Basic IP Routing


Routing is used for taking a packet from one device and sending it through the network to another device on a different network. Routers don’t really care about hosts they only care about networks and the best path to each network. The logical network address of the destination host is used to get packets to a network through a routed network, and then the hardware address of the host is used to deliver the packet from a router to the correct destination host.

If your network has no routers, then it should be apparent that you are not routing. Routers route traffic to all the networks in your internetwork.

To be able to route packets, a router must know, at a minimum, the following:

  1. Destination address
  2. Neighbor routers from which it can learn about remote networks
  3. Possible routes to all remote networks
  4. The best route to each remote network
  5. How to maintain and verify routing information
  6. The router learns about remote networks from neighbor routers or from an administrator.


The router then builds a routing table (a map of the internetwork) that describes how to find the remote networks.

Static Routing

It is used, the administrator is responsible for updating all changes by hand into all routers. Typically, in a large network, a combination of both dynamic and static routing is used.

Dynamic Routing

A protocol on one router communicates with the same protocol running on neighbor routers. The routers then update each other about all the networks they know about and place this information into the routing table. If a change occurs in the network, the dynamic routing protocols automatically inform all routers about the event.

Routing Protocol Basics:-
 

  • Administrative Distances:-

    The administrative distance (AD) is used to rate the trustworthiness of routing information received on a router from a neighbor router. An administrative distance is an integer from 0 to 255, where 0 is the most trusted and 255 means no traffic will be passed via this route.

    If a router receives two updates listing the same remote network, the first thing the router checks is the AD. If one of the advertised routes has a lower AD than the other, then the route with the lowest AD will be placed in the routing table.

    If both advertised routes to the same network have the same AD, then routing protocol metrics (such as hop count or bandwidth of the lines) will be used to find the best path to the remote network. The advertised route with the lowest metric will be placed in the routing table. But if both advertised routes have the same AD as well as the same metrics, then the routing protocol will load-balance to the remote network (which means that it sends packets down each link).

    The default administrative distances that a Cisco router uses to decide which route to take to a remote network.

    Default Administrative Distances
    Route Source Default AD
    Connected interface 0
    Static route 1
    EIGRP 90
    IGRP 100
    OSPF 110
    RIP 120
    External EIGRP 170
    Unknown 255 (this route will never be used)
  • Routing Protocols

    There are three classes of routing protocols:

    Distance vector:-The distance-vector protocols find the best path to a remote network by judging distance. Each time a packet goes through a router, that’s called a hop. The route with the least number of hops to the network is determined to be the best route. The vector indicates the direction to the remote network. Both RIP and IGRP are distance-vector routing protocols. They send the entire routing table to directly connected neighbors.

    Link state:-In link-state protocols, also called shortest-path-first protocols, the routers each create three separate tables. One of these tables keeps track of directly attached neighbors, one determines the topology of the entire internetwork, and one is used as the routing table. Link-state routers know more about the internetwork than any distance vector routing protocol. OSPF is an IP routing protocol that is completely link state. Link state protocols send updates containing the state of their own links to all other routers on the network.

    Hybrid:-Hybrid protocols use aspects of both distance vector and link state for example, EIGRP.

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