Understanding IP Routing
Troubleshooting IP Routing Protocols
offers you a full understanding of invaluable troubleshooting techniques that help keep
your network operating at peak performance. Whether you are looking to hone your
support skills or to prepare for the challenging CCIE exams, this essential
reference shows you how to isolate and resolve common network failures and to sustain
optimal network operation.
The Transmission Control Protocol/Internet Protocol TCP/IP suite of protocols is
the underlying technology for information exchange on the Internet. TCP/IP uses a
layering approach for computer communications similar to the OSI reference model,
but with fewer than seven layers.
IP operates at the Internet layer of the TCP/IP suite, which corresponds to the
network layer of the OSI reference model. IP provides connectionless data-delivery
services, which involve transmission of information from one part of a network to
another in units of data known as packets or datagrams. The essence of the datagram
delivery service model is that a permanent pre-established end-to-end path is not
required for data transfer between two points in a network.
In a packet-based network, each router in the transmission path makes independent
local decisions regarding the optimal forwarding path toward the destination for any
transit packet. The decision-making is based on forwarding intelligence gathered
either dynamically by means of a routing protocol or manually programmed static routes.
Addressing is an important aspect of the data-forwarding process. For any directed
communication, there is a source and a destination. Addressing allows the target
destination to be specified by the source and allows the destination node to also
identify the source. Addressing is even more important in the datagram delivery mode
of operation because, as in IP forwarding, the data path for any transmission is not
nailed through the intermediate nodes between the source and destination.
AS mentioned previously, within the IP datagram services infrastructure, information
that is to be transmitted from one device to another first is broken down into packets.
Each packet has an IP header, a transport layer (TCP or UDP) header, and a payload,
which is a piece of the original information. Each IP packet is self-contained and
independently is forwarded to the destination through the chain of intermediate devices
that might be along the path of transmission.
The routers in the network depend on a routing protocol or static configuration to
forward the datagrams in a stream to their intended destination. For any destination
address, each node in the data path worries about only the outgoing interface or link
along a locally determined optimal path to the destination (or as specified by a special
The IP forwarding process frequently is described as a hop-by-hop destination-based
forwarding mechanism. This means the routers at each hop along the data path normally
forward packets based on the destination address. However, modern routers also can use
policy-based criteria, such as the source address in a packet to direct the forwarding.
At the destination, packets belonging to the same stream are reassembled into the
original information. This process of forwarding a packet from one node to the other
in a connectionless network on the Layer 3 address (IP address, in this case) also is
referred to as routing. Routers are specialized network devices with acquired intelligence.
So how do routers decide where and how to forward packets traversing the internetwork?
Well, this is done in a couple of ways. As alluded to previously, routers can be manually
preprogrammed with predetermined path information known as static routes, or they can
run applications that facilitate the learning and sharing of routing information automatically.
Obtaining routing information by the latter method is referred to as dynamic routing.