On Mon, Nov 9, 2009 at 14:46, Taqdir Singh <[email protected]> wrote: > > Could anyone please answer my below doubts ? > > 1) why BGP is so made with i BGP AD of 200 and eBGP AD of 20 ? > 2) why BGP is so made that eBGP next hop is carrier in local AS ?
The idea behind #1 is really simple - "get the traffic out of my network, as soon as possible". I.e. if you have a prefix from external neighbor that is "equally good" as the one from internal neighbor, or IGP, there is really no point in wasting your own resources to route this someplace else, instead of sending it to another AS. Issue #2 can be debated back and forth, endlessly. Originally, BGP would not advertise prefixes unless they were known in IGP (bgp synchronization, simplified). Hence, next-hop attribute was preserved. It could also happen that some BGP speakers have closer IGP path to that IP than to the advertising edge node, etc. However, the most important thing to note here are route-reflectors. You can have "out-of-band" route-reflectors, that is RRs that are not passing the traffic themselves, they just... reflect routes between clients. In order to support this functionality, you need to preserve next-hop attribute. Hope this helps. -- Marko Milivojevic - CCIE #18427 Senior Technical Instructor - IPexpert Mailto: [email protected] Telephone: +1.810.326.1444 Live Assistance, Please visit: http://www.ipexpert.com/chat eFax: +1.810.454.0130 _______________________________________________ For more information regarding industry leading CCIE Lab training, please visit www.ipexpert.com
