Routing
Routers are important networking devices. They can be used to divide a network into subnets, increasing security and improving available bandwidth. They can be used to interconnect multiple networks locally or through a WAN. They can also be used for providing Internet connectivity.
As such, a router is a relativly intelligent device that ...
- maintains routing tables so it knows how to reach other networks
- uses it's routing table to calculate the best route for forwarding a packet to it's destination
- sends and receives routing information from other routers
- provides basic security through packet-filtering
Routing involves moving information across networks using the most efficient path possible. As an OSI Layer 3 process, routing decisions are made using logical addressing information. This ability to find a route from a source to a destination through logical addresses allow routers to interconnect different networks together regardless of how near or far away they are from each other. Moreover, the type of network or it's size is also irrelevent. Routers can route traffic between all kinds of networks. Thus, when considering the role of a router, we can start with the large picture - autonomous systems containing networks of networks. The role of a router will then be considered for other scenarios such as routing within a single LAN, interconnecting two or more local LAN's or interconnecting LAN's via a WAN.
Before going any further, it is best to be clear in the terminology. The term network is not very precisely defined as it is often used to refer to a LAN or a WAN, or a collection of LAN's. You can think of a network as a communication system belonging to a single entity. An internetwork is just a very large network containing smaller networks.
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Autonomous Systems
The Internet is a collection of many independent networks. A network can be considered to be independent if it is under the control of a single administration entity. These are typically networks controlled by Internet service providers or very large organizations. Each independent network can be considered autonomous and is referred to as an autonomous system (AS.)
A 'globally unique AS number is assigned to each autonomous system through a Regional Internet Registry. This authority is responsible for ensuring numbers are not duplicated. An AS number can be between 1 to 64511 and the next highest unused number is usually assigned to any new autonomous system. Most larger ISPs function as autonomous systems. One example of an autonomous system is the Joint Academic Network (JANET), an ISP that manages the UK's education and research network. JANET's AS number is AS786.
RIPE and ARIN are two examples of Regional Internet Registries. RIPE defines AS numbers as:
"A group of IP networks run by one or more network operators with a single, clearly defined routing policy. When exchanging exterior routing information each AS is identified by a unique number. Exterior routing protocols such as BGP, described in RFC 1771, "A Border Gateway Protocol 4 (BGP-4)", are used to exchange routing information between Autonomous Systems."
In other words, an AS is a connected group of networks that adhere to a single and clearly defined routing policy.
Interior and Exterior Routing Protocols
The routing protocol within one particular autonomous system (AS) can be different to that used on another system. A routing protocol used within an AS is referred to as an Internal Gateway Protocol (IGP). Examples of IGP's in use today are RIP, IGRP, EIGRP, IS-IS and OSPF.
For routing between each autonomous system, another type of routing protocol is required - an Exterior Gateway Protocol (EGP) - often referred to as an inter-AS routing protocol. The most common EGP used today is Border Gateway Protocol (BGP.) BGP uses the autonomous number information to route traffic between autonomous systems.
An autonomous system can contain many separate networks combined into an internetwork. Consider the diagram below. A single autonomous system is shown, containing 4 networks. Each separate network may be allowed to select their own interior routing protocol. The interior routing protocol used between the networks will be defined by the organisation controlling the whole autonomous system, as will the exterior routing protocol used to connect to any other autonomous system.
If it helps, consider the autonomous system above as the JANET system, linking separate college, university or research networks into an internetwork. Each individual network within the system may use it's own interior routing protocol. However, JANET will define the IGP used between the individual networks and also the EGP used to connect to any other autonomous system.
Border Routers
Within any internetwork, you have interior routers and exterior routers. An interior router is any router that does not support a connection to any other network. It is used purely for routing within the network. An exterior router is a router that lies beyond the boundaries of a network. A border router is a router that interconnects a network to other networks. Such a router lies on the border of a particular network, acting as a gateway between the network and the internetwork. A border router may denote a boundary between independent networks or between independent autonomous systems.
The responsibility of a border router is to summarize it's internal network and distribute this information to the rest of the internetwork. Any communication between the network and the internetwork travels through the border router.
~Now try the activity~
| Activity A |
- Why not go to the RIPE website and try to find out your ISP's AS number. In the WHOIS search box, type in the IP address of your Internet connection. Any valid results should list the AS number of your ISP.
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The Role of a Router
Routers are designed to interconnect networks. As such, a router must provide I/O ports for the physical connections of network media, it must allow network traffic to be routed from one place to another, it must be able to communicate with other routers and so determine network topology and possible routing paths and it must provide methods for securing access to and from a network. Thus a router must provide for...
- Physical interconnecivity
- Logical interconnecivity
- Communication with other routers
- Security through access control lists
In order to provide all of these services, a router must do more than push network traffic from one port to another. Although a router looks like a very boring box, it is in fact a fairly complex device and can even be considered as a special type of computer. If you opened up the box you would find a CPU, motherboard, RAM and ROM, I/O ports and an operating system. Just like a regular computer. It is also possible to upgrade various parts, such as the CPU, RAM, firmware and the operating system.
Physical interconnectivity is provided through a variety of physical I/O ports. Plugging suitable media into a port is the relatively easy part of the job of setting up a connection on an interface. A router can come with a variety of different port types - network ports, serial ports, AUX ports, console ports and others. Different port types are used for a different types of connections.
The three main types are:-
- LAN ports
- WAN ports
- Management ports
LAN interfaces are provided for connecting network media such as Ethernet or Token Ring. For example, an eth port on a router is for connecting Ethernet media.
WAN interfaces allow connections to be made through a service provider to distant networks or to the Internet. In both cases, network traffic can flow through these ports. For example, a serial port on a router is a WAN port.
When a router is booted up for the first time, it is not configured for networking services. Thus, it is not possible to connect to an unconfigured router via a network connection. You must connect to the router via a management port and configure the router for networking services before network connections can be made. Two ports on a router can be used for managing and configuring a router, the console port and the auxilliary port (AUX.) These ports are asynchronous serial ports and can only be used for configuring the router - they are not networking ports. The AUX port can be used to configure the router remotely via a modem connection. However, the console port is the preferred port for configuring a router initially and for any subsequent troubleshooting.
Once a router has been configured for networking services, it is also possible to connect and configure a router using HTTP or Telnet over a network connection, provided the router has been configured to allow such as connection. It should be noted though that HTTP and Telnet are inherently insecure connections. Moreover, if the router subsequently fails and a connection cannot be made through a network connection, then the console or AUX ports must be resorted to again.
Configuration of a router involves the logicalal configuration of each network port. Through a router management port the administrator has the task of configuring each port in terms of protocols that will be used, details of any connected networks, IP address that will be used for that port and other such details. Once a router is configured and powered on it will immediately start to monitor the network and communicate with other routers in order to build up a topological picture of the network. It will build up a routing table containing the IP addresses of hosts within the network, determining which hosts are accessible through particular ports.
Through communication with other routers, a router can also determine paths to other networks and using metrics can evaluate the best path from a source to a destination. Routers use routing protocols to communicate with each other. There are a variety of different routing protocols - RIP, IGRP, EIGRP, OSPF and others. Each of these protocols are categorized as distance-vector, link-state or a hybrid of these. Distance-vector routing protocols calculate route costs using hop distance as the main metric. Link-state routing protocols calculate route costs using a combination of metrics such as distance, link load, performance and other metrics. As an administrator, the routing protocol chosen will depend on the network. Factors such as the complexity of the network, routing update convergence times and performance would need consideration. When choosing a routing protocol you would consider...
- Convergence: - if the topology of a network changes, how much time does it take for routing updates to reach all the routers and new routes to be calculated. The time taken for routers to agree on new routes is called convergence. Different routing protocols converge at different rates. If the topology of a network changes frequently then you are going to need a routing protocol with a fast convergence.
- Robustness and Fault Tolerance: - a routing protocol must perform reliably under both normal and extreme network loads. It must not crash or misroute packets even under the most adverse conditions.
- Optimality: - routing protocols often differ in their evaluation of 'best pathways' due to the diferent metrics used by each protocol. One protocol may use hop count as a metric, another may use a more complicated combination of metrics, such as link load, reliability and distance. The performance of a routing protocol in terms of how good that protocol is in finding the 'best pathways' will depend on the specifics of your network.
- Efficiency: - the effectiveness of a routing protocol should also be considered when making a choice. All routing protocols take up router CPU time and RAM space, some require more of these resources than others. Also, routing protocols vary in the amount of network bandwidth they use.
- Scalability: - when choosing a routing protocol you will need to consider the starting size of your network and assess it's potential future growth. Whichever routing protocol you choose, you want it to continue to function as your network expands.
Since routers can be used to segment LAN's, interconnect networks within an internetwork, connect diferents networks together and provide communication between hosts over the Internet, it is essential for routers to provide basic packet-filtering capabilities to secure networks and regulate traffic. A network administrator may want to be able to control the specific type of incoming and outgoing packets between two networks, accepting some but rejecting others. Where there is any connection to the Internet, packet-filtering is essential to block unwanted intrusions into the network. In some cases, it may be desirable to closely regulate and restrict who may access the Internet from within an organization. You may also wish to control which users may access particular resources within the organization itself. All this should be achievable through proper configuration of a router's access control lists.
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Chapter 1 - WANs and Routers
