are migrating
away from RIP to more sophisticated routing protocols.
With the creation of the Interior Gateway Routing Protocol (IGRP) in
the early 1980s, Cisco Systems was the first company to solve the problems
associated with using RIP to route datagrams between interior routers. IGRP
determines the best path through an internet by examining the bandwidth and
delay of the networks between routers. IGRP converges faster than RIP, thereby
avoiding the routing loops caused by disagreement over the next routing hop to
be taken. Further, IGRP does not share RIP's hop count limitation. As a result
of these and other improvements over RIP, IGRP enabled many large, complex,
topologically diverse internetworks to be deployed.
Cisco has enhanced IGRP to handle the increasingly large,
mission-critical networks being designed today. This enhanced version of IGRP
is called Enhanced IGRP. Enhanced IGRP combines the ease of use of traditional
distance vector routing protocols with the fast rerouting capabilities of the
newer link state routing protocols.
Enhanced IGRP consumes significantly less bandwidth than IGRP because
it is able to limit the exchange of routing information to include only the
changed information. In addition, Enhanced IGRP is capable of handling
AppleTalk and Novell IPX routing information, as well as IP routing
OSPF was developed by the Internet Engineering Task Force (IETF) as a
replacement for RIP. OSPF is based on work started by John McQuillan in the
late 1970s and continued by Radia Perlman and Digital Equipment Corporation
(DEC) in the mid-1980s. Every major IP routing vendor supports OSPF.
OSPF is an intradomain, link state, hierarchical routing protocol. OSPF
supports hierarchical routing within an autonomous system. Autonomous systems
can be divided into routing areas. A routing area is typically a collection of
one or more subnets that are closely related. All areas must connect to the
OSPF provides fast rerouting and supports variable length subnet
ISO 10589 (IS-IS) is an intradomain, link state, hierarchical routing
protocol used as the DECnet Phase V routing algorithm. It is similar in many
ways to OSPF. IS-IS can operate over a variety of subnetworks, including
broadcast LANs, WANs, and point-to-point links.
Integrated IS-IS is an implementation of IS-IS for more than just OSI
protocols. Today, Integrated IS-IS supports both OSI and IP protocols.
Like all integrated routing protocols, Integrated IS-IS calls for all
routers to run a single routing algorithm. Link state advertisements sent by
routers running Integrated IS-IS include all destinations running either IP or
OSI network-layer protocols. Protocols such as ARP and ICMP for IP and End
System-to-Intermediate System (ES-IS) for OSI must still be supported by
routers running Integrated IS-IS.
EGPs provide routing between autonomous systems. The two most popular
EGPs in the TCP/IP community are discussed in this section.
The first widespread exterior routing protocol was the Exterior Gateway
Protocol. EGP provides dynamic connectivity but assumes that all autonomous
systems are connected in a tree topology. This was true in the early Internet
but is no longer true.
Although EGP is a dynamic routing protocol, it uses a very simple
design. It does not use metrics and therefore cannot make true intelligent
routing decisions. EGP routing updates contain network reachability
information. In other words, they specify that certain networks are reachable
through certain routers. Because of its limitations with regard to today's
complex internetworks, EGP is being phased out in favor of routing protocols
such as BGP.
BGP represents an attempt to address the most serious of EGP's
problems. Like EGP, BGP is an interdomain routing protocol created for use in
the Internet core routers. Unlike EGP, BGP was designed to prevent routing
loops in arbitrary topologies and to allow policy-based route selection.
BGP was co-authored by a Cisco founder, and Cisco continues to be very
involved in BGP development. The latest revision of BGP, BGP4, was designed to
handle the scaling problems of the growing Internet.
In addition to IP and TCP, the Cisco TCP/IP implementation supports
ARP, RARP, ICMP, Proxy ARP (in which the router acts as an ARP server on behalf
of another device), Echo, Discard, and Probe (an address resolution protocol
developed by Hewlett-Packard Company and used on IEEE 802.3 networks). Cisco
routers also can be configured to use the Domain Name System (DNS) when host
name-to-address mappings are needed.
IP hosts need to know how to reach a router. There are several ways
this can be done:
Add a static route in the host pointing to a
Run RIP or some other IGP on the host.
Run the ICMP Router Discovery Protocol (IRDP) in the
Run Proxy ARP on the router.
Cisco routers support all of these methods.
Cisco provides many TCP/IP value-added features that enhance
applications availability and reduce the total cost of internetwork ownership.
The most important of these features are described in the following
Most networks have reasonably straightforward access requirements. To
address these issues, Cisco implements access lists, a scheme that prevents
certain packets from entering or leaving particular networks.
An access list is a sequential list of instructions to either permit or
deny access through a router interface based on IP address or other criteria.
For example, an access list could be created to deny access to a particular
resource from all computers on one network segment but permit access from all
other segments. Another access list could be used to permit TCP connections
from any host on a local segment to any host in the Internet but to deny all
connections from the Internet into the local net except for electronic mail
connections to a particular designated mail host. Access lists are extremely
flexible, powerful security measures and are available not only for IP, but for
many other protocols supported by Cisco routers.
Other access restrictions are provided by the Department of
Defense-specified security extensions to IP. Cisco supports both the Basic and
the Extended security options as described in
the IP Security Option (IPSO). Support of both access lists and the IPSO makes
Cisco a good choice for networks where security is an issue.
Cisco's TCP/IP implementation includes several schemes that allow
foreign protocols to be tunneled through an IP network. Tunneling allows
network administrators to extend the size of AppleTalk and Novell IPX networks
beyond the size that their native protocols can handle.
The applications that use the TCP/IP protocol suite continue to evolve.
The next set of applications on which a lot of work is being done include those
that use video and audio information. Cisco continues to be actively involved
with the Internet Engineering Task Force (IETF) in defining standards that will
enable network administrators to add audio and video applications to their
existing networks. Cisco supports the Protocol Independent Multicast (PIM)
standard. In addition, Cisco's implementation provides interoperability with
the MBONE, a research multicast backbone that exists today.
IP multicasting (the ability to send IP datagrams to multiple nodes in
a logical group) is an important building block for applications such as video.
Video teleconferencing, for example, requires the ability to send video
information to multiple teleconference sites. If one IP multicast datagram
containing video information can be sent to multiple teleconference sites,
network bandwidth is saved and time synchronization is closer to
In some cases, it may be useful to suppress information about certain
networks. Cisco routers provide an extensive set of configuration options that
allow an administrator to tailor the exchange of routing information within a
particular routing protocol. Both incoming and outgoing information can be
controlled using a set of commands designed for this purpose. For example,
networks can be excluded from routing advertisements, routing updates can be
prevented from reaching certain networks, and other similar actions can be
In large networks, some routers and routing protocols are more reliable
sources of routing information than others. Cisco IP routing software permits
the reliability of information sources to be quantified by the network
administrator with the administrative distance metric. When administrative
distance is specified, the router can select between sources of routing
information based on the reliability of the source. For example, if a router
uses both IGRP and RIP, one might set the administrative distances to reflect
greater confidence in the IGRP information. The router would then use IGRP
information when available. If the source of IGRP information failed, the
router automatically would use RIP information as a backup until the IGRP
source became available again.
Translation between two environments using different routing protocols
requires that routes generated by one protocol be redistributed into the second
routing protocol environment. Route redistribution gives a company the ability
to run different routing protocols in workgroups or areas where each is
particularly effective. By not restricting customers to using only a single
routing protocol, Cisco's route redistribution feature minimizes cost while
maximizing technical advantage through diversity.
Cisco permits routing protocol redistribution between any of its
supported routing protocols. Static route information can also be
redistributed. Further, defaults can be assigned so that one routing protocol
can use the same metric for all redistributed routes, thereby simplifying the
routing redistribution mechanism.
Cisco pioneered the mechanisms that allow network administrators to
build serverless networks. Helper addresses, RARP, and BOOTP allow network
administrators to place servers far away from the workstations that depend on
them, thereby easing network design constraints.
With today's complex, diverse network topologies, a router's ability to
aid the monitoring and debugging process is critical. As the junction point for
multiple segments, a router sees more of the complete network than most other
devices. Many problems can be detected and/or solved using information that
routinely passes through the router.
The Cisco IP routing implementation provides commands that
The current state of the routing table, including the routing
protocol that derived the route, the reliability of the source, the next IP
address to send to, the router interface to use, whether the network is
subnetted, whether the network in question is directly connected, and any
The current state of the active routing protocol process, including
its update interval, metric weights (if applicable), active networks for which
the routing process is functioning, and routing information
The active accounting database, including the number of packets and
bytes exchanged between particular sources and destinations.
The contents of the IP cache, including the destination IP address,
the interface through which that destination is reached, the encapsulation
method used, and the hardware address found at that destination.
IP-related interface parameters, including whether the interface and
interface physical layer hardware are up, whether certain protocols (such as
ICMP and Proxy ARP) are enabled, and the current security
IP-related protocol statistics, including the number of packets and
number of errors received and sent by the following protocols: IP, TCP, User
Datagram Protocol (UDP), EGP, IGRP, Enhanced IGRP, OSPF, IS-IS, ARP, and
Logging of all BGP, EGP, ICMP, IGRP, Enhanced IGRP, OSPF, IS-IS, RIP,
TCP, and UDP transactions.
The number of intermediate hops taken as a packet traverses the
Reachability information between
IP is one of over 20 protocols that can be simultaneously routed and
bridged by any Cisco routers. Cisco has added features to its IP implementation
that optimize the performance of Cisco routers in larger, enterprise-wide