The Destination column is the destination network or host. The gateway column

 

The Destination column is the destination network or host. The Gateway column

indicates the gateway to be used in order to reach the destination. The Flags

column describes certain characteristics of this route. Possible values for Flags are:

 

U- the route is up and operational

H- route to a specific host

G- route uses a gateway

D- route was added because of an ICMP redirect

 

The ref column shows the number of this times the route has been referenced to

establish a connection. The Use column shows the number of packets

transmitted via this route. Finally, the Interface column is the name of the

network interface used by this route.

 

The first entry in the table is a special entry that indicates a loopback route for

the local host. Notice that this is a reserved IP address. This entry is used by the

system to send datagrams to itself. Another unique entry in the table is the

default entry. The gateway specified in this row is the default gateway; it is used

whenever there is no specific route in the table for the destination network

address. Those rows with Flags entry U without a G indicate that the host which

owns the routing table above is directly connected to the network specified in

the destination column. While those with UG are networks directly connected to

the local system.

 

 

What we have discussed is how an individual gateway or host routes data. To

facilitate routing, the Internet uses a routing model that is based on co-equal

collections of autonomous system is called routing domains Routing domains

exchange routing information with other domains using Border Gateway

protocol (BGP). Each routing domain processes the information it receives from

other domains. BGP is an exterior routing protocol and as such is concerned with

routing between networks rather than within them.

 

BGP routing involves two basic activities: determination of optimal routing paths

and the transport of data (typically called packets) through the network. The

transport of packets through the network is relatively straightforward. Path

determination, on the other hand, can be very complex. BGP performs routing

and reachability information with information with other BGP systems.

 

BGP performs three types of routing inter-autonomous system routing, intraautonomous

system routing, and pass-through autonomous system routing.

 

Inter-autonomous system routing occurs between two or more BGP routers in

different autonomous systems. Peer routers in these systems use BGP to

maintain a consistent view of the network topology. BGP neighbors

communicating between autonomous systems must reside in the same physical

network. The Internet serves as an example of an entity that uses this type of

routing because it is comprised of autonomous systems or administrative

domains. Many of these domains represent the various institutions, corporations,

and entities that make up the Internet. BGP is frequently used to provide path

determination for optional routing within the internet

 

Intra-autonomous system routing occurs between two or more BGP routers

located within the same autonomous system. Peer routers within the same

autonomous system use BGP to maintain a consistent view of the system

topology. BGP is also used to determine which router will serve as the connection

point for specific external autonomous systems. Once again, the Internet

provides an example of intra-autonomous systems. Once again, the Internet

 

provides an example of intra-autonomous systems routing. An organization, such

as a university, could make use of BGP to provide optimal routing within its own

administrative domain or autonomous system routing services.

 

Pass-through autonomous system routing occurs between two or more BGP peer

routers that exchange traffic across an autonomous system environment, does

not run BGP. In a pass-through autonomous system that environment, the BGP

traffic did not originate within the autonomous system in question and is not

destined for a node in the autonomous system. BGP must interact with whatever

intra-autonomous system routing protocol is being used to successfully transport

BGP traffic through autonomous system.

 

 

As with any routing protocol, BGP maintains routing tables, transmits routing

updates, and bases routing decisions on routing metrics. The primary function of

a BGP system is to exchange network- reachability information, including

information about the list of autonomous system paths, with other BGP systems.

This information can be used to construct a graph of autonomous system

connectivity from which routing loops can be pruned and with which autonomous

system-level policy decisions can be enforced.

 

Each BGP router maintains a routing table that lists all feasible paths to a

particular network. The router does not refresh the routing table, however.

Instead, routing information received from peer routers is retained until an

incremental update is received.

 

BGP devices exchange routing information upon initial data exchange and after

incremental updates. When a router first connects to the network, BGP routers

exchange their entire BGP routing tables. Similarly, when the routing table

changes, routers send the portion of their routing table that has changed. BGP

routers do not send regularly scheduled routing updates, and BGP routing

updates advertise only the optimal path to a network.

 

BGP uses a single routing metric to determine the best path to a given network.

This metric consists of an arbitrary unit number that specifies the degree of

preference of a particular link. The BGP metric typically is assigned to each link

by the network administrator. The value assigned to a link can be based on any

number of criteria, including the number of autonomous systems through which

the path passes, stability, speed, delay, or cost.

 

An example of a snapshot of an actual BGP routing table raw data is given below. The

table where it came from contains several hundreds of thousands of entries.