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. The routing information a router learns from its routing sources is placed in its routing table. The router will rely on this table to tell it which port to use when forwarding addressed packets
Types of Routes
- Static routes - Routes learned by the router when an administrator manually establishes the route. The administrator must manually update this static route entry whenever an internet work topology change requires an update.
- Dynamic Routes - 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.
Static Route Configuration
A static route allows manual configuration of the routing table. No dynamic changes to this table entry will occur as long as the path is active. The ip route command is used to configure a static route in global configuration mode.
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.
Routing protocols are used between routers to determine paths and maintain routing tables. Dynamic routing relies on a routing protocol to disseminate knowledge.
Characteristics of Routing Protocols
A routing protocol defines the set of rules used by a router when it communicates with neighboring routers. It interprets information in a network layer address to allow a packet to be forwarded to the destination network.
Routing protocols describe:
- How updates are conveyed
- What knowledge is conveyed When to convey knowledge
- How to locate recipients of the updates
Two examples of routing protocols are Routing Information Protocol (RIP) and Interior Gateway Routing Protocol (IGRP).
An autonomous system is a collection of networks under a common administrative domain.
There are two major types of routing protocols used to connect autonomous systems:
- Interior Gateway Protocols (IGP) - Routing Protocols used to exchange routing information within an autonomous system. RIP and IGRP are examples of IGPs.
- Exterior Gateway Protocols (EGP) - used to connect between autonomous systems. Border Gateway Protocol (BGP) is an example of an EGP.
Ranking Routes with 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. Specifying administrative distance values enables the Cisco IOS software to discriminate between sources of routing information. For each destination learned, the IOS always places in the routing table the route from the source with the lowest administrative distance. In general, a routing protocol with a lower administrative distance has a higher likelihood of being used.
Classes of Routing Protocols
Within an autonomous system, most IGP routing algorithms can be classified as conforming to one of three algorithms.
Distance Vector The distance vector routing approach determines the direction (vector) and distance to any link in the internetwork.
Link State The link-state (also called shortest path first) approach re-creates the exact topology of the entire internetwork (or at least the partition in which the router is situated).
Balanced Hybrid A balanced hybrid approach combines aspects of the link-state and distance vector algorithms.
An example of Distance vector protocol is Routing Information protocol(RIP).
Engineers have implemented this link-state concept in Open Shortest Path First (OSPF) routing. An example of a balanced hybrid protocol is Cisco's Enhanced Interior Gateway Routing Protocol (Enhanced IGRP).
Distance Vector Routing Problems
- Distance vector routing protocols maintain routing information by updating routing tables with neighboring routing tables.
- A routing loop is a route where packets never reach their destination, but cycle repeatedly through a series of nodes.
- Defining a maximum routing count prevents infinite loops by defining a limit on the number of hops.
- Split horizon is a technique for solving routing loops that implements not sending information about a route back in the same direction from which it came.
- Route poisoning is a solution to loops in which routers set the distance of routes that have gone down to infinity to make that route unreachable.
- A triggered update is a new routing table that is sent immediately in response to some change. Each receiving router sends a triggered update which creates a wave that propagates across the network.
- Hold-down timers are used to prevent regular update messages from inappropriately reinstating a route that may have gone bad.
- Solutions involving multiple techniques can be implemented on networks with multiple routes.
Discovering Neighbors with CDP
Cisco Discovery Protocol (CDP) is an information gathering tool used by network administrators to get information about directly connected devices. CDP is a proprietary tool that enables network administrators to access a summary of protocol and address information about other devices that are directly connected to the device initiating the command. CDP runs over the data link layer connecting the physical media to the upper-layer protocols. Because CDP operates at this level, two or more CDP devices that support different network-layer protocols (for example, IP and Novell IPX) can learn about each other. Physical media supporting the Subnetwork Access Protocol (SNAP) encapsulation connect CDP devices. These can include all LANs, Frame Relay and other WANs, and ATM networks. A CDP packet can be as small as 80 octets, mostly made up of ASCII strings that represent information such a CDP interfaces, neighbor entries, statistics, etc.
The network administrator uses a show command to display information about the networks directly connected to the switch.
CDP Summary Information
Packets formed by CDP provide the following information about each CDP neighbor device:
- Device identifiers - For example, the switch's configured name and domain name (if any).
- Address list - Up to one address for each protocol supported.
- Port identifier - The name of the local and remote port (in the form of an ASCII character string such as ethernet0).
- Capabilities list - Supported features, for example, the device acts as a source-route bridge as well as a router.
- Platform - The device's hardware platform: for example, Cisco 7000.
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