Static And Dynamic Routing

Static

Static routing is not really a protocol, simply the process of manually entering routes into the routing table via a configuration file that is loaded when the routing device starts up. As an alternative, these routes can be enterd by a network administrator who configures the routes. Since these routes don't change after they are configured (unless a human changes them) they are called 'static' routes.

Static routing is the simplest form of routing, but it is a manual process and does not work well when the routing information has to be changed frequently or needs to be configfured on a large number of routing devices (routers). Static routing also does not handle outages or down connections well because any route that is configured manually must be reconfigured manually to fix or repair any lost connectivity.


Dynamic Routing

Dynamic routing performs the same function as static routing except it is more robust. Static routing allows routing tables in specific routers to be set up in a static manner so network routes for packets are set. If a router on the route goes down the destination may become unreachable. Dynamic routing allows routing tables in routers to change as the possible routes change. There are several protocols used to support dynamic routing including RIP and OSPF.
Routing cost

Counting route cost is based on one of the following calculations:

* Hop count - How many routers the message must go through to reach the recipient.
* Tic count - The time to route in 1/18 seconds (ticks).

Dynamic routing protocols do not change how routing is done. They just allow for dynamic altering of routing tables.
There are two classifications of protocols:

1. IGP - Interior Gateway Protocol. The name used to describe the fact that each system on the internet can choose its own routing protocol. RIP and OSPF are interior gateway protocols.
2. EGP - Exterior Gateway Protocol. Used between routers of different systems. There are two of these, the first having the same name as this protocol description:
1. EGP - Exterior Gateway Protocol
2. BGP - Border Gateway Protocol.

The daemen "routed" uses RIP. The daemon "gated" supports IGP's and EGP's.

Route Discovery Methods

* Distance vector - Periodically sends route table to other routers. Works best on LANs, not WANs.
* Link-state - Routing tables are broadcast at startup and then only when they change. OSPF uses link-state.

Routing Information Protocol (RIP)

The RIP RFC is 1058.
The routing daemon daemon adds a routing policy to the system. If there are multiple routes to a destination, it chooses the best one. The RIP message can con contain information on up to 25 routes. The RIP message contains the following components:

1. Command
2. Version - Normally 1 but set to 2 for RIP version 2.
3. family - Set to 2 for IP addresses.
4. IP address - 32 bit IP address
5. Metrics - Indicate the number of hops to a given network, the hop count.

RIP sends periodically broadcasts its routing table to neighboring routers. The RIP message format contains the following commands:

* 1 - request
* 2 - reply
* 3 & 4 - obsolete
* 5 - poll entry
* 6 - Asks for system to send all or part of routing table

When the daemon "routed" starts, it sends a request out all its interfaces for other router's routing tables. The request is broadcast if the network supports it. For TCP/IP the address family in the message is normally 2, but the initial request has address family set to 0 with the metric set to 16.

Regular routing updates are sent every 30 seconds with all or part of the route table. As each router sends routing tables (advertises routes to networks its NICs interface to) routes are determined to each network.

Drawbacks of RIP:

* RIP has no knowledge of subnet addressing
* It takes a long time to stabilize after a router or link failure.
* Uses more broadcasting than OSPF requiring more network bandwidth.

RIP Version 2

Defined by RFC 1388. It passes further information in some of the fields that are set to 0 for the RIP protocol. These additional fields include a 32 bit subnet mask and a next hop IP address, a routing domain, and route tag. The routing domain is an identifier of the daemon the packet belongs to. The route tags supports EGPs.
Open Shortest Path First (OSPF)

OSPF (RFC 1257) is a link state protocol rather than a distance vector protocol. It tests the status of its link to each of its neighbors and sends the acquired information to them. It stabilizes after a route or link failure faster than a distance vector protocol based system. OSPF uses IP directly, not relying on TCP or UDP. OSPF can:

* Have routes based on IP type of service (part of IP header message) such as FTP or Telnet.
* Support subnets.
* Assign cost to each interface based on reliability, round trip time, etc.
* Distribute traffic evenly over equal cost routes.
* Uses multicasting.

Costs for specific hops can be set by administrators. Adjacent routers swap information instead of broadcasting to all routers.
Border Gateway Protocol (BGP)

Described by RFC 1267, 1268, and 1497. It uses TCP as a transport protocol. When two systems are using BGP, they establish a TCP connection, then send each other their BGP routing tables. BGP uses distance vectoring. It detects failures by sending periodic keep alive messages to its neighbors every 30 seconds. It exchanges information about reachable networks with other BGP systems including the full path of systems that are between them.

Routing Technologies

Routing technologies manage the flow of data between network segments, which are also known as subnets. Windows Server 2003 includes several routing technologies, including demanddial routing, unicast Internet Protocol (IP) routing, IP multicasting, and network address translation (NAT) functionality, which are discussed in this subcollection.

Variable Length Subnet Masks (VLSM)

A Variable Length Subnet Mask (VLSM) is a means of allocating IP addressing
resources to subnets according to their individual need rather than some
general network-wide rule. Of the IP routing protocols supported by Cisco,
OSPF, Dual IS-IS, BGP-4, and EIGRP support "classless" or VLSM routes.
A simple example of a network using variable length subnet masks is found
in Cisco engineering. There are several switches in the engineering
buildings, configured with FDDI and Ethernet interfaces and numbered in
order to support 62 hosts on each switched subnet; in actuality, perhaps
15-30 hosts (printers, workstations, disk servers) are physically attached
to each. However, many engineers also have ISDN or Frame Relay links to
home, and a small subnet there. These home offices typically have a router
or two and an X terminal or workstation; they may have a PC or Macintosh as
well. As such, they are usually configured to support 6 hosts, and a few
are configured for 14. The point to point links are generally unnumbered.

Using "one size fits all" addressing schemes, such as are found in RIP or
IGRP, the home offices would have to be configured to support 62 hosts
each; using numbers on the point to point links would further compound the
address bloat.

One configures the router for Variable Length Subnet Masking by configuring
the router to use a protocol (such as OSPF or EIGRP) that supports this,
and configuring the subnet masks of the various interfaces in the 'ip
address' interface sub-command. To use supernets, one must further
configure the use of 'ip classless' routes.

Transmission Control Protocol/Internet Protocol

the suite of communications protocols used to connect hosts on the Internet. TCP/IP uses several protocols, the two main ones being TCP and IP. TCP/IP is built into the UNIX operating system and is used by the Internet, making it the de facto standard for transmitting data over networks. Even network operating systems that have their own protocols, such as Netware, also support TCP/IP.

Spanning Tree Protocol

Abbreviated STP, a link management protocol that is part of the IEEE 802.1 standard for media access control bridges. Using the spanning tree algorithm, STP provides path redundancy while preventing undesirable loops in a network that are created by multiple active paths between stations. Loops occur when there are alternate routes between hosts. To establish path redundancy, STP creates a tree that spans all of the switches in an extended network, forcing redundant paths into a standby, or blocked, state. STP allows only one active path at a time between any two network devices (this prevents the loops) but establishes the redundant links as a backup if the initial link should fail. If STP costs change, or if one network segment in the STP becomes unreachable, the spanning tree algorithm reconfigures the spanning tree topology and reestablishes the link by activating the standby path. Without spanning tree in place, it is possible that both connections may be simultaneously live, which could result in an endless loop of traffic on the LAN.


For internet.com pages about spanning tree protocol . Also check out the following links!

Related Links


Cisco Systems provides this technically heavy analysis of how spanning tree protocol operates.

OSI Model Concepts

The standard model for networking protocols and distributed applications is the International Standard Organization's Open System Interconnect (ISO/OSI) model. It defines seven network layers.

Short for Open System Interconnection, an ISO standard for worldwide communications that defines a networking framework for implementing protocols in seven layers. Control is passed from one layer to the next, starting at the application layer in one station, proceeding to the bottom layer, over the channel to the next station and back up the hierarchy.

At one time, most vendors agreed to support OSI in one form or another, but OSI was too loosely defined and proprietary standards were too entrenched. Except for the OSI-compliant X.400 and X.500 e-mail and directory standards, which are widely used, what was once thought to become the universal communications standard now serves as the teaching model for all other protocols.

Control is passed from one layer to the next, starting at the application layer in one station, proceeding to the bottom layer, over the channel to the next station and back up the hierarchy.

NETWORK TOPOLOGY

Topology refers to the way in which the network of computers is connected. Each topology is suited to specific tasks and has its own advantages and disadvantages.

The choice of topology is dependent upon

* type and number of equipment being used
* planned applications and rate of data transfers
* required response times
* cost

There are FOUR major competing topologies

* Bus
* Ring
* Star
* FDDI

Most networking software support all topologies.