Dynamic Routing Internet Routing

individual host this is default value if omitted. destination Specifies a destination system where the packet will be routed. It can be either a host name, a network name, an IP address in dot notation, or the keyword default, which signifies the wildcard gateway route. gateway Specifies the gateway router through which the destination is reached; can be either a host name or an IP address in dot notation. count An integer that indicates whether the gateway is a remote host 0 or a local host = 0; this is default value.

15.1.2.2 Dynamic Routing

The routing tables presented here correspond to the situation in which a single router is available; the hosts routing decision for all out−of−network traffic is limited to the only available router. There is no choice between alternative routes to forward data, so the static routing algorithm is quite appropriate. Static routing assumes that, once defined during the system booting, a routing table remains unchanged and valid throughout the systems lifetime. It is recommended that you implement static routing whenever only one router exists in the local network, which is the most common case. A bit of processor time which the system always needs can be saved, as there is no useless routing calculation involved when the output is already known. However, many systems use dynamic routing even under such conditions as in the first example presented. Nothing is fundamentally wrong with that; the system will work properly. But it is fair to say that no real need exists for a dynamic routing when a single router is available. The only pro argument is that such a system is already prepared for any eventual network upgrades which in most cases never happen. A single router in the local network is not acceptable in some network implementations, because a broken router completely isolates the whole network. Introducing a second router could solve this problem. The second router is not only the alternative to a broken first router; two routers can share the routing tasks, making the overall network performance better. In that case we have dynamic routing. Dynamic routing assumes a permanent adaptation of the routing table to current network conditions; it also assumes that the optimal routing decision is always made. How can this highly desirable goal be achieved? A separate process, the gated daemon, is dedicated to this task and runs continuously. There are also other routing daemons, but gated is the most common, and it is a part of the UNIX distribution. The gated daemons running on different remote hosts communicate with one another to update information about the network status; the final output is an updated routing table for each individual host. 15.1.2.3 The gated Daemon Gated is a routing daemon that handles the routing information protocol RIP, border gateway protocol BGP, exterior gateway protocol EGP, and HELLO routing protocol. The gated process can be configured to perform all of these specified routing protocols, or any combination of them. The generic command line is: gated [−c] [−n] [−t trace_option ] [−f config_file ] [ trace_file ] The daemons arguments are: 344 −n Do not modify the kernels routing table; used for testing gated configurations −t trace_option Enable trace flags on startup; see manual pages for details −f config_file Use an alternate configuration file; by default, gated uses etcgated.conf trace_file Trace file in which to place trace information The gated daemon must be started during the system booting through a corresponding rc initialization script, as in the following example this rc startup script is the Korn shell script: if [ −x etcgated ]; then etcgated −f etcgated.conf binecho gated daemon started else binecho Couldnt execute etcgated fi The only condition required to start the gated daemon is the existence of the corresponding executable program; the rest is done with messages sent to console. The configuration file is etcgated.conf, which is used to tune the daemons behavior. Here is an example: cat etcgated.conf redirect yes; rip quiet static { default gateway 146.95.8.249 preference 150; }; This configuration file specifies: redirect ICMP Internet control message protocol messages, i.e., controls routing table changes based on ICMP • quiet specifies that no RIP packets are to be generated • static defines a static route through the second gateway the first gateway is defined in the corresponding rc startup script • This example is just to get a basic idea of what the etcgated.conf file can look like. There are a number of ways to configure the gated daemon.

15.1.3 Multiplexing