Function of router

Routing is the process that a router uses to forward packets toward the destination network. A router makes decisions based upon the destination IP address of a packet.

When routers use dynamic routing, this information is learned from other routers.

When static routing is used, a network administrator configures information about remote networks manually.

Routing protocols use metrics to evaluate what path will be the best for a packet to travel. A metric is a standard of measurement, such as path bandwidth, that is used by routing algorithms to determine the optimal path to a destination. To aid the process of path determination, routing algorithms initialize and maintain routing tables, which contain route information. Route information varies depending on the routing algorithm used.

Below are the functions of routers in the three tier model


Access routers, including SOHO, are located at customer sites such as branch offices that do not need hierarchical routing of their own. Typically, they are optimized for low cost.


Distribution routers take in traffic from multiple access routers, either at the same site, or to collect the data streams from multiple sites to a major location. Distribution routers often are responsible for ennsuring quality of service across a WAN, so they may have considerable memory, multiple WAN interfaces, and substantial processing intelligence.

They may also provide connectivity to groups of servers or to external networks. In the latter application, the router's functionality must be carefully considered as part of the overall security architecture. Separate from the router may be a Firewalled or VPN concentrator, or the router may include these and other security functions.


In enterprises, a core router may provide a "collapsed backbone" interconnecting the distribution tier routers from multiple buildings of a campus, or large enterprise locations. They tend to be optimized for high bandwidth.

When an enterprise is widely distributed with no central location(s), the function of core routing may be subsumed by the WAN service to which the enterprise subscribes, and the distribution routers become the highest tier.

Function of router protocol in design

In terms of which protocol type we should use, it depends on requirements. We are not being asked by a vendor to use a particular protocol,so we are going to used.We will be using a lot of cisco equipment so EIGRP can be brought up with very little manual configuration. If we need interoperability cisco equipment and between others vendors,then OSPF may be a better choice for us. As mentioned in another answer, if you are going to be exchanging routes with a 3rd party,OSPF is the protocol of choice.

Path selection involves applying a routing metric to multiple routes, in order to select (or predict) the best route.

In the case of computer networking, the metric is computed by a routing algorithm, and can cover such information as bandwidth, network delay, hop count, path cost, load,MTU, reliability, and communication cost The routing table stores only the best possible routes, while link-state or topological databases may store all other information as well.

Because we will be using mostly cisco equipment, will be be applying administrative distance routing protocol, multi-protocol routers must use some algorhythm in order to select between routes learned from different routing protocols. Ciscos routers, for example, attribute a value known as the administrative distance to each route, where smaller administrative distances indicate routes learned from a supposedly more reliable protocol.

Choice of routing protocol

Distance Vector

Pure distance vector protocols are rare; the only one really remaining in any sort of use is RIP. EIGRP, a Cisco proprietary protocol, is also technically distance vector, but it makes use of several optimisations that allow it to overcome the traditional shortcomings of distance vector protocols. Distance vector protocols do not distribute any topology information; they simply advertise the next hop to a route, along with a cost.


  • Minimal configuration required.
  • Low CPU/memory overhead.


  • Prone to routing loops (less applicable to EIGRP).
  • Slow convergence time.
  • Different routers may have different perceptions as to the 'state' of the network.

Link state

Link-state protocols function by advertising each router's connected interfaces to every other device in the routing domain. Effectively, each device builds a database of the full network topology, and uses this database to determine the best path to each destination network.

The two major link-state protocols in use are OSPFand IS-IS; both are based around an implementation of Dijkstra's algorithm.OSPF is the more well-known of the two; IS-IS tends to be found more in service provider networks.

Choice of routing protocol continued


  • All routers in the network have a consistent view of the world.
  • Loops are essentially impossible in a link-state network.
  • Fast reconvergence.


  • Higher CPU/memory footprint required.
  • Difficult to filter routes being advertised to specific routers, as link-state algorithms rely on the entire AS having a consistent view of the world.

Protocol choice

In terms of which protocol type you should use, it depends on your requirements. In general, unless you are being forced to do so by a vendor, RIP should not be used. If you are running an all Cisco network, EIGRP can be brought up with very little manual configuration. If interoperability between vendors is a requirement, OSPF may be a better choice. As mentioned in another answer, if you are going to be exchanging routes with a 3rd party, BGP is the protocol of choice.

One last point: dynamic routing protocols are useful only when your topology is redundant, and you need automatic failover. If you have a single router which supports all of your LAN environments and your ISP circuit, then a default route to your ISP would be more than sufficient.

IP address scheme problems

When we looked at our budget we knew we had a limitation on the number of router we could deploy so we decided to use CIDR .Classless Inter-Domain Routing (CIDR) is a replacement for the old process of assigning Class A, B and C addresses. Instead of being limited to network identifiers of 8, 16 or 24 bits, CIDR uses prefixes anywhere from 13 to 27 bits. Blocks of addresses can be assigned to networks as small as 32 hosts or to those with over 500,000 hosts. This allows for address assignments that much more closely fit our needs for the network

A CIDR address includes the standard 32-bit IP address and also information on how many bits are used for the network prefix. For example, in the CIDR address, the "/25" indicates the first 25 bits are used to identify the unique network leaving the remaining bits to identify the specific host.

Problems of contiguous subnets in large design

Variable-length subnet masking

Variable length subnet masking (VLSM) means implementing more than one mask on the same major class of a network. It allows for a more efficient use of IP address space both in terms of hosts and subnets. On a network that does not have an abundance of IP address space it can be essential. In order to implement different masks for the same major network it is required to have a routing protocol that supports VLSM. Such routing protocols are called classless routing protocols. They carry the mask information along with the route advertisements therefore allowing for the support of more than one mask.

Examples of classless routing protocols include OSPF, RIP version 2, Cisco's EIGRP, BGP and IS-IS. Consider an example that employs VLSM. Assume that the Class B address is required to support a network that entails a total of 200 sites. The busiest LANs may support up to 100 hosts and there is a maximum projected total of 400 point-to-point WAN links. Hence there is a requirement for 600 subnets with a maximum of 100 hosts on any subnet. Even with a Class B address there is insufficient address space to meet this requirement without employing VLSM.

Traffic generated by routing updates

Routing protocols have distinct advantages and disadvantages in different situations. A protocol that is supported on old equipment (such as RIP) might not support fast classless operation needed in the core of the network (such as OSPF, IS-IS, or EIGRP). Most organizations use the most appropriate routing protocol in different parts of their network. Organizations running multiple routing protocols need to pass networks learned by one protocol into another. This process is called redistribution.

Each routing protocol within an AS can be thought of as a routing domain. Routes redistributed into a routing domain are termed external routes, while native routes are called internal routes.

The metric is the main method of route selection within a routing protocol. Therefore, it is necessary to define a default seed metric for the networks accepted from other routing protocols.Below I have set out a table to show some common commands which routing updates can be configured


Requires manual redistribution into other routing protocols.ConnectedUnless included in the network command for the routing process, requires manual redistribution.


Requires manual redistribution.EIGRPWill automatically redistribute between IGRP and


if the autonomous system number is the same. Otherwise, it requires manual redistribution.OSPFRequires manual redistribution between different OSPF process IDs and routing protocols.IS-ISRequires manual redistribution between different routing protocols.

BGP Requires manual redistribution between different routing protocols.

Network overhead is an important concept to understand. Understanding overhead is basic to understanding the methodology employed by various technologies to get information from one place to another, and the costs involved. This paper will cover the basics of overhead affiliated with Ethernet, ATM, Frame-Relay, POS, and SONET. Similar information can be found on FDDI, Token Ring, and other technologies, but will not be covered here.

Basic Terminology

BPS - Bits per second (1 Kbps = 1000, 1 Mbps = 1000000, and 1 Gbps = 1000000000)

PPS - Packets per second

PCR - Peak Cell Rate

Byte - 8 bits

Packet/Frame/Cell - terms that refer to discrete data units from different technologies

Layer2 - Refers to the Data/Link Layer of the OSI Network Model

Layer3 - Refers to the Network Layer olf the OSI Network Model

Life of a Data Packet

In this document, TCP/IP will be assumed to be the protocol being utilized for packet data. Much of the world's telecommunication networks are forwarding IP packets to and fro at this very moment. During the course of an IP packet's journey from a transmitter to a receiver, it is encapsulated and de-encapsulated into and out of framing headers and trailers that define how the packet will make it's way to the next hop in the path. This paper is designed to explain what happens when a packet arrives at a router or a switch, is processed, and then forwarded to the next router or switch.


IEEE 802.3 Ethernet, the world's most popular LAN technology, is not going anywhere. It is widely used in 10 Mbps, 100, Mbps, and 1000 Mbps (also called Gigabit Ethernet) varieties. This technology for framing bits can be supported by many types of physical media (Copper wires, Fiber-Optics, Wireless, Coax).

Redundancy for core switch

My two core switches come with dual supervisors for redundancy.

In one of the slots I have placed a 10gb line card.I will have four distribution switches that the core switch will connect to.This allows for through put of 40 Gbps from each distribution slot.

In the fourth slot.there will be a 4 port 10 gb line card.These four slots will be used to connect to the remaining core switch and will allow for 40 Gbps throughout between the 2 core switches.

Distribution switch

Cisco 4606-E

Both of my distribution switches come with dual power supplies and one supervisor card each. Having two distributions switches per network acts as my redundancy as budget constraints could not allow dual supervisors.

In the fourth slot of each switch is a 48 port 2 line card. All access switches in there own network connect to the distribution switches. On the teaching network, this totals 50 fiber links, hence the need for an extra 18 port fiber line card. The administrative network only requires 25 connections per distribution switch.

In the fifth slot lies a 6 port 10 GB line card. Four of these slots will be used to connect to their respective core switchs, allowing for a 40 Gbps throughout between the core and each individual distribution switch.

Equipment selection

Administrative servers

For my two administrative severs I will be using 2 Dell poweredge R610 rack servers on the Unix platform running Solar is 8.I have decided to go with these as they will be used for payroll,accounts and personnel systems.The server is designed for data centers.The other server will be running a windows 98 package called dolphin.see appendix for specifications.

Library server

For the library server I have decided to go with the dell power edge rack server R510.It will be running on the unix platform.This is mainly a retrieval system and will house an online library for curricular research purposes.The server come with sata connections so we can add additional storage for the online library.See appendic for specifications.

Application server

For the application we will be using the dell poweredge rack server R410.The applications such as word and processing,Excel power point are requested by users and then they will be retrieved from the application server.The R410 is used for computer intense applications in space contrained areas.see appendix for specifications

Teaching servers

Foe the teaching server will will be using the dell rack server poweredge rack R210.This server will be mainly used for file access for students to obtain read only versions of coursework.We will portioning the server for the tuition of Unix classes.The server will run on windows server 200 to accommodate to to run the MCP courses in room 312.A futher partition will be made to run MS-dos and autocad 3.0 in rooms 153.The R210 is a server will is mainly used as a specialized application server.see appendix for specifications.

Firewall server

For the firewall server we will be using Barracuda Spam & Virus Firewall. It is compatible with all email servers and can fit into nearly any corporate or small business environment. It is used by small organizations with as few as 10 employees and large organizations with as many as 200,000 employees. A single Barracuda Spam & Virus Firewall handles up to 100,000 active email users. Multiple units can be clustered together for even greater capacity and high availability.See appendix for specifications

Email server

We will be using a dell power edge rack server R210 with Microsoft exchange server installed to manage it.see appendix for specifications.

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