Routing
Routing is the process by which an item gets from one location to another. Many items get routed: for example, mail, telephone calls, and trains. In networking, a router is the device used to route traffic.
Key Information a Router Needs
Destination Address - What is the destination (or address) of the item that needs to be routed?
Identifying sources of information - From which source (other routers) can the router learn the paths to given destinations?
Discovering routes - What are the initial possible routes, or paths, to the intended destinations?
Selecting routes - What is the best path to the intended destination?
Maintaining routing information - A way of verifying that the known paths to destinations are the most current.
•Routed protocols - Any network protocol that provides enough information in its network layer address to allow a packet to be forwarded from host to host based on the addressing scheme. Routed protocols define the format and use of the fields within a packet. Packets generally are conveyed from end system to end system. The Internet protocol IP is an example of a routed protocol.
Here are some examples of Routed Protocols:
•Internet Protocol (IP)
•AppleTalk (AT)
•Novell NetWare Protocol
•Xerox Network Systems (XNS)
•Routing protocols - Supports a routed protocol by providing mechanisms for sharing routing information. Routing protocol messages move between the routers. A routing protocol allows the routers to communicate with other routers to update and maintain tables. examples of routing protocols are RIP,IGRP,EIGRP and OSPF.
Types of Routing
The different types of routing are:
•Static routing
•Default routing
•Dynamic routing
Static Routing
Routes learned by the router when an administrator manually establishes the route. The administrator must manually update this static route entry whenever an internetwork topology change requires an update.
Benefits:
•There is no overhead on the router CPU.
•There is no bandwidth usage between routers
•It adds security
Disadvantage: •The administrator must really understand the internetwork and how each router is connected to configure routes correctly.
•If a network is added to internetwork, the administrator has to add route to it on all routers-by hand
Default Routing
Dynamic Routing
Distance Vector Protocols
Distance vector routing protocols require routers to periodically send all (or a significant portion) of their routing table in routing updates, but only to neighboring routers.
Routing Loop
Routing loops are, simply, the continuous forwarding of packets due to some fault in a network. Packets are continuously looped throughout a particular network or segment.
What Causes Routing Loops?
Routing loops can occur when routing decisions are based on incorrect information, resulting in packets taking paths that return them to already visited routers. They are created due to a variety of circumstances
How Do Routers Prevent Loops?
Routing protocols implement a variety of features designed to prevent routing loops.
•Maximum Hop count
•Split Horizon
•Route Poisoning
•Holddowns
distance vector protocols define infinity as some maximum number. This number refers to a routing metric, such as a hop count.
With this approach, the routing protocol permits the routing loop until the metric exceeds its maximum allowed value. The image shows this defined maximum as 16 hops. Once the metric value exceeds the maximum, network 10.4.0.0 is considered unreachable.
Split Horizon
The rule of split horizon is that it is never useful to send information about a route back in the direction from which the original packet came.
Route Poisoning
With this technique, the router sets a table entry that keeps the network state consistent while other routers gradually converge correctly on the topology change. Used with hold-down timers, which are described soon, route poisoning is a solution to long loops.
Hold-Down
A hold-down timer is a state into which a route is placed so that routers will neither advertise the route nor accept advertisements about the route for a specific length of time (the holddown period). A route is typically placed in holddown when a link in that route fails.
Routing is the process by which an item gets from one location to another. Many items get routed: for example, mail, telephone calls, and trains. In networking, a router is the device used to route traffic.
Key Information a Router Needs
Destination Address - What is the destination (or address) of the item that needs to be routed?
Identifying sources of information - From which source (other routers) can the router learn the paths to given destinations?
Discovering routes - What are the initial possible routes, or paths, to the intended destinations?
Selecting routes - What is the best path to the intended destination?
Maintaining routing information - A way of verifying that the known paths to destinations are the most current.
•Routed protocols - Any network protocol that provides enough information in its network layer address to allow a packet to be forwarded from host to host based on the addressing scheme. Routed protocols define the format and use of the fields within a packet. Packets generally are conveyed from end system to end system. The Internet protocol IP is an example of a routed protocol.
Here are some examples of Routed Protocols:
•Internet Protocol (IP)
•AppleTalk (AT)
•Novell NetWare Protocol
•Xerox Network Systems (XNS)
•Routing protocols - Supports a routed protocol by providing mechanisms for sharing routing information. Routing protocol messages move between the routers. A routing protocol allows the routers to communicate with other routers to update and maintain tables. examples of routing protocols are RIP,IGRP,EIGRP and OSPF.
Types of Routing
The different types of routing are:
•Static routing
•Default routing
•Dynamic routing
Static Routing
Routes learned by the router when an administrator manually establishes the route. The administrator must manually update this static route entry whenever an internetwork topology change requires an update.
Benefits:
•There is no overhead on the router CPU.
•There is no bandwidth usage between routers
•It adds security
Disadvantage: •The administrator must really understand the internetwork and how each router is connected to configure routes correctly.
•If a network is added to internetwork, the administrator has to add route to it on all routers-by hand
Default Routing
A default route is a special type of static route. A default route is a route to use for situations when the route from a source to a destination is not known or when it is unfeasible for the routing table to store sufficient information about the route.
In the image, Cisco B is configured to forward all frames for which the destination network is not explicitly listed in its routing table to Cisco A.
Dynamic Routing
Routes dynamically learned by the router after an administrator configures a routing protocol that helps determine routes. Unlike static routes, once the network administrator enables dynamic routing, route knowledge is automatically updated by a routing process whenever new topology information is received from the internetwork.
Router Metrics
Routing metrics are used by routing algorithms to determine the desirability of a given route to a destination network. Different routing protocols implement different routing metrics. Routing metrics represent network characteristics. Metric information is stored in routing tables. There are a number of commonly used routing metrics, including:
•Path length
•Reliability
•Delay
•Bandwidth
•Load
•Cost
Hop count is a value that counts the number of intermediate systems (such as
routers) through which a packet must pass to travel from the source to the
destination. The path length is the sum of all the hops in the path.
The reliability routing metric can be based on any of a number of network characteristics. These include:
•Bit-error rate (the ratio of received bits that contain errors)
•How often each network link fails, and, once down, how quickly each network link can be repaired.
The delay routing metric is based on the length of time required to move a packet from the source to a destination through the internetwork.
Bandwidth
The bandwidth routing metric is based solely on the available traffic capacity of each network link. However, routes through links with greater bandwidth do not necessarily provide better routes than routes through slower links.
Load
The load routing metric is based on the degree to which a network resource (such as a router) is busy. Load is calculated according to such factors as:
•CPU utilization
•Packets processed per second
Cost
The cost routing metric is based on the monetary cost of using each network link. For example, a slower company-owned link can be configured as preferable over faster public links that cost money for usage time.
Routing protocols are used between routers to determine paths and maintain routing tables. Dynamic routing relies on a routing protocol to disseminate knowledge.
Autonomous Systems
An autonomous system is a collection of networks under a common administrative domain
Adminstrative Distance
Multiple routing protocols and static routes may be used at the same time. If there are several sources for routing information, an administrative distance value is used to rate the trustworthiness of each routing information source.
An Administrative Distance is a rating of the trustworthiness of a routing information source, such as an individual router or a group of routers. It is an integer from 0 to 255.
Router Metrics
Routing metrics are used by routing algorithms to determine the desirability of a given route to a destination network. Different routing protocols implement different routing metrics. Routing metrics represent network characteristics. Metric information is stored in routing tables. There are a number of commonly used routing metrics, including:
•Path length
•Reliability
•Delay
•Bandwidth
•Load
•Cost
Hop count is a value that counts the number of intermediate systems (such as
routers) through which a packet must pass to travel from the source to the
destination. The path length is the sum of all the hops in the path.
The reliability routing metric can be based on any of a number of network characteristics. These include:
•Bit-error rate (the ratio of received bits that contain errors)
•How often each network link fails, and, once down, how quickly each network link can be repaired.
The delay routing metric is based on the length of time required to move a packet from the source to a destination through the internetwork.
Bandwidth
The bandwidth routing metric is based solely on the available traffic capacity of each network link. However, routes through links with greater bandwidth do not necessarily provide better routes than routes through slower links.
Load
The load routing metric is based on the degree to which a network resource (such as a router) is busy. Load is calculated according to such factors as:
•CPU utilization
•Packets processed per second
Cost
The cost routing metric is based on the monetary cost of using each network link. For example, a slower company-owned link can be configured as preferable over faster public links that cost money for usage time.
Routing protocols are used between routers to determine paths and maintain routing tables. Dynamic routing relies on a routing protocol to disseminate knowledge.
Autonomous Systems
An autonomous system is a collection of networks under a common administrative domain
Adminstrative Distance
Multiple routing protocols and static routes may be used at the same time. If there are several sources for routing information, an administrative distance value is used to rate the trustworthiness of each routing information source.
An Administrative Distance is a rating of the trustworthiness of a routing information source, such as an individual router or a group of routers. It is an integer from 0 to 255.
Distance Vector Protocols
Distance vector routing protocols require routers to periodically send all (or a significant portion) of their routing table in routing updates, but only to neighboring routers.
Routing Loop
Routing loops are, simply, the continuous forwarding of packets due to some fault in a network. Packets are continuously looped throughout a particular network or segment.
What Causes Routing Loops?
Routing loops can occur when routing decisions are based on incorrect information, resulting in packets taking paths that return them to already visited routers. They are created due to a variety of circumstances
How Do Routers Prevent Loops?
Routing protocols implement a variety of features designed to prevent routing loops.
•Maximum Hop count
•Split Horizon
•Route Poisoning
•Holddowns
distance vector protocols define infinity as some maximum number. This number refers to a routing metric, such as a hop count.
With this approach, the routing protocol permits the routing loop until the metric exceeds its maximum allowed value. The image shows this defined maximum as 16 hops. Once the metric value exceeds the maximum, network 10.4.0.0 is considered unreachable.
Split Horizon
The rule of split horizon is that it is never useful to send information about a route back in the direction from which the original packet came.
Route Poisoning
With this technique, the router sets a table entry that keeps the network state consistent while other routers gradually converge correctly on the topology change. Used with hold-down timers, which are described soon, route poisoning is a solution to long loops.
Hold-Down
A hold-down timer is a state into which a route is placed so that routers will neither advertise the route nor accept advertisements about the route for a specific length of time (the holddown period). A route is typically placed in holddown when a link in that route fails.
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