Prepending means nothing but just addition... IN BGP you can use prepending in case of attribute of AS_path..
IN BGP when a router advertise a route then it will aa its Autonomous system number in route, so in the same way when this route will pass through neighbor router,neighbor router prepend its AS number in the route..
For Examle:-- Router A belongs to AS no.1, Router B belongs to AS no.2 and router C belongs to AS no.3
When A advertise a route to B then in this route A will install its AS number in the AS_path attribute of the route. now the route is in Router B routing table. When B advertise this route to C then B will prepend its own AS number in this route( means add the AS number before the AS number of router A).
So in the route AS_path attribute will be like this...
IN router C
Advantage:-- When the same route reaches router 1 then router 1 will check its own as number in as_path attribute. when a will find its own AS number it will drop the packet. so no loops will be created.
If I am correct all AS-Prepnding does is simply add its own AS # again to the route out to other interfaces ex: Router a router b router c, router a will send a tcp packet on top of layer 3 (igp) in the following format (as number source)ex: router a 65001 router b 65002 router c 65003 so basically when a router (providing it has a peer relationship) will send a packet like the following (65001, 65002) and when c sends the route outbound the packet will look like this (65001,65002,65003) obviously there is more info here(just showing a simple example here).No when we set the bgp prpend option the same route out of router c will look like this(65001,65002,65003,65001).
The best application for this option is when you have one or more links to an internet isp
AS Path prepending is a technique of deliberately extending the AS Path length of a prefix advertisement by adding additional AS numbers into the AS Path of an advertised prefix. Normally the form of AS Path prepending uses the local AS number to perform the prepending.
Just go thro. this link explaining with an example,
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