what is routing, how it happens click it
Routing Protocol Basic
There are some important things you should know about routing protocols before getting deeper into RIP. Specifically, you need to understand administrative distances, the three different kinds of routing protocols, and routing loops. We will look at each of these in more details.
Administrative Distances
The administrative distance (AD) is used to rate trustworthiness of routing information received on a router from a neighbor router. An administrative distance is an interger from0 to 255, where o 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 network (Which means that it sends packets down each link).
Shows 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 ever be used)
|
If a network is directly connected, the router will always use the interface connected to the network. If you configure a static route, the router will then believe that route over any other learned routes. You can change the administrative distance of static routes, bu by default, they have an AD of 1. This lets us configure routing protocols without having remove the static routes (if we config static route). They'll be used as backup routes in case the routing protocol experiences a failure of some type.
For example, if you have a static route, a RIP-advertised route, and an EIGRP-advertised route listing the same network, then by default, the router will always use the static route unless you change the AD of the static route Which we did.
Routing protocols
There are three classes of routing protocols:
Distance vector The distance-vector protocols in use today find the best path to a remote network by judging distance. For example, in the case of RIP routing, 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 periodically 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 a 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 direcon the network, which is then propagated to their neighbors.
Hybrid Hybrid protocols use aspects of both distance-vector and link-state for example, EIGRP.
Distance-vector Routing Protocols:
The distance-vector routing algorithm passes complete routing table contents to neighboring routers, which then combine the received routing table entries with their own routing table to complete the router's routing table. This is called routing by rumor because a router receiving an update from a neighbor router believes the information about remote networks without actually finding out for itself.
It's possible to have a network that has multiple links to the same remote network, and if that's the case, the administrative distance of each received update is checked first. If the AD is the same, the protocol will have to use metrics to determine the best path to use to that remote network.
Rip uses only hop count to determine the best path to network. If RIP finds more than one link with the same hop count to the same remote network, it will automatically perform a round-robin load balancing. RIP can perform load balancing for up to six equal-cost links(default four).
However, a problem with this type of routing metric arises when the two links to a remote network are different band widths but the same hop count.
 |
Routing loop |
Routing Loops Distance-vector routing protocols keep track of any changes to the internetwork by broadcasting periodic routing updates out all active interface. This broadcast includes the complete routing table. This works just fine, but it's expensive in terms of CPU processing and link bandwidth. And if a network outage happens, real problems can occur. plus, the slow convergence of distance-vector routing protocols can result in inconsistent routing tables and routing loops. routing loop can occur because every router isn't updated simultaneously, or even close to it.
Maximum Hop Count The routing loop problem just described can create an issue called counting to infinity, and it's caused by gossip (broadcasts) and wrong information begin communicated and propagated throughout the internetwork. Without some form of intervention, the hop count increases indefinitely each time a packet passes through a router.
One way of solving this problem is to define a maximum hop count. RIP permits a hop count of up to 15, so anything that requires 16 hops is deemed unreachable. In other words, after a loop of 15 hops. Thus,the maximum hop count will control how long it takes for a routing table entry to become invalid or questionable.
One way of solving this problem is to define a maximum hop count. RIP permits a hop count of up to 15, so anything that requires 16 hops is deemed unreachable. In other words, after a loop of 15 hops. Thus,the maximum hop count will control how long it takes for a routing table entry to become invalid or questionable.
Split Horizon Another solution to the routing loop problem is called split horizon. This reduces incorrect routing information and routing overhead in a distance-vector network by enforcing the rule that routing information cannot be sent back in the direction from which it was received.
In other words, the routing protocol differentiates which interface a network route was learned on, and once this is determined, it won't advertise the route back out that same interface. This would have prevented Router A from sending the update information it received from Router B back to Router B.
Route Poisoning Another way to avoid problems caused by inconsistent updates and stop network loops is route poisoning. For example, when Network 5 goes down, RouterE initiating by advertising Network 5 with a hop count of 16, or unreachable (sometimes referred to as infinite).
The poisoning of the route to network 5 keeps RouterC from being susceptible to incorrect updates about the route to Network 5. When RouterC receives a route poisoning from RouterE,it sends an update, called a poison reverse, back to RouterE. This ensures that all routers on the segment have received the poisoned route information.
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