ROUTING FIRST STEP

ROUTING. - Routing is the process of moving packets across a network from one host to another. It is usually performed by dedicated devices called routers.

Classes of Routing Protocols.
Distance Vector
Link State
Advanced distance vector
Routed vs. Routing Protocols
A routing protocol sends and receives routing information packets to and from other routers.
A routed protocol can be routed by a router, which means that it can be forwarded from one router to another.
A routed protocol contains the data elements required for a packet to be sent outside of its host network or network segment. In other words, a routed protocol can be routed.
Protocols used to communicate routing information between routers within an autonomous system are Interior Gateway Protocols (IGP), which are routing protocols, but not routed protocols.
Examples of routed protocols are IP and IPX, and examples of routing protocols are RIP and IGRP.

• Static routes - are fixed routes that are manually entered by the administrator into the router's configuration.
• Dynamic routing - is the process by which a network adapts automatically to the changes in topology or traffic as those changes occur.
• Default routes - are very much like static route. The administrator enters the default route, and it becomes the default path the router uses to forward packets for which it knows no other route to use. Without a default route, packets with unknown destinations are dripped.




Overview
"Distance Vector" and "Link State" are terms used to describe routing protocols which are used by routers to forward packets between networks. The purpose of any routing protocol is to dynamically communicate information about all network paths used to reach a destination and to select the from those paths, the best path to reach a destination network. The terms distance vector and link state are used to group routing protocols into two broad categories based on whether the routing protocol selects the best routing path based on a distance metric (the distance) and an interface (the vector), or selects the best routing path by calculating the state of each link in a path and finding the path that has the lowest total metric to reach the destination.
DISTANCE VECTOR
Distance
Distance is the cost of reaching a destination, usually based on the number of hosts the path passes through, or the total of all the administrative metrics assigned to the links in the path.
Vector
From the standpoint of routing protocols, the vector is the interface traffic will be forwarded out in order to reach an given destination network along a route or path selected by the routing protocol as the best path to the destination network.
Distance vector protocols use a distance calculation plus an outgoing network interface (a vector) to choose the best path to a destination network. The network protocol (IPX, SPX, IP, Appletalk, DECnet etc.) will forward data using the best paths selected.
Common distance vector routing protocols include:
• Appletalk RTMP
• IPX RIP
• IP RIP
• IGRP
Advantages of Distance Vector Protocols
Well Supported
Protocols such as RIP have been around a long time and most, if not all devices that perform routing will understand RIP.

LINK STATE
Link State protocols track the status and connection type of each link and produces a calculated metric based on these and other factors, including some set by the network administrator. Link state protocols know whether a link is up or down and how fast it is and calculates a cost to 'get there'. Since routers run routing protocols to figure out how to get to a destination, you can think of the 'link states' as being the status of the interfaces on the router. Link State protocols will take a path which has more hops, but that uses a faster medium over a path using a slower medium with fewer hops.
Because of their awareness of media types and other factors, link state protocols require more processing power (more circuit logic in the case of ASICs) and memory. Distance vector algorithms being simpler require simpler hardware.

A Comparison: Link State vs. Distance Vector
See Fig. 1-1 below. If all routers were running a Distance Vector protocol, the path or 'route' chosen would be from A B directly over the ISDN serial link, even though that link is about 10 times slower than the indirect route from A C D B.
A Link State protocol would choose the A C D B path because it's using a faster medium (100 Mb ethernet). In this example, it would be better to run a Link State routing protocol, but if all the links in the network are the same speed, then a Distance Vector protocol is better.
FIG. 1-1



IGP vs EGP
An Interior gateway protocol (IGP) is one which exchanges reachability information for destinations inside its AS. E.g. RIP, EIGRP, OSPF , IS-IS
An Exterior gateway protocol (EGP) is one which exchanges reachability information for destinations outside its AS. E.g. BGP
• Interior Gateway Protocols
o within a single autonomous system
 single network administration
 unique routing policy
 make best use of network resources
• Exterior Gateway Protocols
o among different autonomous systems
 independent administrative entities
 communication between independent network infrastructures
Default Administrative Distance Value Table
This table lists the administrative distance default values of the protocols that Cisco supports:
Route Source Default Distance Values
Connected interface 0
Static route 1
Enhanced Interior Gateway Routing Protocol (EIGRP) summary route 5
External Border Gateway Protocol (BGP) 20
Internal EIGRP 90
IGRP 100
OSPF 110
Intermediate System-to-Intermediate System (IS-IS) 115
Routing Information Protocol (RIP) 120
Exterior Gateway Protocol (EGP) 140
On Demand Routing (ODR) 160
External EIGRP 170
Internal BGP 200
Unknown* 255
If the administrative distance is 255, the router does not believe the source of that route and does not install the route in the routing table.
Each routing protocol has a default AD; you can, however, change the AD for specific routes or for a routing protocol. Earlier when I added 150 to the end of the static route entry, I changed the AD for that specific route. If I want to change the AD for a routing protocol, I must use the distance command.