SOME BASIC CONCEPT OF ROUTING

What is Cisco Discovery Protocol (CDP)

The Cisco Discovery Protocol (CDP) is a Cisco proprietary Layer 2 (Data Link Layer) network protocol developed by Cisco to share information about other directly connected Cisco devices, such as the operating system version and IP address.

Cisco Discovery Protocol (CDP) messages received from a neighbor Cisco device are not forwarded to any other devices by default. This means that Cisco Discovery Protocol (CDP) is passed only to directly connected Cisco devices. Each Cisco device (which supports Cisco Discovery Protocol (CDP)) stores the messages received from neighbor devices in a table that can be viewed using the show cdp neighbors command.

Cisco devices send Cisco Discovery Protocol (CDP) messages to the multicast destination address 01:00:0C:CC:CC:CC. CDP messages are sent every 60 seconds on interfaces that support Subnetwork Access Protocol (SNAP) headers.

Types of Routes Static Routes and Dynamic Routes, Difference between static route and dynamic route

Routing is the process of selecting paths in a network along which to send network traffic and route is the path to send the network traffic.

static routes - 

 A static route is a route that is created manually by a network administrator. Static routes are typically used in smaller networks. In static routing, the Router's routing table entries are populated manually by a network administrator.The major advantages of static routing are reduced routing protocol router overhead and reduced routing protocol network traffic. The major disadvantages of static routing are network changes require manual reconfiguration in routers and network outages cannot be automatically routed around.

Default Route - 

A Default Route (also known as the gateway of last resort) is a special type of static route. Where a static route specifies a path a router should use to reach a specific destination, a default route specifies a path the router should use if it doesn’t know how to reach the destination. 

Routable protocol -

 A Routable protocol is a network protocol which can carry data from one network and can pass through the router to reach another network and be delivered to a computer in that remote network.

Non-routable protocols-

A non-routable protocol’s data cannot be passed through a router to reach a remote network. This is mainly because of the lack of capability of protocol (almost all non-routable protocols are designed long back which will not fit well in current networks) and the addressing scheme the non-routable protocol is using.  

What is Autonomous System | What is Autonomous System Number

An Autonomous System (AS) is a group of networks under a single administrative control which could be an Internet Service Provider (ISP) or a large Enterprise Organization. An Interior Gateway Protocol (IGP) refers to a routing protocol that handles routing within a single autonomous system. 

 When BGP (Border Gateway Protocol) was at development and standardization stage, a 16-bit binary number was used as the Autonomous System Number (ASN) to identify the Autonomous Systems. 16-bit Autonomous System Number (ASN) is also known as 2-Octet Autonomous System Number (ASN). By using a 16 bit binary number, we can represent (2 ¹⁶) numbers, which is equal to 65536 in decimals.

Adiministrative Distance

Administrative Distance (AD) is a value that routers use in order to select the best path when there are two or more different routes to the same destination from two different routing protocols. Administrative Distance counts the reliability of a routing protocol. Administrative Distance (AD) is a numeric value which can range from 0 to 255. A smaller Administrative Distance (AD) is more trusted by a router, therefore the best Administrative Distance (AD) being 0 and the worst, 255.

Administrative Distance (AD)	Route Type
0	Connected interface
0 or 1	Static Route
90	Internal EIGRP Route (within the same Autonomu system)
100	IGRP Route
110	OSPF Route
115	IS-IS
120	RIP Route
255	Unknown Route

Metric Value

If the router has more than one route found by two different routing protocols, for the same destination network, the router chooses Adiministrative Distance to choose the best one.

But in some cases, there will be two paths found by the same protocol, to the same destination network. Here the Routing protocol will use Route Metric value to find the best path.

Distance Vector Routing Protocols

Distance Vector protocols are the simplest amoung Routing Protocols. Distance vector routing protocols use the distance and direction (vector) to find paths to destinations.

A router which is running a Distance Vector routing protocol informs its neighbours about the network topology changes periodically, using limited brodcast using destination IP address 255.255.255.255.

Distance Vector protocols use the Bellman-Ford algorithm for finding best paths to destinations.

Routers running Distance Vector protocols learn who their neighbours are by listening for routing broadcasts on their interfaces. Distance Vector protocols periodically send local limited brodcast (255.255.255.255) to share routing information.

Distance Vector algorithms pass routing table updates to their immediate neighbors in all directions. At each exchange, the router increments the distance value received for a route, thereby applying its own distance value to it.

Link State Routing Protocols

Link state routing protocols maintain complete road map of the network in each router running a link state routing protocol. Each router running a link state routing protocol originates information about the router, its directly connected links, and the state of those links. This information is sent to all the routers in the network as multicast messages. Link-state routing always try to maintain full networks topology by updating itself incrementally whenever a change happen in network.

Each router in the network keeps a copy of it, without changing it. After obtaining the complete picture of network topology, each router will independently calculate its own best paths to reach the destination networks.